Portable hip exoskeleton improves walking economy for stroke survivors

Domain: neuroscience
Published: 2026-02-14
Editorial outcome: major_revision

Abstract
Increased metabolic cost of walking after stroke limits mobility and quality of life for millions of individuals. Existing portable assistive devices, primarily targeting the ankle joint, have failed to alleviate this burden. Here, we tested a portable, lightweight hip exoskeleton providing bilateral assistance during walking for individuals with chronic post-stroke hemiparesis. The exoskeleton significantly reduced the net metabolic cost of walking by 18 ± 2% (mean ± standard error, p = 0.0002) in seven participants during treadmill walking—a reduction sufficient to potentially lessen fatigue and extend walking duration—compared to walking without the device. This improvement was associated with a 29 ± 6% reduction in positive biological hip work (p = 0.0052), indicating effective offloading of the hip joints. These results provide the first evidence that portable hip exoskeleton assistance can improve walking economy in stroke survivors, offering a promising therapeutic strategy to enhance real-world mobility and functional recovery in this large clinical population.

Introduction
Millions of individuals experience reduced mobility and quality of life after a stroke1,2. Hemiparesis affects 80% of stroke survivors and is the leading cause of disability in the United States3. This condition involves impaired motor control4, muscle weakness5, and spasticity on one side of the body6, which collectively contribute to challenges in community ambulation7,8.

Independent walking is essential for enhancing community participation and quality of life after stroke8,9. Unfortunately, even after receiving the best rehabilitation and physical therapy, many stroke survivors face permanent impairments. Typically, individuals with hemiparesis have slow walking speeds10, limited endurance11, and high fall risk12, diminishing their walking independence. These challenges primarily result from the inefficiency of hemiparetic gait patterns. The metabolic cost of walking in individuals with hemiparesis is 60% higher than in healthy populations13, creating a significant physiological burden that directly limits functional endurance. This excessive effort often restricts individuals’ ability to walk distances required for community engagement, contributes to debilitating fatigue, and ultimately diminishes independence and quality of life2. Therefore, reducing this metabolic burden is a critical clinical goal, and new devices and interventions are urgently needed to improve walking economy for those affected by hemiparesis.

Individuals with hemiparesis exhibit abnormal gait patterns due to the weakness of their affected side10. Ankle weakness typically leads to impaired push-off during late stance, decreasing propulsion4,14. Ankle weakness also contributes to a lack of dorsiflexion during swing (foot drop), reducing foot clearance and increasing fall risk15. Knee weakness leads to instability during early stance, forcing individuals with hemiparesis to hyperextend their knee for stability4,16. This hyperextension increases loading on the knee, which can increase pain and the risk of secondary conditions, such as osteoarthritis17,18. Furthermore, knee spasticity can lead to stiff knee gait, characterized by a lack of knee flexion during swing14. Finally, hip weakness impairs swing initiation, leading to shorter steps and decreased gait symmetry19,20. Compensatory movements, such as hip hiking and circumduction, are commonly used to compensate for foot drop and stiff knee gait21. Moreover, the unaffected side must compensate for the weakness and lack of coordination of the affected side, making hemiparetic gait highly asymmetric. These abnormal gait patterns result in a substantial increase in the metabolic cost of walking22–24. Thus, new devices and interventions that directly target leg weakness and abnormal gait could improve metabolic cost of walking in the hemiparetic population.

Powered exoskeletons are a potential solution to this problem25. Exoskeletons can supplement the affected joint’s residual strength using power from motors and have been previously studied to assist hemiparetic gait26–28. However, achieving substantial metabolic cost reductions with portable devices in clinical populations such as stroke survivors remains a significant challenge25.

Substantial research efforts have focused on using powered ankle exoskeletons or exosuits to improve paretic ankle propulsion29–31. One study demonstrated that improving propulsion decreased the metabolic cost of walking in individuals with hemiparesis27. However, this study used a tethered device to offset the mass of the actuators, battery, and control electronics, making the device unusable outside the lab. The additional mass of an exoskeleton increases metabolic cost proportionally to its distance from the user’s center of mass, so carrying mass at the ankle is four times more costly than carrying mass at the trunk32. To partly address this problem, portable versions of the ankle exosuit located the actuation, battery, and electronics in a hip pack, which minimizes the metabolic penalty of the additional mass33. This portable ankle exosuit demonstrated increased overground walking speed27,34. However, it did not decrease the metabolic cost of walking or transport34–36. As such, no portable ankle exoskeleton has successfully reduced the metabolic cost of walking for individuals with hemiparesis, most likely due to the added exoskeleton weight.

Powered hip exoskeletons can be very lightweight37–40, minimizing the metabolic penalty of the additional mass. Furthermore, the mass of a hip exoskeleton creates less burden on the body because it is located closer to the body’s center of mass, thereby minimizing the metabolic energy cost of carrying it32. The peak torque and power are lower at the hip joint than at the ankle joint41, allowing hip exoskeletons to have smaller motors and batteries. In contrast to ankle exoskeletons, which can only inject energy during stance when the foot is in contact with the ground, hip exoskeletons can inject additional energy during swing. Pathological conditions that result in decreased ankle push-off demand greater positive hip flexion and extension power42–44, which is metabolically more costly than positive ankle power45. Thus, powered hip exoskeletons are a promising solution to assist hemiparetic gait.

Although many studies have demonstrated the metabolic benefits of powered hip exoskeleton assistance in healthy individuals39,40,46–56, only a handful of studies have explored hip exoskeletons to assist individuals with hemiparesis57–60. Unilateral hip exoskeleton assistance to the hemiparetic side has shown improved walking speed57,58. Moreover, bilateral assistance to both hips has shown greater improvements in walking speed than unilateral assistance to the affected hip joint only58,59. A case series with three individuals with hemiparesis subjects suggested that unilateral hip flexion assistance could improve the metabolic cost of walking61. However, a subsequent study with nine hemiparetic subjects showed no significant improvement in the metabolic cost of walking with bilateral hip assistance60. Unfortunately, these studies did not assess the impact of hip assistance on hemiparetic gait biomechanics. As a result, it remains unknown whether concurrent hip flexion and extension assistance provided by a powered hip exoskeleton can effectively compensate for impaired ankle propulsion and reduce metabolic cost.

Biomechanical analysis has shown that using a hip exoskeleton to assist with hip flexion and extension can help compensate for reduced ankle propulsion associated with a passive leg prosthesis62 thereby decreasing the metabolic cost of walking in individuals with above-knee amputation63. Based on these findings, we hypothesize that a hip exoskeleton could offset the impaired ankle propulsion in individuals with hemiparesis by simultaneously assisting the affected and unaffected hip joints, leading to reduced metabolic cost and positive biological work of both hip joints. To test this hypothesis, we assessed the metabolic cost of treadmill walking at a constant self-selected speed, with and without a lightweight, portable, bilateral hip exoskeleton designed to supplement the user’s hip flexion and extension, while measuring the positive biological work performed by both hip joints.

Results
We conducted experiments with individuals with hemiparesis after stroke (n = 7) walking with and without a lightweight, portable, bilateral hip exoskeleton (Fig. 1a, b). The exoskeleton provided flexion assistance during late stance and swing and extension assistance during early-to-mid stance to the affected and unaffected hip joints (Supplementary Fig. 1). The assistance was manually tuned for each participant and each side independently, based on the participant’s subjective feedback and the experimenter’s judgment. The total weight of the exoskeleton was 2.63 kg. Participants walked on a fully instrumented, split-belt treadmill (Bertec) for six minutes at a time with and without the powered hip exoskeleton. The treadmill speed was based on the participants’ overground walking speed, measured using the 10-meter walk test. Data were recorded during the last 2 min of each treadmill walking trial. Pairwise t tests were performed to assess significant differences in our three primary outcome measures: metabolic cost of walking, total positive biological work, and positive biological hip work, with and without the exoskeleton. See Materials and Methods for more details.

Metabolic cost
The metabolic cost of walking was calculated for each participant as the total metabolic energy expenditure minus the metabolic consumption during relaxed standing32,64. The average net metabolic rate was 2.8 ± 0.1 W kg−1 (mean ± standard error of the mean) without the exoskeleton and decreased to 2.3 ± 0.1 W kg−1 with the exoskeleton assistance (Fig. 1c). Thus, assistance from the powered exoskeleton reduced the metabolic cost of walking by 18 ± 2% on average across the seven participants (two-tailed paired t test, p = 0.0002, n = 7). All participants showed a reduction in the metabolic cost with the exoskeleton compared to walking without the exoskeleton (Fig. 1c).

To account for the metabolic cost of wearing the exoskeleton, we calculated the metabolic penalty of adding exoskeleton mass to the thigh and pelvis64,65. The exoskeleton mass added to each thigh was 0.67 kg, resulting in a metabolic penalty of 7.44 W (5.55 W kg−1 × 0.67 kg/side × 2 sides). The exoskeleton mass added to the pelvis was 1.29 kg, resulting in a metabolic penalty of 4.23 W (3.33 W kg−1 × 1.29 kg). Thus, the total metabolic penalty of wearing our portable hip exoskeleton was 11.76 W, which, normalized by the body weight of each participant and averaged, is equivalent to 0.148 W kg−1. Accounting for the estimated metabolic penalty of the exoskeleton mass, the theoretical net metabolic savings compared to wearing the unpowered exoskeleton would be 0.653 W kg−1, compared to the measured 0.505 W kg−1 without the exoskeleton. Given that the average positive mechanical power for both exoskeleton sides combined was 0.317 W kg−1, the average ratio between the weight-adjusted metabolic saving and the average positive bilateral exoskeleton power across subjects was 2.06 (Supplementary Fig. 2). Thus, if we account for the metabolic penalty of the exoskeleton mass, the average modified exoskeleton performance index65,66 was 0.515, which is about half the theoretical maximum of 1 (Supplementary Table 1).

To assess the metabolic cost of walking in a speed-normalized form, we calculated the cost of transport67. The average cost of transport with the exoskeleton was 3.6 ± 0.4 J m−1 kg−1, down from 4.4 ± 0.5 J m−1 kg−1 without the exoskeleton. As expected from the metabolic rate analysis, all participants decreased their cost of transport with the exoskeleton. However, the reduction in the cost of transport was inversely proportional to the walking speed (Supplementary Fig. 3).

Positive joint work
As is commonly done in the field46,68, we analyzed positive work as it has been shown to have a substantially higher metabolic cost than negative or total work69. Positive biological work (Fig. 2a, b) was calculated by integrating positive biological power. Biological hip joint power (Fig. 2c) with the exoskeleton is the total hip joint power calculated through inverse dynamics minus the exoskeleton power.

The total positive work done by the hip exoskeleton (affected and unaffected sides) per stride was 0.12 ± 0.01 J kg−1. This exoskeleton assistance significantly reduced the total positive biological work by 14% ± 4% on average across the seven participants (two-tailed paired t test, p = 0.0167, n = 7). Specifically, the total positive biological work done by the affected and unaffected hip, knee, and ankle joints per stride decreased from 0.76 ± 0.07 J kg−1 without the exoskeleton to 0.66 ± 0.07 J kg−1 with the exoskeleton (Fig. 2a).

Exoskeleton assistance significantly reduced the positive biological hip work by 29% ± 6% on average across the seven participants (two-tailed paired t test, p = 0.0052, n = 7). Specifically, the positive biological hip work per stride decreased from 0.33 ± 0.05 J kg−1 without the exoskeleton to 0.23 ± 0.05 J kg−1 with the exoskeleton. In contrast, no significant differences in positive biological work were observed for the knee or ankle joints (Fig. 2a).

Without the exoskeleton, the total positive work per stride was 0.26 ± 0.03 J kg−1 on the affected side and 0.51 ± 0.05 J kg−1 on the unaffected side. With the exoskeleton, the total positive work increased by 0.02 ± 0.02 J kg−1 (8% ± 7%) on the affected side and was virtually unchanged on the unaffected side (−1% ± 5%).

On the affected side, the exoskeleton provided 21% ± 3% of the total positive work, and the biological hip joint contribution to total positive work decreased from 48% ± 6% to 35% ± 6% (Fig. 2b). On the unaffected side, the exoskeleton contributed 14% ± 1% of the total positive work, and the contribution of the unaffected hip joint decreased from 40% ± 4% to 25% ± 2% with the exoskeleton.

Hip work during stance
The exoskeleton assistance noticeably affected the biological hip joint power during different phases of the gait cycle (Fig. 2c). We assessed the effects of exoskeleton assistance on positive biological hip work within different gait phases: initial double support, single support, terminal double support, and swing (Fig. 3). Initial double support lasts from the ipsilateral heel strike to the contralateral toe-off, then single support occurs during the contralateral swing phase, and finally, terminal double support occurs from the contralateral heel strike until the ipsilateral toe-off.

When the unaffected side is in initial double support, the affected side is in terminal double support (Fig. 3). During this critical step-to-step transition, the exoskeleton provides extension assistance to the unaffected side and flexion assistance to the affected side, decreasing the positive biological hip work by 0.026 ± 0.008 J kg−1 (60% ± 18%) and 0.022 ± 0.013 J kg−1 (49% ± 26%), respectively, per stride. During the next phase of gait, when the unaffected side was in single support and the affected side was in swing, the positive biological hip work also decreased by 0.035 ± 0.013 J kg−1 (64% ± 19%) and 0.014 ± 0.014 J kg−1 (55% ± 27%) on the unaffected and affected side, respectively. During the following step-to-step transition (unaffected side trailing), the positive biological hip work decreased slightly by 0.005 ± 0.009 J kg−1 (83% ± 25%) on the unaffected side but increased by 0.014 ± 0.009 J kg−1 (118% ± 22%) on the affected side. In the final phase, when the unaffected side is in swing and the affected side is in single support, the positive biological hip work decreased by 0.017 ± 0.004 J kg−1 (71% ± 17%) on the unaffected side and increased slightly by 0.004 ± 0.003 J kg−1 (168% ± 22%) on the affected side.

Foot clearance
Minimum foot clearance trajectories were calculated by taking the minimum trajectory of the toe and heel during swing (Supplementary Fig. 4). Hip exoskeleton assistance increased foot clearance on both the affected and unaffected sides. On the affected side, peak foot clearance increased by 4 ± 2 mm (10% ± 6%) with the exoskeleton. On the unaffected side, peak foot clearance increased by 3 ± 1 mm (7% ± 3%) with the exoskeleton.

Kinematics
Kinematic trajectories during walking with and without the exoskeleton are represented in Supplementary Fig. 5. Hip exoskeleton assistance slightly increased the affected hip joint’s average range of motion (Supplementary Fig. 6), primarily by increasing peak hip flexion by 2° ± 1° (8% ± 3%). The unaffected hip joint’s average range of motion decreased by 2° ± 1° (8% ± 2%), decreasing in both peak flexion and extension. Exoskeleton extension assistance slightly increased the average peak knee extension angle by 2° ± 1° (31% ± 18%) on both sides. Finally, with the exoskeleton, the affected ankle joint was slightly more plantarflexed throughout the gait cycle, while the unaffected ankle joint was slightly more dorsiflexed throughout the gait cycle. These differences were generally less than 2° and resulted in a slightly decreased range of motion at the affected ankle joint (Supplementary Fig. 6). Statistical Parametric Map (SPM) analysis demonstrated that there were no statistically significant differences (two-tailed paired t test, α = 0.05) in the kinematic trajectories of each joint during walking with the exoskeleton compared to without the exoskeleton (Supplementary Fig. 7). Thus, hip exoskeleton assistance resulted in only minor, non-clinically significant and non-statistically significant kinematic changes.

Symmetry was evaluated by calculating a symmetry index (SI) where 0% indicates perfect symmetry36. Hip exoskeleton assistance improved symmetry in the hip joint range of motion (−30% ± 10% without the exoskeleton vs. −18% ± 9% with the exoskeleton) but had only minor effects on the knee and ankle range of motion symmetry (Supplementary Fig. 8). Exoskeleton assistance slightly improved symmetry in stance time (13% ± 3% asymmetry without the exoskeleton vs. 12% ± 4% asymmetry with the exoskeleton), but increased asymmetry in step length (−8.2% ± 6% without the exoskeleton vs. −14% ± 5% with the exoskeleton) (Supplementary Fig. 9). Generally, during walking with and without the exoskeleton, participants spent longer in stance on their unaffected side and took longer steps onto their affected side.

To evaluate gait stability, we compared the extrapolated center of mass and step width during walking with and without the exoskeleton (Supplementary Fig. 10). The minimum mediolateral distance from the extrapolated center of mass to the average center of pressure of the affected and unaffected sides was relatively unchanged, decreasing by 0.2 ± 0.3 cm (5% ± 8%) on the affected side and by 0.06 ± 0.34 cm (4% ± 11%) on the unaffected side with the exoskeleton. Furthermore, the step width was 20 ± 2 cm during walking with and without the exoskeleton.

Kinetics
Kinetic trajectories, including the biological hip, knee, and ankle joint moment and vertical ground reaction force (GRF) trajectories are presented in Supplementary Fig. 11. On the affected side, exoskeleton assistance increased the average peak biological hip flexion and extension moments on the affected side by 0.07 ± 0.09 Nm kg−1 (3% ± 17%) and 0.07 ± 0.04 Nm kg−1 (38% ± 27%), respectively. On the unaffected side, exoskeleton assistance increased the peak biological hip flexion moment by 0.13 ± 0.09 Nm kg−1 (27% ± 19%) and decreased the peak biological hip extension moment by 0.10 ± 0.04 Nm kg−1 (20% ± 7%). Similarly, on the affected side, hip exoskeleton assistance increased the peak knee flexion moment by 0.10 ± 0.03 Nm kg−1 (21% ± 9%) and decreased the peak knee extension moment by 0.09 ± 0.04 Nm kg−1 (48% ± 23%). In contrast, on the unaffected side, exoskeleton assistance decreased the peak knee flexion moment by 0.0002 ± 0.0300 Nm kg−1 (0.6% ± 8.4%) and increased the peak knee extension moment by 0.10 ± 0.05 Nm kg−1 (27% ± 17%). Finally, the ankle moments were unchanged on both the affected and unaffected sides during walking with exoskeleton assistance compared to walking without the exoskeleton. The greater peak joint moments with hip exoskeleton assistance are likely related to the greater observed vertical ground reaction force. The peak vertical GRF increased by 0.04 ± 0.006 N kg−1 (4.4% ± 0.6%) on both the affected and unaffected sides (Supplementary Fig. 12).

Exoskeleton assistance
Many participants preferred similar exoskeleton tuning on each side, resulting in similar generated torque profiles on each side. Notably, participant 6 received greater peak flexion torque on their affected side compared to their unaffected side. Generally, the peak flexion torque provided by the exoskeleton was greater than the peak extension torque, with greater extension torque supplied to the unaffected side (Supplementary Fig. 1). Each peak of exoskeleton torque resulted in a positive exoskeleton power peak, with flexion torque resulting in a greater peak power than extension torque. The peak power generated on the unaffected side was greater than that generated on the affected side for both flexion and extension assistance. Specifically, the peak power generated on the unaffected side (0.17 ± 0.02 W kg−1 flexion, 0.09 ± 0.01 W kg−1 extension) was greater than that generated on the affected side (0.13 ± 0.02 W kg−1 flexion, 0.08 ± 0.01 W kg−1 extension) for both flexion and extension assistance.

Handrail forces
For safety, all participants were instructed to rest their unaffected hand on a force-instrumented handrail mounted on the side of the treadmill, using only the minimum support needed. The average total handrail force was generally low, ranging from 1% to 5% of the participant’s body weight (Table 1), and there was no statistically significant difference between the average total handrail force applied with and without the exoskeleton. Moreover, average handrail forces in the lateral, frontal, and vertical axes were very similar with and without the exoskeleton (Supplementary Fig. 13). However, the peak lateral handrail force occurred slightly earlier in the gait cycle when using the exoskeleton. The average handrail forces per stride in each axis remained the same with and without the exoskeleton. Some participants showed small differences in peak forces in each direction of their handrail forces, which can be seen when examining each plane of motion separately (Supplementary Fig. 14). The largest differences in handrail use were observed for participant 2, whose peak vector total handrail force increased by 2.2%, and participant 6, whose peak vector total handrail force decreased by 2.5%, with the exoskeleton per stride. On average, three participants used slightly more force with the exoskeleton (S2, S3, S4), while four participants used slightly less force (S1, S5, S6, S7).

Methods
Participants
We recruited seven individuals with hemiparesis from local clinics to participate in this study. Participant inclusion criteria included the following: age between 18 and 85 years, at least 6 months post-stroke, hemiparesis due to stroke, and the ability to walk on a treadmill for six minutes. Participants were excluded from the study if they weighed more than 250 lbs or had a cognitive deficiency that precluded the ability to give informed consent, were pregnant, or had a co-morbidity that interfered with the study. Following statistical power analysis, we enrolled seven participants (Table 1).

Ethics
The Institutional Review Board at the University of Utah approved the study protocol (IRB00120712). Participants provided written informed consent to participate in the study. Participants also provided written consent for the publication of pictures and videos of the experiments.

Exoskeleton training and assistance tuning
Participants underwent extensive training with the hip exoskeleton before the metabolic cost evaluation and motion capture experiment (Fig. 4). Training with the exoskeleton allows users to adapt and maximize the benefits of the assistance86. It can take hours of training for individuals with mobility challenges to experience maximum benefits from exoskeleton assistance87. Due to the neurological impact of a stroke, motor adaptation is critical for individuals with hemiparesis to optimize the benefits of exoskeleton assistance88. To allow sufficient training, participants came in for exoskeleton training sessions and completed at least 30 min of continuous walking with the exoskeleton before data collection (Fig. 4b).

The first training session included fitting the exoskeleton for each participant. The experimenter helped the participants don the powered hip exoskeleton and adjusted the straps to ensure a proper fit while alleviating potential discomfort. After fitting the device, the participants walked on a treadmill, and the experimenter manually tuned the assistive controller by adjusting the timing and peak of the flexion and extension torques. To maximize assistance, the experimenter watched a real-time plot of the estimated joint power and adjusted the controller parameters while the participant walked with the exoskeleton. This tuning procedure was done independently for the affected and unaffected sides. Exoskeleton assistance parameters were tuned based on the experimenter’s training and experience via visual inspection of gait during the walking sessions89. The experimenters also watched a live estimate of exoskeleton power based on the exoskeleton torque and velocity, and adjusted timing and peak torque parameters to maximize positive power generation, which is critical for reducing the metabolic cost of healthy subjects47,70. After each adjustment, the participant was asked if the assistance felt better or worse. If it felt better, the experimenter would continue adjusting that parameter in that direction until it felt worse. If the change felt worse, the parameter was reverted to its previous value, and a different change was made. Comfort was assessed by asking the participants for their subjective feedback and visually inspecting the exoskeleton interfaces for excessive bending or movements. Participants walked for approximately half an hour, less than 10 min at a time, with rest between walking trials.

Each additional training session was completed approximately one week apart. These sessions included at least half an hour of total walking with the device, where participants walked for less than 10 min at a time with rest in between walking trials. The experimenter continued tuning the exoskeleton assistance as necessary. The experimenter determined when to advance the participant in the protocol based on their experience and the participant’s feedback on their comfort and confidence with the device. All participants completed at least one training session before completing data collection.

The amount of exoskeleton training was not standardized across participants in this study to accommodate differences in walking ability between participants. Some participants exhibited less consistent walking patterns, which made exoskeleton tuning less consistent and necessitated more tuning and training. Furthermore, some participants needed more rest and walked less each training day, so they had more training days than participants who could walk longer in one session. On average, participants completed 2 ± 1 additional training sessions (not including the fit/tune session) before data collection. Participants generally completed a session once a week, completing training over 4 ± 1 weeks on average. The experimenter started with the converged parameters from the prior training session and tuned as necessary from there. Parameters were retuned in each training session based on any changes in user preference or comfort, and any gait changes observed by the experimenter. The exoskeleton parameters were kept the same as the final tuned parameters from the final training session during the metabolic cost evaluation and motion capture experiments.

Metabolic cost evaluation
Approximately one week after the participants completed the exoskeleton training, we evaluated their metabolic cost of walking with and without the powered exoskeleton (Fig. 4c). Participants donned a heart rate monitor (Polar). They performed a 10-meter walk test twice to determine their self-selected walking speed. Participants then donned a portable indirect calorimetry measurement system (K5, COSMED) and performed the final stage of the metabolic system calibration90.

At the beginning of the experiment, participants stood still for 2 min while their baseline standing metabolic rate was collected. Next, the participants walked on a fully instrumented, split-belt treadmill (Bertec) at their self-selected walking speed for 6 min to determine the metabolic cost of walking without the exoskeleton. Data were collected during the last 2 min of the walking trial.

Participants rested seated until their heart rate reached their baseline resting rate. Then, the participants donned the powered hip exoskeleton. After ensuring proper fit, the participants walked with the exoskeleton on the treadmill at the same walking speed as without the exoskeleton. To allow sufficient time for acclimatization88, participants performed three treadmill walking trials while wearing the exoskeleton, as in other metabolic studies28,63. Each walking trial consisted of a slow increase in assistance for 1 min, followed by 6 min of walking. Thus, each participant walked with the exoskeleton assistance for 21 min. Data were collected during the last 2 min of the third walking test. Before each test, the participants rested seated for at least 5 min, or until their heart rate reached their baseline resting rate. Upon completion of the three exoskeleton walking trials, the participants rested. After the final rest, we measured their standing metabolic expenditure to check for fatigue.

For safety, all participants were instructed to rest their unaffected hand on a side-mounted handrail during all treadmill walking tests. We instructed participants to use the smallest amount of handrail support needed, and the level of support was different for each participant. The average vector total handrail force per stride ranged between 1% and 5% of each participant’s body weight (Table 1). Participants were generally consistent with the level of support they used with and without the exoskeleton, and only two participants showed meaningful differences between exoskeleton conditions. Participant 2 increased their peak vector total handrail force by 2.2% with the exoskeleton, while participant 6 decreased their peak vector total handrail force by 2.5% with the exoskeleton. Participants wore a harness, but no body weight was supported.

The appearance of the conditions was not randomized, and participants always walked without the exoskeleton first. Randomizing the appearance of the exoskeleton and no exoskeleton conditions could have disrupted the adaptation period and introduced after effects59,91. Moreover, it would have been impractical because it would have required participants to repeatedly don and doff the exoskeleton, requiring additional time to ensure the proper fit of the exoskeleton and recalibrate the motion capture system, contributing to fatigue. The lack of randomization may have introduced bias. For example, if the participants became fatigued during the session, we would expect that their metabolic cost would have increased between the first and last 6-minute walking trials. To ensure fatigue was not a problem, we compared the initial standing baseline taken before the walking tests to the standing baseline after the tests with the exoskeleton. This also allowed us to assess potential drifts in the respiratory measurements. There was no statistical difference between the standing metabolic expenditure measured at the beginning and end of the protocol. Thus, respiratory measurements showed no significant drifts, and participants did not experience fatigue.

Motion capture experiment
Compared to our previous metabolic study with individuals who have above-knee amputations63, we added an experimental session to perform a comprehensive biomechanical analysis using full-body motion capture with an instrumented treadmill and instrumented handrails. This additional analysis provided a mechanistic explanation for the observed metabolic reduction, which is essential to assess the benefits of the proposed assistive approach.

Approximately one week after the metabolic cost evaluation, the final testing session was performed in a motion capture lab using a 12-camera Vicon system (Vicon Motion Systems, Ltd., Oxford, U.K.) and a fully instrumented split-belt treadmill (Bertec, Ohio, USA) (Fig. 4d). Participants wore tight-fitting clothing with reflective markers placed on anatomical bony landmarks. Markers were placed following a modified Plug-in-Gait model with redundant markers placed on the hip, thighs, shins, and feet for more reliable tracking92.

Each participant completed a static calibration trial, a functional joint center calibration trial, and a functional dynamic capture. These calibration routines were repeated twice, once without the exoskeleton and once with the exoskeleton. The static calibration created a subject-specific motion capture model. The functional joint center calibration located the centers of rotation for the hip and knee joint axes using the Symmetric Center of Rotation Estimation (SCoRE)93 and Symmetrical Axis of Rotation Analysis (SARA)94. The functional dynamic capture improved the ability of the software to build and label the model for each additional capture.

Upon completion of the calibration trials, the participants repeated the series of 6-minute walking trials completed during the metabolic cost evaluation at the same speed and with the same exoskeleton tuning. Data were recorded during the last 2 min of the test without the exoskeleton and the third test with the exoskeleton. Marker trajectories were sampled at 200 Hz, and ground reaction forces were sampled at 1000 Hz. All data were synchronized using a lock sync box (Vicon Motion Systems, Ltd., Oxford, U.K.).

We conducted the metabolic cost evaluation and motion capture sessions on separate days to minimize the burden on participants and the potential for fatigue. To reduce the impact of day-to-day variability, we implemented exoskeleton training sessions before data collection and used the same treadmill walking speed and exoskeleton tuning during both experiments. A multi-day protocol also reduced the impact of data collection failures due to problems with the exoskeleton, metabolic, or motion capture equipment.

Powered hip exoskeleton
The powered hip exoskeleton used in this study was previously presented and validated in healthy individuals37, individuals with above-knee amputation63, and one individual with hemiparesis57. The exoskeleton provides bilateral flexion and extension torque to the user’s hip joints while allowing for passive abduction and adduction. All interfaces, actuation, electronics, and the battery are integrated into the exoskeleton, which is worn on the user’s pelvis and thighs. The total weight of the exoskeleton was up to 2.63 kg.

Compared to our previous metabolic study with individuals who have above-knee amputations63, we made substantial upgrades to the powered hip exoskeleton hardware. First, we moved from a unilateral to a bilateral configuration in which assistance is provided to both hip joints. Then, we redesigned the pelvis interface and thigh cuff, moving from a soft textile-like structure to a semi-rigid interface sized to fit each user. Finally, we used more reliable cables and connectors to interface the main electronic board, located in the pelvis interface, with the actuators in the exoskeleton thigh segments. These design changes are described in our preliminary validation study with healthy individuals37.

Each actuator features a five-bar linkage based on an offset slider crank and includes a high-efficiency ball screw (Fig. 4a). The hip exoskeleton has a peak torque of 45 Nm in flexion and extension. The pelvis interface integrates the electronics and battery into a rigid frame that connects to the user through BOA straps (BOA Technology Inc.) and flexible 3D printed orthoses. The thigh interface includes an anterior stiff 3D printed frame and a flexible wrap-around cuff with a posterior BOA closure. The passive degrees of freedom in the powered hip exoskeleton create a self-aligning mechanism similar to the system described in our previous work95, which has been shown to improve users’ comfort and performance96. The 3D printed interfaces can be adjusted to fit different users, and the BOA straps feature ratcheting dials to provide customizable tightening levels.

Assistive controller
As in our previous studies37,62,63, the hip exoskeleton’s embedded control system runs an assistive controller based on adaptive frequency oscillators97–99 (Fig. 4b). The controller has a 0–100% gait cycle with a flexion torque gaussian profile and extension torque gaussian profile. The magnitude, width, and location of each Gaussian in the gait cycle can be independently controlled for each side. These parameters were tuned for each participant before the start of the experiment. Tuning was completed independently for the participants’ affected and unaffected hip joints. Tuning was based on feedback from the participants and the experimenter’s judgment.

Data processing and statistical analyses
The modified Brockway equation was used to calculate body weight normalized metabolic power over the last two minutes of the standing and walking trials64. Net metabolic power was obtained by subtracting the average metabolic power measured during the standing baseline trial from the average metabolic power measured during each walking trial. The metabolic cost of transport was calculated for each walking trial by dividing the metabolic power of each participant by their walking speed67.

The exoskeleton performance index quantifies the ratio between the change in metabolic power and average exoskeleton positive mechanical power provided, scaled by the muscular efficiency factor of 0.2566. We use this muscular efficiency factor based on work indicating that human muscles perform positive mechanical work with a muscular efficiency that asymptotically approaches 0.2569.

We evaluated interlimb asymmetry during both walking trials using asymmetry index (SI). The SI is calculated by taking the difference between the spatiotemporal data of the affected and unaffected sides, divided by one-half the sum of the spatiotemporal data from both sides, multiplied by 100%36. A positive SI indicates asymmetry towards the subject’s unaffected side, while a negative SI indicates asymmetry towards the subject’s affected side. Zero indicates perfect symmetry.

Motion capture data were analyzed offline using Nexus 2.16 (Vicon Motion Systems, Ltd., Oxford, U.K.), Visual 3D (C-Motion, Maryland, USA), and MATLAB 2024b (MathWorks, Inc., Massachusetts, USA). Core processing in Nexus produced 3D trajectories and joint centers from the raw marker data and calibrated joint positions using SCoRE and SARA. The marker trajectories, force plate analog data, and exoskeleton data were imported into Visual 3D. Marker trajectories and force plate measurements were digitally lowpass filtered using bidirectional, fourth-order Butterworth filters at 6 Hz and 15 Hz, respectively. Kinematics and kinetics were computed using the V3D Composite Pelvis100. All inverse dynamic calculations were computed in Visual 3D and imported into MATLAB for subsequent analysis. Trajectories from the inverse dynamic calculations were digitally lowpass filtered using a bidirectional, second-order Butterworth filter at 6 Hz.

As is commonly done in the powered exoskeleton field30,101,102, we calculated the biological torque produced by the participants by assuming that the exoskeleton perfectly transfers its desired assistance to the user. Thus, the biological hip torque was calculated by subtracting the assistive torque provided by the exoskeleton from the total hip torque calculated through inverse dynamics.

After stride-time normalization, we first calculated the average kinematic and kinetic profiles for each subject across strides, and then the average across the resultant subject means. We also calculated the average maximum values for each subject across strides, and then the average across the resultant subject means. Inter-participant means and standard errors are reported unless otherwise indicated.

Joint work was calculated as the time integral of power during each stride. Energetic analyses of the affected and unaffected hip joints were further divided into the subphases of a stride. Specifically, these phases are identified as initial double support, single support, terminal double support, and swing. Initial double support lasts from the ipsilateral heel strike to the contralateral toe-off, then single support occurs until the contralateral heel strike. Terminal double support occurs until the ipsilateral toe-off, and finally, swing occurs from the ipsilateral toe-off to heel strike.

Two-tailed paired t tests evaluated effects of exoskeleton assistance on our primary outcome measures: metabolic power, total positive biological work (affected and unaffected hip, knee, and ankle joints, summed), and positive biological hip work (affected and unaffected hip joints, summed). We set α = 0.05 for these analyses and applied a Holm–Bonferroni correction to adjust for multiple comparisons. We did not statistically compare any secondary outcome measures (hip work during stance, foot clearance, kinematics, kinetics). We used a two-tailed t test because we did not have a prior assumption that the difference between conditions (walking with or without the exoskeleton) would be positive or negative. Finally, the data were normally distributed and there were no outliers, meeting the requirements for a two-tailed paired t test.

To evaluate changes in kinematic trajectories during walking with the exoskeleton compared to walking without the exoskeleton, we performed a 1-dimensional Statistical Parametric Map (SPM) analysis, using the spm1d 0.4.10 package in Matlab 2024b103. SPM was run on the kinematic trajectories of each joint (affected and unaffected hip, knee, and ankle) during walking with the exoskeleton compared to walking without the exoskeleton to identify statistically significant changes (two-tailed paired t tests with α = 0.05).

Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper introduces a portable hip exoskeleton robot aimed at improving the quality of life for stroke patients by reducing their walking metabolism, which is highly significant in the current field of exoskeleton robotics. However, due to the complex and distinct walking kinematics, dynamics, and lower-limb musculoskeletal characteristics of stroke patients compared to healthy individuals, focusing solely on metabolic improvement is not comprehensive enough. For exoskeleton robots designed for stroke patients, it is desirable to enhance their mobility and metabolism without imposing additional burdens on other lower-limb joints and muscles. Overall, this paper holds substantial research value, but requires more comprehensive analysis of lower-limb joints and muscles in assisted stroke patients to confirm that hip exoskeleton assistance can effectively improve their quality of life.\n1. The statement in the third paragraph, “This hyperextension increases loading on the knee, which can increase pain and the risk of secondary conditions, such as osteoarthritis,” indicates that elevated joint force may heighten the risk of joint pathologies (e.g., osteoarthritis). If assistance is applied at the hip joint, how would the joint force at the hip, knee, and ankle change? Would this impose greater force on the joints, potentially causing further harm to patients?\n2. Gait asymmetry is a defining characteristic of stroke patients, as elaborated in the fourth paragraph: “Moreover, the unaffected side must compensate for the weakness and lack of coordination of the affected side, making hemiparetic gait highly asymmetric.” However, this critical feature was not analyzed in the study’s results. Could a comparative analysis be conducted on gait symmetry before and after hip exoskeleton assistance, alongside existing assistive strategies for stroke patients?\n3. Although no prior studies have specifically targeted metabolic reduction in stroke patients, hip exoskeletons have been extensively investigated for reducing walking metabolic rate in healthy populations. Does the methodology of this study fundamentally differ from those previous works? What adaptations were made specifically for stroke patients? Does its application in stroke patients demonstrate superior efficacy compared to existing research?\n4. This study shares similarities in research methodology with \"Powered hip exoskeleton improves walking economy in individuals with above-knee amputation.\" Could you elaborate in detail on the methodological improvements in this study, as well as the advancements in the exoskeleton hardware?", "reviewer_2": "Manuscript Number: NatComm – 617508\nPortable Hip Exoskeleton Improves Walking Economy for Stroke Survivors\nSUMMARY OF WORK:\nThis manuscript examines whether a bilateral, powered, autonomous hip exoskeleton (Fig 1a.) can reduce the metabolic cost of fixed-speed treadmill walking in people with stroke (Table 1). The authors designed and conducted a rigorous and comprehensive within-participant experimental design comparing metabolic and lower-limb joint mechanical effort without (No exo) and with (With exo) the hip exo providing clinician supervised ‘self-tuned’ timing and magnitude (ED Fig. 1) on each leg (paretic and non-paretic) (Fig. 1b, Fig 4.) The results indicate that hip exo assistance can reduce metabolic effort (W/kg) by 18% (Fig. 1c) and this is accompanied by significant reductions in users’ hip joint (29%) and total combined-lower-limb (14%) mechanical work. In addition, the authors detail how the exoskeleton contributes to hip work for each leg and phase of the gait cycle (Fig. 3) and show that the exo increases paretic limb toe clearance (ED Fig. 2).\nOverall, the manuscript is excellent and presents the first evidence that hip exoskeleton can reduce metabolic effort post-stroke. The writing and presentation are logical and concise – with clearly stated objectives, aims and hypotheses. The analysis is already comprehensive and mechanistic as it stands, however, there are some key limitations in its ability to support the data with full rigor. The reader is left with some key questions that when resolved will increase the confidence in what will be a surefire landmark finding in the field. I offer my suggestions below in an effort to help strengthen the findings and ultimately the impact of the work.\nCOMMENTS/SUGGESTIONS:\nThere are two major issues that need to be addressed.\n1. The authors mention that a number of the participants used handrails. This is fine as long as the handrail support is not measurably different in the No exo vs. With exo conditions (i.e. handrail support is not more/less on average in No exo vs. With exo condition). The authors address in their results stating:\n‘For safety, all participants were instructed to rest their unaffected hand on a side-mounted handrail during all treadmill walking tests, using the smallest amount of handrail support needed. There was no statistical difference between the force applied while walking without the exoskeleton compared to with the exoskeleton. The handrail force was also low, ranging from 1 - 4% of each participant’s body weight.”\nThis is super. But, please add detail on the level of the forces measured over the gait cycle (e.g., participant average hand rail force time series graphs and a table showing avg. values per participant in ED.)\n2. Based on documented efficiency of muscular positive mechanical work - user metabolic savings should not exceed ~ 4x user mechanical savings due to exoskeleton mechanical input. For this study the net metabolic power savings was ~0.5 W/kg which would suggest savings in positive stride cycle average mechanical power across the combined lower-limb of ~0.125 W/kg. However, the authors report metabolic results in W/kg and mechanical results in J/kg making this ‘back of the envelope’ gut check difficult without knowledge of the stride frequency. To add rigor and physiological grounding to the metabolic result please formally relate user metabolic savings (change in user input) to the user mechanical savings (change in user output and/or the exoskeleton mechanical input (device input) in consistent units and report for each individual. The preferred approach would be to plot these data in a scatter graph – and fit a line to the data – analyzing the value of the slope of the fit. Versions of this have been documented in the literature and termed ‘apparent efficiency’/ ‘performance index’ (Sawicki-Ferris), or ‘augmentation factor’ (Mooney-Rouse-Herr, Siviy-Walsh).\nSome minor critiques.\n3. Given that the participants vary widely in their walking speed – and metabolic rate is function of walking speed it would be a nice addition to present the data in speed normalized form as a function of speed (i.e. cost of transport (net met power/speed = J/m/kg vs. speed graph) for No exo vs. With Exo and use it to discuss what would happen if these participants were free to increase/decrease walking speed With Exo.\n4. Given that the participants vary widely in their ‘tuned’ exoskeleton torque assistance profile, it would be a nice addition to add some discussion (if not analysis) on what features of the exo torque primarily drove the mechanical and metabolic changes.\n5. It would be nice to indicate on the key results Fig. 1c what the ‘added mass penalty’ of the device is (i.e. Exo No Power). In other words- how much of the exoskeleton work went to offset it’s own mass? This would help more clearly define the performance ceiling of a more streamlined design.\n6. Has this device been tested on able bodied controls? If so, what are the results and how do they compare to the stroke cohort. This would be especially interesting to note with respect to the ‘apparent efficiency’ (see comment 2). Discussion point?", "reviewer_3": "In this work, authors presented a hip exoskeleton that provides active mechanical assistance bilaterally and bidirectionally to the hip joint during treadmill walking in seven individuals with hemiparesis. The use of their exoskeleton was able to reduce the metabolic cost of walking by 18%, likely by reducing positive biological hip work. This work has great potential to offer a portable solution for individuals with mild-to-moderate hemiparesis that enables them to walk for longer duration, leading to improved recovery and integration into the real world. Presentation of the work can be substantially improved by reorganizing the introduction and discussion, providing more detail in results (particularly in comparisons of non-metabolics related outcomes) and methods (particularly in training and tuning), and a more comprehensive discussion synthesizing results of different outcomes evaluated.\nIntroduction\nThe intro should be restructured to improve its cohesiveness. In this submission, authors jump back and forth between the discussion of post-stroke impairment and exoskeleton. Instead, consider centering the introduction around impairment-specific challenges related to metabolic cost, followed by discussion of demonstrated benefits and limitations of currently explored exoskeleton solutions to target these challenges, and why the authors’ solution - bilateral and bidirectional hip exoskeleton - is a promising alternative solution.\nPlease use people-first language when appropriate. “Individuals with above-knee amputation” is people-first, while “above-knee amputees” is not.\nResults\nConsider moving information from Extended Data Fig 1 into the main text when discussing tuning for both legs. From evaluating the figure it appears most individuals were tuned to receive similar level of assistance between the two legs, except for one participant. Results would be more comprehensive to include this quantification.\nThere seems to be a mix of statistical model-driven results and qualitative results. If this is the case, the authors should clarify which comparisons were statistically compared and which were not by distinguishing their language between these sets of comparisons. If all comparisons were statistically driven, then these comparisons should also include relevant statistical output (method, p value).\nOn a similar note as the previous comment, it is unclear whether the authors compared any hip work during stance, foot clearance, and kinematic variables with and without exo assistance using statistical methods.\nReporting percentage of change alongside actual numbers is helpful in sensing whether the change in outcome due to assistance is a big or a small change. The authors should include this information throughout - currently not included in hip work during stance, foot clearance, and kinematics.\nIn “exoskeleton assistance,” please provide quantifiable results that lead to authors’ claim that “peak power generated on the unaffected side was greater than that generated on the affected side” for both hip motion.\nDiscussion\nIn the discussion of symmetry restoration through exo, consider linking this back to metabolic cost reduction (i.e. relationship between asymmetry and increased metabolic cost)\nHowever, the solution is unique since this device provides bilateral assistance. Authors should consider reframing this statement to highlight the advantages and rationale for choosing a bilateral solution with active assistance bidirectionally compared to other approaches.\nDiscussion paragraph acknowledging the smaller sample size and lack of difference between different groupings could be better polished by synthesizing these results more. While further studies with increased sample are certainly warranted, consider the possibility that these results suggest this exo could be a viable solution for people with wide range of impairment, size or sex, and use of other assistive devices.\nBroadly speaking, the organization of the discussion could be improved, similar to the reviewer comment with the introduction. Some suggestions include: 1) Inclusion of a limitation paragraph towards the end of the discussion so they appear together (small sample size, weight, tuning method), 2) more transitions between paragraphs, and 3) grouping ideas together.\nThe discussion of manual tuning and potential direction for optimization seems incomplete. First of all, these two sections can be combined (but there is currently an unrelated paragraph in between - see notes on organization above). Second, the discussion would be more comprehensive with authors’ comments on how they think the decision to manual tune impacted their results, and why they decided to do it this way. After that, the authors could acknowledge automatic optimization pipeline as a potential future direction.\nMethods\nPlease provide more detail on the training sessions individuals received - what was the average number of sessions and average duration over which time period? For individuals who received more than one training, did the authors tune the parameters fresh? Or start at the converged parameter from the previous training? How did the authors accommodate if user preference and comfort has changed between training?\nPlease provide more detail on how the authors used “feedback from participants and the experimenter’s judgment” to tune the assistance parameters.\nIn the last paragraph of methods (discussion of stats), please include additional information regarding how the authors determined the assumptions of a two-tailed paired t-test were evaluated and met for the data."}, "author_response": "RESPONSE TO REVIEWER 1\nThis paper introduces a portable hip exoskeleton robot aimed at improving the quality of life for stroke\npatients by reducing their walking metabolism, which is highly significant in the current field of exoskeleton\nrobotics. However, due to the complex and distinct walking kinematics, dynamics, and lower-limb\nmusculoskeletal characteristics of stroke patients compared to healthy individuals, focusing solely on\nmetabolic improvement is not comprehensive enough. For exoskeleton robots designed for stroke patients,\nit is desirable to enhance their mobility and metabolism without imposing additional burdens on other lower-\nlimb joints and muscles. Overall, this paper holds substantial research value, but requires more\ncomprehensive analysis of lower-limb joints and muscles in assisted stroke patients to confirm that hip\nexoskeleton assistance can effectively improve their quality of life.\nThank you for your thoughtful review and feedback. We have carefully reviewed your suggestions and\nfind these insights highly valuable. Below are point-by-point responses to your comments. Revisions to\nthe manuscript are included in red.\nThe statement in the third paragraph, “This hyperextension increases loading on the knee, which can\nincrease pain and the risk of secondary conditions, such as osteoarthritis,” indicates that elevated joint force\nmay heighten the risk of joint pathologies (e.g., osteoarthritis). If assistance is applied at the hip joint, how\nwould the joint force at the hip, knee, and ankle change? Would this impose greater force on the joints,\npotentially causing further harm to patients?\nThank you, this is a great question. Although our study did not directly assess joint contact forces, in\nthe revised paper, we added an analysis of the joint torques and ground reaction forces, which have been\nshown to correlate with joint contact forces85. Specifically, we added a kinetic results section and\nextended data figure 7 showing the biological joint torques calculated through inverse dynamics, and\nthe vertical ground reaction forces, with and without the exoskeleton. This additional analysis shows\ninconsistent results for the joint torque, with some joints exhibiting higher and others showing lower\ntorque when using the exoskeleton, although vertical ground reaction forces were slightly higher with\nthe exoskeleton. We also included implementing musculoskeletal modeling to directly analyze the\neffects of hip exoskeleton assistance on joint forces in the revised discussion as a future research\ndirection.\nResults: Kinetics\n“Kinetic trajectories, including the biological hip, knee, and ankle joint moment and vertical ground\nreaction force (GRF) trajectories are presented in Extended Data Fig. 7. On the affected side,\nexoskeleton assistance increased the average peak biological hip flexion and extension moments on\nthe affected side by 0.07 + 0.09 Nm kg-1 (30% + 12%) and 0.07 + 0.04 Nm kg-1 (41% + 27%),\nrespectively. On the unaffected side, exoskeleton assistance increased the peak biological hip flexion\nmoment by 0.13 + 0.09 Nm kg-1 (44% + 12%) and decreased the peak biological hip extension\nmoment by 0.10 + 0.04 Nm kg-1 (23% + 6%). Similarly, on the affected side, hip exoskeleton\nassistance increased the peak knee flexion moment by 0.10 + 0.03 Nm kg-1 (30% + 2%) and\ndecreased the peak knee extension moment by 0.09 + 0.04 Nm kg-1 (49% + 23%). In contrast, on the\nunaffected side, exoskeleton assistance decreased the peak knee flexion moment by 0.002 + 0.030 Nm\nkg-1 (4% + 17%) and increased the peak knee extension moment by 0.10 + 0.05 Nm kg-1 (30% +\n16%). Finally, the ankle moments were unchanged on both the affected and\nunaffected sides during walking with exoskeleton assistance compared to walking without the\nexoskeleton. The greater peak joint moments with hip exoskeleton assistance are likely related\nto the greater observed vertical ground reaction force. The peak vertical GRF increased by 0.04\n+ 0.005 N kg-1 (4% + 1%) on both the affected and unaffected sides. “\nDiscussion\n“Previous studies in healthy young individuals suggest that powered exoskeletons have the potential\nto reduce joint loading85, which has been previously linked to the increased incidence of osteoarthritis.\nIn this study, we could not perform this analysis due to the lack of electromyography, which is needed\nto validate the biomechanical simulations estimating joint loading. Future studies should evaluate the\neffects of exoskeleton assistance on hip, knee, and ankle joint loading in individuals with hemiparesis.\nFinally, future research should examine the long-term effects of hip exoskeleton assistance, including\npain and risk of secondary conditions.”\nExtended Data Fig. 1 | Kinetic trajectories. Moving top to bottom: body weight\nnormalized hip moment, knee moment, ankle moment, and vertical ground reaction force\n(GRF) trajectories for the affected (left) and unaffected (right) sides without the exoskeleton\n(gray) and with the exoskeleton (red). Flexion is positive for the hip and knee, and\ndorsiflexion is positive for the ankle. The solid line represents the mean, and the shaded\nregion in the associated color represents the standard error. The circle represents when toe\noff occurs during the gait cycle. The light purple shaded area in the background represents a\nnormative dataset39.\nGait asymmetry is a defining characteristic of stroke patients, as elaborated in the fourth paragraph:\n“Moreover, the unaffected side must compensate for the weakness and lack of coordination of the affected\nside, making hemiparetic gait highly asymmetric.” However, this critical feature was not analyzed in the\nstudy’s results. Could a comparative analysis be conducted on gait symmetry before and after hip\nexoskeleton assistance, alongside existing assistive strategies for stroke patients?\nThank you for pointing this out. To address this comment, we revised the results section, including one\nspatial (step length) and one temporal (stance time) symmetry outcome (Extended Fig. 6). We also\nexpanded our discussion of spatiotemporal asymmetries to compare our results with existing assistive\nstrategies for stroke patients.\nResults: Kinematics\n“Symmetry was evaluated by calculating a symmetry index (SI) where 0% indicates perfect\nsymmetry34. Exoskeleton assistance slightly improved symmetry in stance time (13% + 3%\nasymmetry without the exoskeleton vs. 12% + 4% asymmetry with the exoskeleton), but increased\nasymmetry in step length (-8.2% + 6% without the exoskeleton vs. -14% + 5% with the exoskeleton)\n(Extended Data Fig. 6). Generally during walking with and without the exoskeleton, participants spent\nlonger in stance on their unaffected side and took longer steps onto their affected side.”\nDiscussion\n“Hip exoskeleton assistance had a small effect on the hip joint range of motion for both sides (Extended\nData Fig. 5). Specifically, the exoskeleton assistance increased the affected hip joint’s range of motion\nby 2° and decreased the unaffected hip joint’s range of motion by 2°, slightly improving symmetry in\nthe hip joint range of motion. Hip exoskeleton assistance slightly improved symmetry in stance time\nbut decreased symmetry in step length (Extended Data Fig. 6). The observed increase in step length\nasymmetry indicates that the participants take longer steps onto their affected side with the exoskeleton\nassistance. This result is likely due to the assisted hip flexion during swing. Overall, we did not observe\nany consistent trends in spatiotemporal variables across participants while walking with the\nexoskeleton compared to walking without the exoskeleton. Although passive80 and powered24 ankle-\nfoot orthoses/exosuit have shown consistent improvements in symmetry for stance time and step length,\nprevious powered hip exoskeleton studies have not56– 58. Our results seem to confirm that portable\npowered hip exoskeletons do not consistently increase symmetry. This result may be due to the assistive\ncontroller being primarily tuned to provide positive power as needed to reduce metabolic rate.”\nMethods: Data processing and statistical analyses\n“We evaluated interlimb asymmetry during both walking trials using a symmetry index (SI). The SI is\ncalculated by taking the difference between the spatiotemporal data of the affected and unaffected sides,\ndivided by one-half the sum of the spatiotemporal data from both sides, multiplied by 100%34. A positive\nSI indicates asymmetry towards the subject’s unaffected side, while a negative SI indicates asymmetry\ntowards the subject’s affected side. Zero indicates perfect symmetry. “\nExtended Data Fig. 2 | Spatiotemporal symmetry. Symmetry in stance time (left) and step length (right) without the exoskeleton (gray) and\nwith the exoskeleton (red). The bars represent the mean, and the error bars represent the standard error. The blue dots represent values for the\nindividual participants. 0 is perfect symmetry. The symmetry index is calculated by taking the difference between the affected and unaffected\nsides, divided by one-half the sum of the affected and unaffected sides, multiplied by 100%34. A positive symmetry index in stance time\nindicates the participant spends longer in stance on their unaffected side. A positive symmetry index in step length indicates the participant\ntakes a longer step onto their unaffected side.\nAlthough no prior studies have specifically targeted metabolic reduction in stroke patients, hip exoskeletons\nhave been extensively investigated for reducing walking metabolic rate in healthy populations. Does the\nmethodology of this study fundamentally differ from those previous works? What adaptations were made\nspecifically for stroke patients?\nThis is a great question. The methodology of this study did not differ fundamentally from that used in\nhealthy populations. However, we adapted the experimental protocol to account for the decreased\nendurance and lack of sensation and communication resulting from the neurological injury. We include\nmore details about the exoskeleton tuning process in the revised methods section.\nMethods: Exoskeleton training and assistance tuning:\n“The first training session included fitting the exoskeleton for each participant. The experimenter\nhelped the participants don the powered hip exoskeleton and adjusted the straps to ensure a proper fit\nwhile alleviating potential discomfort. After fitting the device, the participants walked on a treadmill,\nand the experimenter manually tuned the assistive controller by adjusting the timing and peak of the\nflexion and extension torques. To maximize assistance, the experimenter watched a real-time plot of\nthe estimated joint power and adjusted the controller parameters while the participant walked with the\nexoskeleton. This tuning procedure was done independently for the affected and unaffected sides.\nExoskeleton assistance parameters were tuned based on the experimenter’s training and experience\nvia visual inspection of gait during the walking sessions89. The experimenters also watched a live\nestimate of exoskeleton power based on the exoskeleton torque and velocity, and adjusted timing and\npeak torque parameters to maximize positive power\ngeneration, which is critical for reducing the metabolic cost of healthy subjects45,79. After each\nadjustment, the participant was asked if the assistance felt better or worse. If it felt better, the\nexperimenter would continue adjusting that parameter in that direction until it felt worse. If the\nchange felt worse, the parameter was reverted to its previous value, and a different change was\nmade. Comfort was assessed by asking the participants for their subjective feedback and visually\ninspecting the exoskeleton interfaces for excessive bending or movements. Participants walked for\napproximately half an hour, less than 10 minutes at a time, with rest between walking trials.”\nDoes its application in stroke patients demonstrate superior efficacy compared to existing research?\nWe are not entirely sure what the reviewer means. This study shows an 18% metabolic reduction, which is\namong the highest observed with portable hip exoskeletons. However, we believe that comparing metabolic\nreductions in healthy individuals and individuals with hemiparesis is not beneficial for the field. In our\nopinion, this study shows superior efficacy by providing the first and only demonstration of a portable\nexoskeleton significantly reducing the metabolic cost of walking in individuals with hemiparesis. This\nmetabolic reduction is expected to have a considerable effect on mobility, given that the observed 18%\nmetabolic reduction is equivalent to removing a 13.6 kg backpack from an average healthy person. We\nemphasize this point in the revised discussion section:\nDiscussion\n“The reduction in the metabolic cost of walking obtained in this study using a portable hip\nexoskeleton (18%) is substantially greater than the metabolic reduction previously achieved using\nan ankle exosuit tethered to a benchtop actuator (10%)25. A case series with three individuals with\nhemiparesis suggested potential for unilateral hip flexion assistance to reduce the metabolic cost of\nwalking59. Still, they found a much lower reduction compared to this study (8.6%), and no follow-up\nstudies confirmed its statistical significance. Thus, our analysis suggests that bilateral, bidirectional\nhip assistance may be more effective than unilateral, unidirectional hip assistance and ankle\nassistance in improving the metabolic cost of walking with a portable exoskeleton or exosuit.”\nThis study shares similarities in research methodology with \"Powered hip exoskeleton improves walking\neconomy in individuals with above-knee amputation.\" Could you elaborate in detail on the methodological\nimprovements in this study, as well as the advancements in the exoskeleton hardware?\nThank you for pointing this out. In the revised paper, we describe the difference in the exoskeleton\nhardware as well as methodological changes compared to our previous metabolic study.\nMethods: Motion capture experiment\n“Compared to our previous metabolic study with individuals who have above-knee amputations61,\nwe added an experimental session to perform a comprehensive biomechanical analysis using full-\nbody motion capture with an instrumented treadmill and instrumented handrails. This additional\nanalysis provided a mechanistic explanation for the observed metabolic reduction, which is\nessential to assess the benefits of the proposed assistive approach.”\nMethods: Powered hip exoskeleton\n“The powered hip exoskeleton used in this study was previously presented and validated in healthy\nindividuals35, individuals with above-knee amputation61, and one individual with hemiparesis55. The\nexoskeleton provides bilateral flexion and extension torque to the user’s hip joints while allowing for\npassive abduction and adduction. All interfaces, actuation, electronics, and the battery are integrated\ninto the exoskeleton, which is worn on the user’s pelvis and thighs. The total weight of the exoskeleton\nwas up to 2.63 kg.\nCompared to our previous metabolic study with individuals who have above-knee amputations61,\nwe made substantial upgrades to the powered hip exoskeleton hardware. First, we moved from a\nunilateral to a bilateral configuration in which assistance is provided to both hip joints. Then, we\nredesigned the pelvis interface and thigh cuff, moving from a soft textile-like structure to a semirigid\ninterface sized to fit each user. Finally, we used more reliable cables and connectors to interface\nthe main electronic board located in the pelvis interface to the actuator in the exoskeleton thigh\nsegments. These design changes are described in our preliminary validation study with healthy\nindividuals .”\nReviewer #2 (Remarks to the Author):\nSUMMARY OF WORK:\nThis manuscript examines whether a bilateral, powered, autonomous hip exoskeleton (Fig 1a.) can reduce the\nmetabolic cost of fixed-speed treadmill walking in people with stroke (Table 1). The authors designed and\nconducted a rigorous and comprehensive within-participant experimental design comparing metabolic and\nlower-limb joint mechanical effort without (No exo) and with (With exo) the hip exo providing clinician\nsupervised ‘self-tuned’ timing and magnitude (ED Fig. 1) on each leg (paretic and non-paretic) (Fig. 1b, Fig 4.)\nThe results indicate that hip exo assistance can reduce metabolic effort (W/kg) by 18% (Fig. 1c) and this is\naccompanied by significant reductions in users’ hip joint (29%) and total combined-lower-limb (14%)\nmechanical work. In addition, the authors detail how the exoskeleton contributes to hip work for each leg and\nphase of the gait cycle (Fig. 3) and show that the exo increases paretic limb toe clearance (ED Fig. 2).\nOverall, the manuscript is excellent and presents the first evidence that hip exoskeleton can reduce\nmetabolic effort post-stroke. The writing and presentation are logical and concise – with clearly stated\nobjectives, aims and hypotheses. The analysis is already comprehensive and mechanistic as it stands,\nhowever, there are some key limitations in its ability to support the data with full rigor. The reader is left\nwith some key questions that when resolved will increase the confidence in what will be a surefire landmark\nfinding in the field. I offer my suggestions below in an effort to help strengthen the findings and ultimately\nthe impact of the work.\nThank you for your positive feedback and for bringing our attention to the remaining questions in this\nwork. Below, we provide a point-by-point response addressing each of your comments. Revisions to\nthe manuscript are included in red.\nCOMMENTS/SUGGESTIONS:\nThere are two major issues that need to be addressed.\nThe authors mention that a number of the participants used handrails. This is fine as long as the handrail\nsupport is not measurably different in the No exo vs. With exo conditions (i.e. handrail support is not\nmore/less on average in No exo vs. With exo condition). The authors address in their results stating:\n“For safety, all participants were instructed to rest their unaffected hand on a side-mounted handrail during all\ntreadmill walking tests, using the smallest amount of handrail support needed. There was no statistical\ndifference between the force applied while walking without the exoskeleton compared to with the exoskeleton.\nThe handrail force was also low, ranging from 1 - 4% of each participant’s body weight.”\nThis is super. But, please add detail on the level of the forces measured over the gait cycle (e.g., participant\naverage hand rail force time series graphs and a table showing avg. values per participant in ED.)\nThank you for pointing this out. To address this comment, we reported the handrail forces as time series for\neach subject, both with and without the exoskeleton (Extended Data Fig. 9). We present this analysis\nin the revised results and methods sections, including two additional extended data figures. We also\nincluded the average handrail force per participant in the revised Table 1. Relevant manuscript sections\nare copied below.\nResults: Handrail forces\n“For safety, all participants were instructed to rest their unaffected hand on a force-instrumented\nhandrail mounted on the side of the treadmill, using only the minimum support needed. The average\ntotal handrail force was generally low, ranging from 1% to 5% of the participant’s body weight (Table\n1), and there was no statistically significant difference between the average total handrail force\napplied with and without the exoskeleton. Moreover, average handrail forces in the lateral, frontal,\nand vertical axes were very similar with and without the exoskeleton (Extended Data Fig. 8).\nHowever, the peak lateral handrail force occurred slightly earlier in the gait cycle when using the\nexoskeleton. The average handrail forces per stride in each axis remained the same with and without\nthe exoskeleton. Some participants showed small differences in peak forces in each direction of their\nhandrail forces, which can be seen when examining each plane of motion separately (Extended Data\nFig. 9). The largest differences in handrail use were observed for participant 2, whose peak vector\ntotal handrail force increased by 2.2%, and participant 6, whose peak vector total handrail force\ndecreased by 2.5%, with the exoskeleton per stride. On average, three participants used slightly more\nforce with the exoskeleton (S2, S3, S4), while four participants used slightly less force (S1, S5, S6,\nS7).”\nMethods: Metabolic cost evaluation\n“For safety, all participants were instructed to rest their unaffected hand on a side-mounted\nhandrail during all treadmill walking tests, using the smallest amount of handrail support needed.\nWe instructed participants to use the smallest amount of handrail support needed, and the level of\nsupport was different for each participant. The average vector total handrail force per stride\nranged between 1% and 5% of each participant’s body weight (Table 1). Participants were\ngenerally consistent with the level of support they used with and without the exoskeleton, and only\ntwo participants showed meaningful differences between exoskeleton conditions. Participant 2\nincreased their peak vector total handrail force by 2.2% with the exoskeleton, while participant 6\ndecreased their peak vector total handrail force by 2.5% with the exoskeleton. Participants wore a\nharness, but no body weight was supported.”\nTable 1. Participant baseline characteristics.\nWalking Avg handrail\nParetic Chronicity Weight\nParticipant Sex Age (years) Orthosis speed Height (m) force (% BW)\nside (years) (kg)\n(m s-1) no exo / exo\nExtended Data Fig. 3 | Average handrail forces. Average handrail force per stride as a percent of body weight (top)\nwithout the exoskeleton (gray) and with the exoskeleton (red). The bars represent the mean, and the error bars represent\nthe standard error. The blue dots represent values for the individual participants. Average handrail force trajectories\n(bottom) without the exoskeleton (gray) and with the exoskeleton (red). The solid lines represent the mean, and the\nshaded regions represent the standard error. Lateral (left), frontal (middle), and vertical (right) forces are presented\nseparately.\nExtended Data Fig. 4 | Participant handrail force trajectories. Moving top to bottom: lateral, frontal, vertical, and vector total force trajectories as a\npercent of body weight for each participant (columns left to right) without the exoskeleton (gray) and with the exoskeleton (red). The solid lines represent\nthe mean, and the shaded regions represent the standard error.\nBased on documented efficiency of muscular positive mechanical work - user metabolic savings should not\nexceed ~ 4x user mechanical savings due to exoskeleton mechanical input. For this study the net metabolic\npower savings was ~0.5 W/kg which would suggest savings in positive stride cycle average mechanical\npower across the combined lower-limb of ~0.125 W/kg. However, the authors report metabolic results in\nW/kg and mechanical results in J/kg making this ‘back of the envelope’ gut check difficult without\nknowledge of the stride frequency. To add rigor and physiological grounding to the metabolic result please\nformally relate user metabolic savings (change in user input) to the user mechanical savings\n(change in user output and/or the exoskeleton mechanical input (device input) in consistent units and report for each\nindividual. The preferred approach would be to plot these data in a scatter graph – and fit a line to the data\n– analyzing the value of the slope of the fit. Versions of this have been documented in the literature and\ntermed ‘apparent efficiency’/ ‘performance index’ (Sawicki-Ferris), or ‘augmentation factor’ (Mooney-\nRouse-Herr, Siviy-Walsh).\nThank you for suggesting this analysis. As recommended by the reviewer, we calculated the ratio between\nthe net metabolic power saving and the average positive mechanical power provided by the exoskeleton\nand calculated the apparent exoskeleton efficiency. We performed this analysis with and without adjusting\nfor the metabolic penalty of wearing the exoskeleton to address the fact that we did not perform a no-\nassistance condition (transparent mode). As suggested, we created a scatter plot showing the net metabolic\npower saving (with the exoskeleton mass adjustment) and positive exoskeleton mechanical power for each\nsubject. Below are the relevant manuscript sections and figures.\nResults: Metabolic cost\n“To account for the metabolic cost of wearing the exoskeleton, we calculated the metabolic penalty of\nadding exoskeleton mass to the thigh and pelvis62,63. The exoskeleton mass added to each thigh was\n0.67 kg, resulting in a metabolic penalty of 7.44 W (5.55 W kg-1 x 0.67 kg/side x 2 sides). The\nexoskeleton mass added to the pelvis was 1.29 kg, resulting in a metabolic penalty of 4.23 W (3.33 W\nkg-1 x 1.29 kg). Thus, the total metabolic penalty of wearing our portable hip exoskeleton was 11.76\nW, which, normalized by the body weight of each participant and averaged, is equivalent to 0.148 W\nkg-1.\nAccounting for the estimated metabolic penalty of the exoskeleton mass, the theoretical net\nmetabolic saving compared to wearing the unpowered exoskeleton would be 0.653 W kg-1,\ncompared to the measured 0.505 W kg-1 without the exoskeleton. Given that the average positive\nmechanical power for both exoskeleton sides combined was 0.317 W kg-1, the average ratio\nbetween the weight-adjusted metabolic saving and the average positive bilateral exoskeleton power\nacross subjects was 2.06 (Extended Data Fig. 2). Thus, if we account for the metabolic penalty of\nthe exoskeleton mass, the average modified exoskeleton performance index63,64 was 0.515, which\nis about half the theoretical maximum of 1.\nTo assess the metabolic cost of walking in a speed-normalized form, we calculated the cost of\ntransport65. The average cost of transport with the exoskeleton was 3.6 + 0.4 J kg-1, down from\n4.4 + 0.5 J kg-1 without the exoskeleton. As expected from the metabolic rate analysis, all\nparticipants decreased their cost of transport with the exoskeleton. However, the reduction in the\ncost of transport was inversely proportional to the walking speed (Extended Data Fig. 3).”\nDiscussion:\n“The metabolic penalty of wearing our portable hip exoskeleton was estimated at 11.76 W, similar\nto the average metabolic penalty of other advanced hip exoskeletons (6.93 – 17.7 W, Extended Data\nTable 1). When accounting for this metabolic penalty, the performance index of our exoskeleton\nwas 0.515, well below the theoretical maximum of one, which would indicate perfect energy transfer\nbetween the exoskeleton and the user. The apparent efficiency of our device is notably lower than\nthat of portable power hip exoskeletons measured in previous studies with healthy subjects (0.515\nversus 0.74-0.92, Extended Data Table 1). This difference may indicate that our exoskeleton is less\nefficient in transferring mechanical energy to the user compared to other powered exoskeletons.\nHowever, it may also indicate that greater positive mechanical energy is necessary to achieve the\nsame metabolic benefits in individuals with hemiparesis compared to healthy subjects.\nNotably, the performance index calculation relies on two crucial assumptions that have not been\nverified63,64. The first assumption is that the metabolic penalty of adding mass to different leg segments\nin individuals with hemiparesis is the same as in healthy individuals63 and does not change with walking\nspeed. The second assumption is that muscles generate positive mechanical work with a muscular\nefficiency that asymptotically approaches 0.2564,67 for both healthy individuals and those with\nhemiparesis, regardless of the walking speed. Future studies should examine the validity of these\nassumptions by comparing the exoskeleton performance index in healthy individuals and individuals\nwith hemiparesis using the same device and assistive strategy.”\nMethods: Data processing and statistical analyses\n“The exoskeleton performance index quantifies the ratio between the change in metabolic power\nand average exoskeleton positive mechanical power provided, scaled by the muscular efficiency\nfactor of 0.2564. We use this muscular efficiency factor based on work indicating that human muscles\nperform positive mechanical work with a muscular efficiency that asymptotically approaches\n0.2567.”\nExtended Data Fig. 5 | Metabolic reduction vs. exoskeleton power. The\nmetabolic reduction of each participant, modified to include the metabolic penalty\nof the exoskeleton mass, versus the positive bilateral exoskeleton power provided\nfor each subject. Including the metabolic burden of wearing the exoskeleton, the\nratio between the user's metabolic savings and the exoskeleton's power input is 2.06.\nThus, for every W of positive power provided by the exoskeleton, the user is\nexpected to save 2.06 W of metabolic power.\nSome minor critiques.\nGiven that the participants vary widely in their walking speed – and metabolic rate is function of walking\nspeed it would be a nice addition to present the data in speed normalized form as a function of speed (i.e. cost\nof transport (net met power/speed = J/m/kg vs. speed graph) for No exo vs. With Exo and use it to discuss\nwhat would happen if these participants were free to increase/decrease walking speed With Exo.\nThank you for suggesting this analysis. Following your recommendation, we calculated the cost of\ntransport for each subject with and without the exoskeleton. We also calculated the reduction of\nmetabolic cost of transport on average across subjects and for each subject separately. Relevant\nmanuscript sections and figures are copied below.\nResults: Metabolic cost\n“To assess the metabolic cost of walking in a speed-normalized form, we calculated the cost of\ntransport65. The average cost of transport with the exoskeleton was 3.6 + 0.4 J kg-1, down from 4.4\n+ 0.5 J kg-1 without the exoskeleton. As expected from the metabolic rate analysis, all\nparticipants decreased their cost of transport with the exoskeleton. However, the reduction in the\ncost of transport was inversely proportional to the walking speed (Extended Data Fig. 3).”\nDiscussion\n“All participants reduced their metabolic cost of walking with the exoskeleton. However, the self-\nselected walking speeds varied widely among participants (0.37 m s-1 – 1.00 m s-1). To account for\nthese differences in speed, we analyzed the cost of transport, which is a speed-normalized\nrepresentation of metabolic walking effort65. As expected, the cost of transport decreased with\nwalking speed both with and without the exoskeleton (Extended Data Fig. 3). Interestingly, the\nslowest participants exhibited the greatest reductions in the cost of transport with exoskeleton\nassistance, even though the metabolic rate reductions were approximately the same (Fig. 1). On\naverage, the cost of transport decreased by 0.8 J m-1 kg-1 with the exoskeleton, which is equivalent\nto increasing walking speed by 0.16 m s-1, assuming a linear relationship between cost of transport\nand walking speed within the observed range of speeds. This result suggests that the proposed\nassistive exoskeleton could enable hemiparetic subjects to increase their walking speed, in\nagreement with previous studies55–57.”\nMethods: data processing and statistical analyses\n“The modified Brockway equation was used to calculate body weight normalized metabolic power over\nthe last two minutes of the standing and walking trials62. Net metabolic power was obtained by\nsubtracting the average metabolic power measured during the standing baseline trial from the average\nmetabolic power measured during each walking trial. The metabolic cost of transport was calculated\nfor each walking trial by dividing the metabolic power of each participant by their walking speed65. “\nExtended Data Fig. 6 | Metabolic cost of transport. Average metabolic cost of transport (left) without the exoskeleton (gray, solid circles)\nand with the exoskeleton (red, open circles). The bars represent the mean, and the error bars represent the standard error. The colored dots\nrepresent values for the individual participants. The metabolic cost of transport of each participant versus their walking speed (middle) without\nthe exoskeleton (solid circles) and with the exoskeleton (open circles). The linear relationship between the metabolic cost of transport without\nthe exoskeleton and walking is represented by the equation y = 7.97 – 5.12*x, as plotted. The reduction in the metabolic cost of transport of\neach participant with the exoskeleton versus their walking speed (right).\nGiven that the participants vary widely in their ‘tuned’ exoskeleton torque assistance profile, it would be a\nnice addition to add some discussion (if not analysis) on what features of the exo torque primarily drove the\nmechanical and metabolic changes.\nFollowing your recommendation, we added a discussion of the difference in exoskeleton assistive\nprofiles.\nDiscussion\n“The exoskeleton assistance was manually tuned for each participant. The experimenter adjusted the\nassistance parameters to maximize positive exoskeleton power generation based on the online estimates\nshown in the exoskeleton graphical user interface while ensuring patient safety and comfort through visual\ninspection and subjective feedback from the participants. The exoskeleton provided predominantly positive\npower on both sides, although peak power was greater on the participants’ unaffected side than on their\naffected side for both flexion and extension, likely due to the smaller range of motion of the affected side.\nIn agreement with previous studies showing that positive exoskeleton power is critical for reducing the\nmetabolic cost of walking45,79, the participant with the exoskeleton assistance profile with the most negative\npower also had the smallest metabolic cost reduction (S3, 8%). Although we did not observe a strong\ncorrelation between positive exoskeleton power and metabolic cost reduction, this study confirms that\npositive power is key to reducing metabolic rate.\nGenerally, participants preferred greater flexion torque than extension torque and greater torque on\ntheir unaffected side than their affected side (Extended Data Fig. 1). Despite these general trends, the\nexoskeleton torque profiles vary widely between subjects. Participant 3, who showed the smallest\nmetabolic reduction, had the latest timing for peak flexion and extension assistance during the gait\ncycle, which may have contributed to the generation of large amounts of negative power from the\nassistance profiles. Participants 2 and 3 received the lowest peak flexion and extension torques and had\nthe smallest reductions in metabolic cost, which could indicate a relationship between peak assistive\ntorque and metabolic reduction. On the other hand, participant 6 received the greatest peak flexion\ntorques but did not achieve the largest reduction in metabolic cost, indicating that a more nuanced\ninterpretation is needed.”\nIt would be nice to indicate on the key results Fig. 1c what the ‘added mass penalty’ of the device is (i.e.\nExo No Power). In other words- how much of the exoskeleton work went to offset it’s own mass? This\nwould help more clearly define the performance ceiling of a more streamlined design.\nThat’s a good point. As suggested, we performed the analysis of the exoskeleton mass penalty and reported\nthe estimated value (11.76 W, 0.148 W kg-1) in the revised result section (see below). However,\nwe decided not to include this information in Figure 1c because the mass penalty data is obtained from\nestimates and not from actual measurements, like the data showed in Fig. 1c.\nResults:\n“To account for the metabolic cost of wearing the exoskeleton, we calculated the metabolic penalty of\nadding exoskeleton mass to the thigh and pelvis62,63. The exoskeleton mass added to each thigh was\n0.67 kg, resulting in a metabolic penalty of 7.44 W (5.55 W kg-1 x 0.67 kg/side x 2 sides). The exoskeleton\nmass added to the pelvis was 1.29 kg, resulting in a metabolic penalty of 4.23 W (3.33 W kg-1 x 1.29\nkg). Thus, the total metabolic penalty of wearing our portable hip exoskeleton was 11.76 W, which,\nnormalized by the body weight of each participant and averaged, is equivalent to 0.148 W kg-1.”\nHas this device been tested on able-bodied controls? If so, what are the results and how do they compare to\nthe stroke cohort. This would be especially interesting to note with respect to the ‘apparent efficiency’\n(see comment 2). Discussion point?\nUnfortunately, this device has not been tested on able-bodied controls, so we are not sure how the\napparent efficiency would compare between healthy and hemiparetic subjects. We include a new table\ncomparing the performance index of different exoskeleton studies in healthy populations (Extended\nData Table 1).\nDiscussion:\n“The metabolic penalty of wearing our portable hip exoskeleton was estimated at 11.76 W, similar\nto the average metabolic penalty of other advanced hip exoskeletons (6.93 – 17.7 W, Extended Data\nTable 1). When accounting for this metabolic penalty, the performance index of our exoskeleton was\n0.515, well below the theoretical maximum of one, which would indicate perfect energy transfer\nbetween the exoskeleton and the user. The apparent efficiency of our device is notably lower than\nthat of portable power hip exoskeletons measured in previous studies with healthy subjects (0.515\nversus 0.74-0.92, Extended Data Table 1). This difference may indicate that our exoskeleton is less\nefficient in transferring mechanical energy to the user compared to other powered exoskeletons.\nHowever, it may also indicate that greater positive mechanical energy is necessary to achieve the\nsame metabolic benefits in individuals with hemiparesis compared to healthy subjects.\nNotably, the performance index calculation relies on two crucial assumptions that have not been\nverified63,64. The first assumption is that the metabolic penalty of adding mass to different leg segments\nin individuals with hemiparesis is the same as in healthy individuals63 and does not change with walking\nspeed. The second assumption is that muscles generate positive mechanical work with a muscular\nefficiency that asymptotically approaches 0.2564,67 for both healthy individuals and those with\nhemiparesis, regardless of the walking speed. Future studies should examine the validity of these\nassumptions by comparing the exoskeleton performance index in healthy individuals and individuals\nwith hemiparesis using the same device and assistive strategy.”\nExtended Data Table 2. Benchmark with state-of-the-art portable hip exoskeletons. Values are estimated to the best of our ability based on\nthe information provided in the referenced studies. All studies were performed with healthy participants unless otherwise noted.\nMetabolic Average\nPeak Metabolic Device mass Performance\nDevice penalty positive power Note\ntorque/force reduction [kg] index\n[W] [W kg-1]\n13%\nSeo 2016 38 9.8 Nm 0.400 W kg-1 2.8 total 9.32** 0.16 0.87*\n13%\nY Lee 2017 51 12 Nm 0.404 W kg-1 2.6 total 8.66** 0.44 0.92\n7%\nH Lee 2017 50 ~ 6 Nm ~ 0.500 W kg-1 2.8 total 9.32** 0.165 0.76 Elderly\n14.5%\nSeo 2017 37 12 Nm 0.450 W kg-1 2.4 total 7.92** 0.16 0.91*\n16.8%\nLim 2019 46 10.1 Nm 0.520 W kg-1 2.1 total 6.93** ~ 0.16 0.81\n9.3% 4.5 waist Extension\nJ Kim 2019 49 300 N 0.368 W kg-1 0.46 thighs 17.5 0.125 0.74 only\n2.8 waist\n13%\nCao 2020 52 150 N 0.500 W kg-1 0\n0\n.7\n.5\n0\n0\nt h\nf\ni\ne\ng\ne\nh\nt\ns 20.6 N/A N/A\n7.2% 1.87 waist Flexion\nJ Kim 2022 53 127 N 0.230 W kg-1 0.44 thighs 8.67 0.077 0.75 only\n24% 2.6 waist\nLuo 2024 54 14 Nm 0.720 W kg-1 0.60 thighs 12.0 N/A N/A\n18% 1.29 waist\nThis study 12 Nm 11.76 0.317 0.515* Stroke\n0.505 W kg-1 1.34 thighs\n* Includes the metabolic penalty from device mass\n** Assumes all mass is attached to the waist\nRESPONSE TO REVIWER 3\nReviewer #3 (Remarks to the Author):\nIn this work, authors presented a hip exoskeleton that provides active mechanical assistance bilaterally and\nbidirectionally to the hip joint during treadmill walking in seven individuals with hemiparesis. The use of\ntheir exoskeleton was able to reduce the metabolic cost of walking by 18%, likely by reducing positive\nbiological hip work. This work has great potential to offer a portable solution for individuals with mild-to-\nmoderate hemiparesis that enables them to walk for longer duration, leading to improved recovery and\nintegration into the real world. Presentation of the work can be substantially improved by\nreorganizing the introduction and discussion, providing more detail in results (particularly in comparisons of\nnon-metabolics related outcomes) and methods (particularly in training and tuning), and a more comprehensive\ndiscussion synthesizing results of different outcomes evaluated.\nThank you for reviewing our manuscript and providing key suggestions on how to improve it. We\nprovide a point-by-point response to each of your comments below. Revisions to the manuscript are\nincluded in red.\nIntroduction\nThe intro should be restructured to improve its cohesiveness. In this submission, authors jump back and\nforth between the discussion of post-stroke impairment and exoskeleton. Instead, consider centering the\nintroduction around impairment-specific challenges related to metabolic cost, followed by discussion of\ndemonstrated benefits and limitations of currently explored exoskeleton solutions to target these challenges,\nand why the authors’ solution - bilateral and bidirectional hip exoskeleton - is a promising alternative\nsolution.\nThank you for your feedback. We have revised the introduction to improve clarity and flow. Following\nyour recommendation, the revised introduction starts by presenting the challenges associated with the high\nmetabolic cost of walking in the hemiparetic population. Then, we talk about leg weakness and\ncompensatory gait strategies. After that, we discuss exoskeletons, summarizing the positive results and\nidentifying open challenges. Finally, we present the rationale for the proposed assistive approach and the\nhypothesis we tested in this study.\nPlease use people-first language when appropriate. “Individuals with above-knee amputation” is people-\nfirst, while “above-knee amputees” is not.\nThank you for bringing this to our attention. We revised the manuscript to use people-first language.\nIntroduction\n“Biomechanical analysis has shown that using a hip exoskeleton to assist with hip flexion and\nextension can help compensate for reduced ankle propulsion associated with a passive leg\nprosthesis60 thereby decreasing the metabolic cost of walking in individuals with above-knee\namputation .”\n“Compared to our previous metabolic study with individuals who have above-knee\namputations61....”\nResults\nConsider moving information from Extended Data Fig 1 into the main text when discussing tuning for both\nlegs. From evaluating the figure it appears most individuals were tuned to receive similar level of assistance\nbetween the two legs, except for one participant. Results would be more comprehensive to include this\nquantification.\nThank you for your suggestion. As recommended by the reviewer, we have added more detail in the\nmethods and discussion sections to emphasize the different assistance tuning between participants.\nHowever, we chose not to move Extended Data Fig. 1 to the main manuscript because we felt it might\ndistract from our primary outcome related to the hypothesis.\nResults: Exoskeleton assistance\n“Many participants preferred similar exoskeleton tuning on each side, resulting in similar\ngenerated torque profiles on each side. Notably, participant 6 received greater peak flexion torque\non their affected side compared to their unaffected side. Generally, the peak flexion torque provided\nby the exoskeleton was greater than the peak extension torque, with greater extension torque\nsupplied to the unaffected side (Extended Data Fig. 1). Each peak of exoskeleton torque resulted in\na positive exoskeleton power peak, with flexion torque resulting in a greater peak power than\nextension torque. The peak power generated on the unaffected side was greater than that generated\non the affected side for both flexion and extension assistance. Specifically, the peak power generated\non the unaffected side (0.17 + 0.02 W kg-1 flexion, 0.09 + 0.01 W kg-1 extension) was greater than\nthat generated on the affected side (0.13 + 0.02 W kg-1 flexion, 0.08 + 0.01 W kg-1extension) for\nboth flexion and extension assistance.”\nThere seems to be a mix of statistical model-driven results and qualitative results. If this is the case, the\nauthors should clarify which comparisons were statistically compared and which were not by distinguishing\ntheir language between these sets of comparisons. If all comparisons were statistically driven, then these\ncomparisons should also include relevant statistical output (method, p value).\nThank you; this is an important point to clarify. We only conducted statistical tests on our three primary\noutcomes: the metabolic cost of walking, the total positive biological work performed by the affected\nand unaffected hip, knee, and ankle joints, and the positive biological hip work by the affected and\nunaffected hip joints. Due to the smaller sample size (N=7) and the associated statistical power, we\ndecided to analyze only these primary outcomes and to correct for multiple comparisons. In the revised\npaper, we included the relevant statistical output after reporting the main results. We also reworded\nseveral sections to clearly indicate which outcomes were compared statistically.\nIntroduction\n“We hypothesize that the exoskeleton assistance will decrease the positive biological work done by\nboth hip joints, thereby reducing the metabolic cost of walking. Thus, our primary outcome\nmeasures will be the metabolic rate, the total positive biological work calculated as the sum of the\naffected and unaffected hip, knee, and ankle joints, and the positive biological hip work, calculated\nas the sum of the affected and unaffected hip joints.”\nFig. 2 caption\n“... c) represent the standard error. Brackets indicate that statistical tests were performed to\ncompare the two conditions, and asterisks indicate significant differences.”\nResults\n“Data were recorded during the last 2 minutes of each treadmill walking trial. Pairwise t-tests\nwere performed to assess significant differences in our three primary outcome measures:\nmetabolic cost of walking, total positive biological work, and positive biological hip work, with\nand without the exoskeleton. See Materials and Methods for more details.”\nResults: Metabolic cost\n“Thus, assistance from the powered exoskeleton reduced the metabolic cost of walking by 18 +\n2.0% on average across the seven participants (two-tailed paired t-test, P = 0.0002).”\nResults: Positive joint work\n“This exoskeleton assistance significantly reduced the total positive biological work by 14 + 4% on\naverage across the seven participants (two-tailed paired t-test, P = 0.0167). This exoskeleton\nassistance significantly reduced the positive biological hip work by 29 + 6% on average across the\nseven participants (two-tailed paired t-test, P = 0.0052).”\nMethods: Data processing and statistical analysis\n“Two-tailed paired t-tests evaluated effects of exoskeleton assistance on our primary outcome\nmeasures: metabolic power, total positive biological work (affected and unaffected hip, knee, and\nankle joints, summed), and positive biological hip work (affected and unaffected hip joints,\nsummed). We set α = 0.05 for these analyses and applied a Holm-Bonferroni correction to adjust\nfor multiple comparisons. We did not statistically compare any secondary outcome measures (hip\nwork during stance, foot clearance, kinematics). We used a two-tailed t-test because we did not have\na prior assumption that the difference between conditions (walking with or without the exoskeleton)\nwould be positive or negative. Finally, the data were normally distributed and there were no\noutliers, meeting the requirements for a two-tailed paired t-test.”\nOn a similar note as the previous comment, it is unclear whether the authors compared any hip work during\nstance, foot clearance, and kinematic variables with and without exo assistance using statistical methods.\nAs noted above, we only ran statistics on our primary outcome measures (metabolic power, total positive\nbiological work (affected and unaffected hip, knee, and ankle joints, summed), and positive biological\nhip work (affected and unaffected hip joints, summed), so we did not statistically compare hip work\nduring stance. This secondary biomechanical analysis was to provide more context and\nunderstanding of when the hip exoskeleton assistance was most effective, which was different on each\nside. As a secondary outcome, we did not run a statistical analysis. We hope this is clarified in the\nrevised manuscript sections (as above).\nReporting percentage of change alongside actual numbers is helpful in sensing whether the change in\noutcome due to assistance is a big or a small change. The authors should include this information throughout\n- currently not included in hip work during stance, foot clearance, and kinematics.\nWe agree that adding the percent change can be helpful. Thank you for the suggestion. We have\nensured that percentage changes are included in all major outcomes. The revised results are below:\nResults: Hip work during stance\n“When the unaffected side is in initial double support, the affected side is in terminal double support\n(Fig. 3). During this critical step-to-step transition, the exoskeleton provides extension assistance to\nthe unaffected side and flexion assistance to the affected side, decreasing the positive biological hip\nwork by 0.027 + 0.009 J kg-1 (41% + 15%) and 0.022 + 0.013 J kg-1 (28% + 19%), respectively, per\nstride. The positive biological hip work also decreased by 0.033 + 0.013 J kg-1(63% + 19%) and 0.014\n+ 0.014 J kg-1 (129% + 27%) on the unaffected and affected side, respectively, during the next phase\nof gait, when the unaffected side was in single support and the affe cted side was in swing. During the\nfollowing step-to-step transition (unaffected side trailing), the positive biological hip work decreased\nslightly by 0.005 + 0.009 J kg-1 (83% + 25%) on the unaffected side but increased by 0.014 + 0.009 J\nkg-1 (118% + 22%) on the affected side.\nIn the final phase, when the unaffected side is in swing and the affected side is in single support, the\npositive biological hip work decreased by 0.017 + 0.004 J kg-1 (71% + 17%) on the unaffected side\nand increased slightly by 0.004 + 0.003 J kg-1 (168% + 22%) on the affected side.”\nResults: Foot clearance\n“Minimum foot clearance trajectories were calculated by taking the minimum trajectory of the toe and\nheel during swing (Extended Data Fig. 4). Hip exoskeleton assistance increased foot clearance on both\nthe affected and unaffected sides. On the affected side, peak foot clearance increased by 4 + 2 mm\n(12% + 7%) with the exoskeleton. On the unaffected side, peak foot clearance increased by 2 + 1 mm\n(6% + 3%) with the exoskeleton.”\nResults: Kinematics\n“Hip exoskeleton assistance slightly increased the affected hip joint’s average range of motion,\nprimarily by increasing peak hip flexion by 2° + 1° (8% + 3%). The unaffected hip joint’s average\nrange of motion decreased by 2° + 1° (8% + 2%), decreasing in both peak flexion and extension.\nExoskeleton extension assistance slightly increased the average peak knee extension angle by 2° + 1°\n(31% + 18%) on both sides. Finally, with the exoskeleton, the affected ankle joint was slightly more\nplantarflexed throughout the gait cycle, while the unaffected ankle joint was slightly more dorsiflexed\nthroughout the gait cycle. These differences were generally less than 2° and did not affe ct the ankle\njoint range of motion. Thus, hip exoskeleton assistance resulted in only minor, non-clinically\nsignificant kinematic changes (Extended Data Fig. 5).”\nIn “exoskeleton assistance,” please provide quantifiable results that lead to authors’ claim that “peak power\ngenerated on the unaffected side was greater than that generated on the affected side” for both hip motion.\nThank you for pointing this out. We have revised the results to include the peak power generated on\neach side for each direction of motion:\nResults: Exoskeleton assistance\n“Many participants preferred similar exoskeleton tuning on each side, resulting in similar\ngenerated torque profiles on each side. Notably, participant 6 received greater peak flexion torque\non their affected side compared to their unaffected side. Generally, the peak flexion torque provided\nby the exoskeleton was greater than the peak extension torque, with greater extension torque\nsupplied to the unaffected side (Extended Data Fig. 1). Each peak of exoskeleton torque resulted in\na positive exoskeleton power peak, with flexion torque resulting in a greater peak power than\nextension torque. The peak power generated on the unaffected side was greater than that generated\non the affected side for both flexion and extension assistance. Specifically, the peak power generated\non the unaffected side (0.17 + 0.02 W kg-1 flexion, 0.09 + 0.01 W kg-1 extension) was greater than\nthat generated on the affected side (0.13 + 0.02 W kg-1 flexion, 0.08 + 0.01 W kg-1extension) for\nboth flexion and extension assistance.”\nDiscussion\nIn the discussion of symmetry restoration through exo, consider linking this back to metabolic cost reduction\n(i.e. relationship between asymmetry and increased metabolic cost)\nThank you for your feedback. As suggested by the reviewer, we revised the results to include analysis\non two gait symmetry outcomes, one spatial (step length) and one temporal (stance time), and included\na figure in the extended data (Extended Data Fig. 6). We also revised the discussion to connect these\nsymmetry results back to metabolic cost.\nResults: Kinematics\n“Symmetry was evaluated by calculating a symmetry index (SI) where 0% indicates perfect\nsymmetry34. Exoskeleton assistance slightly improved symmetry in stance time (13% + 3%\nasymmetry without the exoskeleton vs. 12% + 4% asymmetry with the exoskeleton), but increased\nasymmetry in step length (-8.2% + 6% without the exoskeleton vs. -14% + 5% with the exoskeleton)\n(Extended Data Fig. 6). Generally during walking with and without the exoskeleton, participants spent\nlonger in stance on their unaffected side and took longer steps onto their affected side.”\nDiscussion\n“Hip exoskeleton assistance had a small effect on the hip joint range of motion for both sides (Extended\nData Fig. 5). Specifically, the exoskeleton assistance increased the affected hip joint’s range of motion\nby 2° and decreased the unaffected hip joint’s range of motion by 2°, slightly improving symmetry in\nthe hip joint range of motion. Hip exoskeleton assistance slightly improved symmetry in stance time but\ndecreased symmetry in step length (Extended Data Fig. 6). The observed increase in step length\nasymmetry indicates that the participants take longer steps onto their affected side with the exoskeleton\nassistance. This result is likely due to the assisted hip flexion during swing. Overall, we did not observe\nany consistent trends in spatiotemporal variables across participants while walking with the exoskeleton\ncompared to walking without the exoskeleton. Although passive80 and powered24 ankle-foot\northoses/exosuit have shown consistent improvements in symmetry for stance time and step length,\nprevious powered hip exoskeleton studies have not\n56–\n. Our results seem to confirm that portable powered hip exoskeletons do not consistently increase\n58\nsymmetry. This result may be due to the assistive controller being primarily tuned to provide positive\npower as needed to reduce metabolic rate.”\nMethods: Data processing and statistical analyses\n“We evaluated interlimb asymmetry during both walking trials using a symmetry index (SI). The SI is\ncalculated by taking the difference between the spatiotemporal data of the affected and unaffected sides,\ndivided by one-half the sum of the spatiotemporal data from both sides, multiplied by 100%34. A positive\nSI indicates asymmetry towards the subject’s unaffected side, while a negative SI indicates asymmetry\ntowards the subject’s affected side. Zero indicates perfect symmetry. “\nExtended Data Fig. 7 | Spatiotemporal symmetry. Symmetry in stance time (left) and step length (right) without the exoskeleton (gray) and\nwith the exoskeleton (red). The bars represent the mean, and the error bars represent the standard error. The blue dots represent values for the\nindividual participants. 0 is perfect symmetry. The symmetry index is calculated by taking the difference between the affected and unaffected\nsides, divided by one-half the sum of the affected and unaffected sides, multiplied by 100%34. A positive symmetry index in stance time\nindicates the participant spends longer in stance on their unaffected side. A positive symmetry index in step length indicates the participant\ntakes a longer step onto their unaffected side.\nindividuals with hemiparesis using a wearable unilateral hip-assisting robot in the discussion of prior work.\nTherefore, statement “this result has never been replicated in individuals with hemiparesis” is untrue.\nHowever, the solution is unique since this device provides bilateral assistance. Authors should consider\nreframing this statement to highlight the advantages and rationale for choosing a bilateral solution with\nactive assistance bidirectionally compared to other approaches.\nThank you for bringing this to our attention. As suggested by the reviewer, we revised our manuscript to\ndiscuss this case series, including considerations related to the unilateral and unidirectional assistive\napproach, as well as the lack of statistical analysis. Also, as suggested by the reviewer, we removed the\nstatement “this result has never been replicated in individuals with hemiparesis” in the discussion.\nIntroduction\n“Although many studies have demonstrated the metabolic benefits of powered hip exoskeleton assistance\nin healthy individuals37,38,44–54, only a handful of studies have explored hip exoskeletons to assist\nindividuals with hemiparesis55–58. Unilateral hip exoskeleton assistance to the hemiparetic side has\nshown improved walking speed55,56. Moreover, bilateral assistance to both hips has shown greater\nimprovements in walking speed than unilateral assistance to the affected hip joint only56,57. A case series\nwith three individuals with hemiparesis subjects suggested that unilateral hip flexion assistance could\nimprove the metabolic cost of walking59. However, a subsequent study with nine hemiparetic subjects\nshowed no significant improvement in the metabolic cost of walking with bilateral hip assistance58.\nUnfortunately, these studies did not assess the impact of hip assistance on hemiparetic gait\nbiomechanics. As a result, it remains unknown whether concurrent hip flexion and extension assistance\nprovided by a powered hip exoskeleton can effectively compensate for impaired ankle propulsion and\nreduce metabolic cost.”\nDiscussion\n“The reduction in the metabolic cost of walking obtained in this study using a portable hip exoskeleton\n(18%) is substantially greater than the metabolic reduction previously achieved using an ankle exosuit\ntethered to a benchtop actuator (10%)25. A case series with three individuals with\nhemiparesis suggested potential for unilateral hip flexion assistance to reduce the metabolic cost of\nwalking59. Still, they found a much lower reduction compared to this study (8.6%), and no follow-\nup studies confirmed its statistical significance. Thus, our analysis suggests that bilateral,\nbidirectional hip assistance may be more effective than unilateral, unidirectional hip assistance\nand ankle assistance in improving the metabolic cost of walking with a portable exoskeleton or\nexosuit.”\nDiscussion paragraph acknowledging the smaller sample size and lack of difference between different\ngroupings could be better polished by synthesizing these results more. While further studies with increased\nsample are certainly warranted, consider the possibility that these results suggest this exo could be a viable\nsolution for people with wide range of impairment, size or sex, and use of other assistive devices.\nThank you for emphasizing a key value of this study. We revised the discussion section to include a\nparagraph on the limitations, specifically emphasizing that the exoskeleton can assist a diverse range of\nindividuals with hemiparesis.\nAlthough this study includes only a small sample size (N=7), the sample was diverse. The two female\nparticipants (S1 and S2) had an average metabolic cost reduction similar to that of the male\nparticipants (18% women vs 18% men). Two participants (S2, S6) used ankle-foot orthoses (AFO). The\naverage metabolic cost reduction for these two participants was slightly less than for the others (17%\nvs 19%). This result suggests that individuals with ankle-foot orthoses may experience slightly less\nmetabolic benefit from hip exoskeleton assistance, and it could be due to the AFOs restricting ankle\npush-off and limiting gait adaptability77,78. We did not observe any other trends in the age, chronicity,\nweight, or other biological factors of the participants presented in Table 1. Our results indicate that\nhip exoskeletons can assist a wide range of individuals with hemiparesis based\non level of impairment, size, sex, and use of other assistive devices. Further studies with a larger\nsample size are needed to assess important clinical and biological factors that may impact the\nefficacy of hip exoskeleton assistance.\nBroadly speaking, the organization of the discussion could be improved, similar to the reviewer comment\nwith the introduction. Some suggestions include: 1) Inclusion of a limitation paragraph towards the end of\nthe discussion so they appear together (small sample size, weight, tuning method), 2) more transitions\nbetween paragraphs, and 3) grouping ideas together.\nThank you for this suggestion. Following your feedback, we revised the discussion to group ideas in the\nfollowing order: main paper contribution/significance, metabolic rate/cost of transport, total/joint positive\nwork, assistance power/torque, range of motion, and foot clearance. We also included a limitation section\nat the end of the discussion and improve transitions between paragraphs.\nMethods\nPlease provide more detail on the training sessions individuals received – what was the average number of\nsessions and average duration over which time period? For individuals who received more than one training,\ndid the authors tune the parameters fresh? Or start at the converged parameter from the previous training? How\ndid the authors accommodate if user preference and comfort has changed between training?\nThis is an important point to clarify. Thank you for bringing it to our attention. We include additional\ninformation on the training and tuning process in the revised methods section:\nMethods: Exoskeleton training and assistance tuning\n“The amount of exoskeleton training was not standardized across participants in this study to\naccommodate differences in walking ability between participants. Some participants exhibited less\nconsistent walking patterns, which made exoskeleton tuning less consistent and necessitated more\ntuning and training. Furthermore, some participants needed more rest and walked less each\ntraining day, so they had more training days than participants who could walk longer in one session.\nOn average, participants completed 2+] additional training sessions (not including the fit/tune\nsession) before data collection. Participants generally completed a session once a week, completing\ntraining over 4+] weeks on average. The experimenter started with the converged parameters from\nthe prior training session and tuned as necessary from there. Parameters were retuned in each\ntraining session based on any changes in user preference or comfort, and any gait changes observed\nby the experimenter. The exoskeleton parameters were kept the same as the final tuned parameters\nfrom the final training session during the metabolic cost evaluation and motion capture\nexperiments.”\nPlease provide more detail on how the authors used “feedback from participants and the experimenter’s\njudgment” to tune the assistance parameters.\nThank you, this is another important point for clarification. We revised the methods section as follows\nto include more detail on the tuning process:\nMethods: Exoskeleton training and assistance tuning:\n“This tuning procedure was done independently for the affected and unaffected sides. Exoskeleton\nassistance parameters were tuned based on the experimenter’s training and experience via visual\ninspection of gait during the walking sessions89. The experimenters also watched a live estimate of\nexoskeleton power based on the exoskeleton torque and velocity, and adjusted timing and peak torque\nparameters to maximize positive power generation, which is critical for reducing the metabolic cost\nof healthy subjects45,79. After each adjustment, the participant was asked if the assistance felt better\nor worse. If it felt better, the experimenter would continue adjusting that parameter in that direction\nuntil it felt worse. If the change felt worse, the parameter was reverted to its previous value, and a\ndifferent change was made. Comfort was assessed by asking the participants for their subjective\nfeedback and visually inspecting the exoskeleton interfaces for excessive bending or movements.\nParticipants walked for approximately half an hour, less than 10 minutes at a time, with rest between\nwalking trials.”\nIn the last paragraph of methods (discussion of stats), please include additional information regarding how\nthe authors determined the assumptions of a two-tailed paired t-test were evaluated and met for the data.\nWe added this information to the revised methods section.\nMethods: Data processing and statistical analysis\n“Two-tailed paired t-tests evaluated effects of exoskeleton assistance on our primary outcome\nmeasures: metabolic power, total positive biological work (affected and unaffected hip, knee, and\nankle joints, summed), and positive biological hip work (affected and unaffected hip joints,\nsummed). We set α = 0.05 for these analyses and applied a Holm-Bonferroni correction to adjust\nfor multiple comparisons. We did not statistically compare any secondary outcome measures (hip\nwork during stance, foot clearance, kinematics). We used a two-tailed t-test because we did not have\na prior assumption that the difference between conditions (walking with or without the exoskeleton)\nwould be positive or negative. Finally, the data were normally distributed and there were no\noutliers, meeting the requirements for a two-tailed paired t-test.”\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThis study demonstrates that applying a hip exoskeleton to stroke patients effectively reduces the metabolic\ncost of walking. While this metabolic improvement is a valuable outcome, it is of paramount importance\nfor clinical populations to ensure that the assistive torque does not introduce additional burdens or potential\nharms. The current evidence—specifically, the absence of EMG data, the observed increase in ground\nreaction forces (Extended Data Fig. 1), and the lack of significant improvement in gait symmetry (Extended\nData Fig. 2)—makes it difficult to conclusively rule out such adverse effects. Therefore, I would\nrecommend a more comprehensive analysis of the existing data to evaluate changes in other critical patient-\noriented metrics, such as gait stability, joint trajectory symmetry, and other clinically relevant indicators,\nfollowing the application of exoskeleton assistance.\nThank you for your recommendation. Following your suggestion, we performed additional analysis to\nevaluate critical patient-oriented metrics and clinically relevant indicators. Specifically, we evaluated\nstability by calculating the extrapolated center of mass and stride width with and without the\nexoskeleton. This analysis shows no changes in stability. In addition, we evaluated joint trajectory\nsymmetry by calculating the symmetry index for the joint range of motion and by performing 1-\ndimensional spatial parametric mapping with and without the exoskeleton (Extended Data Figs 6-7).\nAlthough there were no statistically significant differences in joint kinematics, we observed an\nimprovement in hip joint range of motion symmetry. Based on this additional analysis, we can\nconclusively rule out adverse effects. We present the results of this additional analysis in the results\nsection of the revised manuscript and extended data.\nResults: Kinematics\nKinematic trajectories during walking with and without the exoskeleton are presented in\nExtended Data Fig. 5. Hip exoskeleton assistance slightly increased the affected hip joints’\naverage range of motion (Extended Data Fig. 6), primarily by increasing peak hip flexion by\n2° + 1° (7% + 3%). The unaffected hip joints’ average range of motion decreased by 2° + 1°\n(5% + 3%), decreasing in both peak flexion and extension. Exoskeleton extension assistance\nslightly increased the average peak knee extension angle by 2° + 1° (33% + 18%) on both\nsides. Finally, with the exoskeleton, the affected ankle joint was slightly more plantarflexed\nthroughout the gait cycle, while the unaffected ankle joint was slightly more dorsiflexed\nthroughout the gait cycle. These differences were generally less than 2° and resulted in a\nslightly decreased range of motion at the affected ankle joint (Extended Data Fig. 6). Spatial\nParametric Map (SPM) analysis demonstrated that there were no statistically significant\ndifferences (two-tailed paired t-test, α= 0.05) in the kinematic trajectories of each joint during\nwalking with the exoskeleton compared to without the exoskeleton (Extended Data Fig. 7).\nThus, hip exoskeleton assistance resulted in only minor, non-clinically significant and non-\nstatistically significant kinematic changes.\nSymmetry was evaluated by calculating a symmetry index (SI) where 0% indicates perfect\nsymmetry34. Hip exoskeleton assistance improved symmetry in the hip joint range of motion\n(-30% + 10% without the exoskeleton vs. -18% + 9% with the exoskeleton) but had only\nminor effects on the knee and ankle range of motion symmetry (Extended data Fig. 8) ....\nTo evaluate gait stability, we evaluated the extrapolated center of mass and step width\nduring walking with and without the exoskeleton (Extended Data Fig. 10). The minimum\nmediolateral distance from the extrapolated center of mass to the average center of pressure\nof the affected and unaffected sides was relatively unchanged, decreasing by 0.2 + 0.3 cm\n(5% + 8%) on the affected side and by 0.06 + 0.34 cm (4% + 11%) on the unaffected side\nwith the exoskeleton. Furthermore, the step width was 20 + 2 cm during walking with and\nwithout the exoskeleton.\nDiscussion\nHip exoskeleton assistance had a small effect on the hip joint range of motion for both sides\n(Extended Data Fig. 5-6). Specifically, the exoskeleton assistance increased the affected hip\njoints’ range of motion by 2° and decreased the unaffected hip joints’ range of motion by 2°,\nimproving symmetry in hip joint range of motion by 12% (Extended Data Fig. 7). The\nobserved kinematic changes are small and below the thresholds for clinical and statistical\nsignificance (Extended Data Fig. 8). Furthermore, we did not observe any meaningful\nchanges in the extrapolated center of mass and step width, indicating that hip exoskeleton\nassistance did not adversely affect stability (Extended Data Fig. 10).\nMethods\nTo evaluate changes in kinematic trajectories during walking with the exoskeleton compared to\nwalking without the exoskeleton, we performed a 1-dimensional Statistical Parametric Map\n(SPM) analysis, using the spm1d 0.4.10 package in Matlab 2024b103. SPM was run on the\nkinematic trajectories of each joint (affected and unaffected hip, knee, and ankle) during walking\nwith the exoskeleton compared to walking without the exoskeleton to identify statistically\nsignificant changes (two-tailed paired t-tests with α = 0.05).\nNo exo With exo\nS1 S2 S 3 S4 S5 S6 S7\nAffected side Unaffected side\ng ]edn\n[ootimof\ne gnapr\nHig]edn[ootimof\ne gnaer\neK\ng ]edn[oomtiof\ne g naerk\nnlA\nExtended Data Fig. 6 | Joint range of motion. The range of motion of the\naffected (left) and unaffected (right) side hip (top), knee (middle), and ankle\n(bottom) joints during walking without the exoskeleton (gray) and with the\nexoskeleton (red). The bars represent the mean, and the error bars represent the\nstandard error. The blue dots represent values for the individual participants.\nNo exo With exo\nS1 S2 S 3 S4 S5 S6 S7\nHip Knee Ankle\n]%[\nyrtemmys\nnotoim\nfo egnaR\nExtended Data Fig. 7| Joint range of motion symmetry index. The symmetry index for the range of motion of the\nhip (left), knee (middle), and ankle (right) joints during walking without the exoskeleton (gray) and with the exoskeleton\n(red). The bars represent the mean, and the error bars represent the standard error. The blue dots represent values for\nthe individual participants. The symmetry index is calculated by taking the difference between the affected and\nunaffected sides, divided by one-half the sum of the affected and unaffected sides, multiplied by 100%36. At perfect\nsymmetry, the index would equal 0%. A negative symmetry index indicates the participant has a larger range of motion\non their unaffected side.\nTrajectory difference Significance threshold\nAffected side Unaffected side\n5\n]ged[\nelgna\npiH]ged[\nelgna\neenK\n]ged[\nelgna\nelknA\n0\n-5\n5\n0\n-5\n5\n0\n-5\nGait cycle [%] Gait cycle [%]\nExtended Data Fig. 7 Spatial parametric map analysis. The difference in the\nkinematic trajectories of the affected (left) and unaffected (right) hip (top),\nknee (middle), and ankle (bottom) joints during walking with the exoskeleton compared\nto walking without the exoskeleton (red, solid lines). The threshold for a statistically\nsignificant difference at each joint is indicated by the dashed lines (gray). No\nkinematic differences exceed the respective threshold for statistical significance\n(two-tailed paired t-test, α= 0.05).\nNo exo Wth exo\nS1 S2 S 3 S4 S5 S6 S7\nAffected side Unaffected side\nExtended Data Fig 10 | Stability analysis. The minimum mediolateral distance from the extrapolated center\nof mass to the center of pressure for the affected and unaffected sides (left) and the step width (right) during\nwalking without the exoskeleton (gray) and with the exoskeleton (red). The bars represent the mean, and the\nerror bars represent the standard error. The blue dots represent values for the individual participants.\n]mc[\nhtdiw\npetS\necnatsid\nlaretaloidem\nmuminiM\n]mc[\nPOC\not\nMOC\ndetalopartxe\nAdditionally, a specific clarification is needed regarding Extended Data Fig. 1 (Vertical GRF). After the\ntoe-off event, the ground reaction force remains non-zero for a period of time. Could the authors explain\nthe reason for this?\nThank you for pointing this out. To address this discrepancy, we conducted additional analysis by\nexamining the results of each participant separately (see Extended Data Fig. 12 below). This analysis\nshows that the visual discrepancy is an artifact resulting from averaging the toe off timing across\nparticipants. As you can see from the figure below, the participants had different toe off timing with\ngreater variability on the unaffected side compared to the affected side. As a result, the average GRF\nvalue at the average toe off timing is greater than that observed in individual participants. This analysis\nexplains why the ground reaction force remains non-zero for a period of time after toe off.\nResults: Kinetics\nThe peak vertical GRF increased by 0.04 + 0.006 N kg-1 (4.4% + 0.6%) on both the affected\nand unaffected sides (Extended Data Fig. 12).\nAverage Toe off\nS1 S2 S3 S4 S5 S6 S7 Toe off\n1.2\n1\n0.8\n0.6\n0.4\nNo exo With exo\n0 5 0 1 0 0 0 5 0 1 0 0\nGait cycle [%] Gait cycle [%]\nExtended Data Fig. 12 | Vertical ground reaction force (GRF) trajectories. The vertical GRF trajectories\nof each participant's unaffected (top) and affected (bottom) sides during walking without the exoskeleton\n(left) and with the exoskeleton (right). Each participant's average toe off location is marked with a square in\nthe corresponding color. The average trajectory across subjects is shown in the dashed black line, with the\naverage toe off location across participants represented as a solid circle.\nedis\ndetceffanU\n-gk\nN[\nFRG\nlacitreV\n0.2\n0\n1.2\n1\n0.8\n0.6\n0.4\n0.2\n0\nedis\ndetceffA\n]1-gk\nN[\nFRG\nlacitreV\nReviewer #2 (Remarks to the Author):\nThe authors have done an excellent job of addressing my comments/feedback from the original review. I\nhave no further feedback. This is an excellent paper and I look forward to citing it.\nThank you for your previous feedback, which has improved the quality of our paper. We are pleased\nto hear that you are satisfied with our revisions.\nReviewer #3\nThe revision is improved with complete analyses are more complete and clear writing. The authors have\naddressed prior reviews thoroughly. A few minor comments should still be addressed before the work can\nbe accepted for publication.\nThank you for your suggestions. You can find a point-by-point answer below.\nIntroduction\n Consolidate the hypothesis. The last paragraph states “we hypothesize that ...” twice. Please merge into a\nsingle, stand-alone hypothesis that cleanly motivates the study.\n Relocate the outcomes sentence. The paragraph also ends with outcome definitions. The sentence, “Thus,\nour primary outcome measures will be ...”, reads like Methods/Results rather than a concluding line of the\nIntroduction. Reword it to close the rationale succinctly or move it to the Results.\nAs recommended by the reviewer, we consolidated the two hypotheses into one and removed the\nfinal sentence in the paragraph. The revised paragraph is copied below:\n“Based on these findings, we hypothesize that a hip exoskeleton could offset the impaired\nankle propulsion in individuals with hemiparesis by simultaneously assisting the affected and\nunaffected hip joints, leading to reduced metabolic cost and positive biological work on both\nhip joints. To test this hypothesis, we assessed the metabolic cost of treadmill walking at a\nconstant self-selected speed, with and without a lightweight, portable, bilateral hip\nexoskeleton designed to supplement the users’ hip flexion and extension, while measuring the\npositive biological work performed by both hip joints.”\nResults\n Make the gait-phase reporting fully parallel. In the “Hip work during stance” subsection, use a symmetric\nstructure across all four phases and begin each with “During ...” for immediate readability. In addition, present\nall four phases in one paragraph rather than splitting three in one paragraph and the last in a separate paragraph\n– unless the authors intentionally isolate the final phase, in which case add a brief interpretation.\nThank you for this suggestion. We merged the paragraphs and used parallel structure as\nrecommended.\n“When the unaffected side is in initial double support, the affected side is in terminal double\nsupport (Fig. 3). During this critical step-to-step transition, the exoskeleton provides extension\nassistance to the unaffected side and flexion assistance to the affected side, decreasing the\npositive biological hip work by 0.026 + 0.008 J kg-1 (60% + 18%) and 0.022 + 0.013 J kg-1\n(49% + 26%), respectively, per stride. During the next phase of gait, when the unaffected side\nwas in single support and the affected side was in swing, the positive biological hip work also\ndecreased by 0.035 + 0.013 J kg-1 (64% + 19%) and 0.014 + 0.014 J kg-1 (55% + 27%) on\nthe unaffected and affected side, respectively. During the following step-to-step transition\n(unaffected side trailing), the positive biological hip\nwork decreased slightly by 0.005 + 0.009 J kg-1 (83% + 25%) on the unaffected side but\nincreased by 0.014 + 0.009 J kg-1 (118% + 22%) on the affected side. In the final phase, when\nthe unaffected side is in swing and the affected side is in single support, the positive biological\nhip work decreased by 0.017 + 0.004 J kg-1 (71% + 17%) on the unaffected side and increased\nslightly by 0.004 + 0.003 J kg-1 (168% + 22%) on the affected side.”\nDiscussion\n Streamline the opening. The first four sentences largely repeat the Introduction. Condense or remove them\nso the first paragraph directly summarizes the main findings and their magnitude.\nWe condensed the opening paragraph as recommended.\n“Individuals with hemiparesis have a significantly higher metabolic cost of walking than healthy\nindividuals2,22,24, limiting physical activity8, community participation, and reducing overall health\nand quality of life1,3,9. Portable powered exoskeletons have successfully improved the metabolic\ncost of walking in young, healthy individuals39,40,46–51,53–56, but have not yet done so in individuals\nwith hemiparesis25,60. Here we show that assistance from a lightweight, portable, bilateral hip\nexoskeleton decreased the metabolic cost of walking in individuals with hemiparesis (n=7) by\n18%, equivalent to removing a 13.5-kg backpack from a healthy individual32 (Fig. 1c). The\nobserved metabolic cost reduction is substantial and has the potential to improve mobility and\nquality of life after stroke.”\n Move speculative long-term implications. The paragraph on potential long-term benefits is speculative; it\nbelongs later, after core findings and comparisons to prior work.\nWe moved the paragraph to the end of the discussion as recommended.\n Ensure no new data is presented in discussion. Reference Extended Data Table 1 in the Results, not only\nin the Discussion\nThanks for catching that. We moved that reference to the results section, where it should have\nbeen. We also double-checked that no new data is presented in the discussion.\n“Thus, if we account for the metabolic penalty of the exoskeleton mass, the average\nmodified exoskeleton performance index65,66 was 0.515, which is about half the theoretical\nmaximum of 1 (Extended Data Table 1).”\n Clarify the comparison of performance index. The last two sentences currently hedge between two\nexplanations for the lower performance index (device efficiency vs. pathology-related demands). Please\ntake a position and support it: (1) If the authors attribute it to exoskeleton design, indicate which design\nfeatures plausibly reduce energy transfer (e.g., mass distribution, timing, peak torque limits, interface\ncompliance, etc.) and why. (2) If the authors attribute it to hemiparetic gait, cite literature indicating higher\npositive mechanical work requirements or altered muscle efficiency in hemiparesis and explain how that\nmaps to your findings.\nThanks for this comment. Upon further reflection, we decided that the most likely cause for the\ndifference in performance index is not the exoskeleton but the hemiparetic gait cycle. We revised\nthe paragraph accordingly. We clarify this gap in the subsequent paragraph.\n“The metabolic penalty of wearing our portable hip exoskeleton was estimated at 11.76 W,\nsimilar to the average metabolic penalty of other advanced hip exoskeletons (6.93 – 17.7 W,\nExtended Data Table 1). When accounting for this metabolic penalty, the performance index\nof our exoskeleton was 0.515, well below the theoretical maximum of 1, which would\nindicate perfect energy transfer between the exoskeleton and the user. The apparent\nefficiency of our device is notably lower than that of portable power hip exoskeletons\nmeasured in previous studies with healthy subjects (0.515 versus 0.74-0.92, Extended Data\nTable 1). This difference may indicate that greater positive mechanical energy is necessary\nto achieve the same metabolic benefits in individuals with hemiparesis compared to healthy\nsubjects.”\nFurthermore, the subsequent paragraph questions the performance index’s validity in pathological gait.\nReporting it and then questioning it reads as internally inconsistent. Since this metric is still informative,\nkeep it succinctly, acknowledge the key assumptions, and fold this limitation into the prior paragraph that\ndiscusses the performance index rather than a stand-alone paragraph.\nWe respectfully disagree that questioning the validity of the performance index is an internal\ninconsistency.\n Reposition the sample-size paragraph. The paragraph expanding on applicability and addressing sample\nsize is useful but not tied to specific results. Move it to the paragraph immediately preceding Limitations.\nWe moved the paragraph as recommended.\nMethods\n No comment\nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the\nNature Communications initiative to facilitate training in peer review and to provide appropriate recognition\nfor Early Career Researchers who co-review manuscripts.\nThank you for reviewing our work. Your comments helped us improve our manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "This study demonstrates that applying a hip exoskeleton to stroke patients effectively reduces the metabolic cost of walking. While this metabolic improvement is a valuable outcome, it is of paramount importance for clinical populations to ensure that the assistive torque does not introduce additional burdens or potential harms. The current evidence—specifically, the absence of EMG data, the observed increase in ground reaction forces (Extended Data Fig. 1), and the lack of significant improvement in gait symmetry (Extended Data Fig. 2)—makes it difficult to conclusively rule out such adverse effects. Therefore, I would recommend a more comprehensive analysis of the existing data to evaluate changes in other critical patient-oriented metrics, such as gait stability, joint trajectory symmetry, and other clinically relevant indicators, following the application of exoskeleton assistance.\nAdditionally, a specific clarification is needed regarding Extended Data Fig. 1 (Vertical GRF). After the toe-off event, the ground reaction force remains non-zero for a period of time. Could the authors explain the reason for this?", "reviewer_2": "The authors have done an excellent job of addressing my comments/feedback from the original review. I have no further feedback. This is an excellent paper and I look forward to citing it.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "RESPONSE TO REVIWER 3\nReviewer #3 (Remarks to the Author):\nIn this work, authors presented a hip exoskeleton that provides active mechanical assistance bilaterally and\nbidirectionally to the hip joint during treadmill walking in seven individuals with hemiparesis. The use of\ntheir exoskeleton was able to reduce the metabolic cost of walking by 18%, likely by reducing positive\nbiological hip work. This work has great potential to offer a portable solution for individuals with mild-to-\nmoderate hemiparesis that enables them to walk for longer duration, leading to improved recovery and\nintegration into the real world. Presentation of the work can be substantially improved by\nreorganizing the introduction and discussion, providing more detail in results (particularly in comparisons of\nnon-metabolics related outcomes) and methods (particularly in training and tuning), and a more comprehensive\ndiscussion synthesizing results of different outcomes evaluated.\nThank you for reviewing our manuscript and providing key suggestions on how to improve it. We\nprovide a point-by-point response to each of your comments below. Revisions to the manuscript are\nincluded in red.\nIntroduction\nThe intro should be restructured to improve its cohesiveness. In this submission, authors jump back and\nforth between the discussion of post-stroke impairment and exoskeleton. Instead, consider centering the\nintroduction around impairment-specific challenges related to metabolic cost, followed by discussion of\ndemonstrated benefits and limitations of currently explored exoskeleton solutions to target these challenges,\nand why the authors’ solution - bilateral and bidirectional hip exoskeleton - is a promising alternative\nsolution.\nThank you for your feedback. We have revised the introduction to improve clarity and flow. Following\nyour recommendation, the revised introduction starts by presenting the challenges associated with the high\nmetabolic cost of walking in the hemiparetic population. Then, we talk about leg weakness and\ncompensatory gait strategies. After that, we discuss exoskeletons, summarizing the positive results and\nidentifying open challenges. Finally, we present the rationale for the proposed assistive approach and the\nhypothesis we tested in this study.\nPlease use people-first language when appropriate. “Individuals with above-knee amputation” is people-\nfirst, while “above-knee amputees” is not.\nThank you for bringing this to our attention. We revised the manuscript to use people-first language.\nIntroduction\n“Biomechanical analysis has shown that using a hip exoskeleton to assist with hip flexion and\nextension can help compensate for reduced ankle propulsion associated with a passive leg\nprosthesis60 thereby decreasing the metabolic cost of walking in individuals with above-knee\namputation .”\n“Compared to our previous metabolic study with individuals who have above-knee\namputations61....”\nResults\nConsider moving information from Extended Data Fig 1 into the main text when discussing tuning for both\nlegs. From evaluating the figure it appears most individuals were tuned to receive similar level of assistance\nbetween the two legs, except for one participant. Results would be more comprehensive to include this\nquantification.\nThank you for your suggestion. As recommended by the reviewer, we have added more detail in the\nmethods and discussion sections to emphasize the different assistance tuning between participants.\nHowever, we chose not to move Extended Data Fig. 1 to the main manuscript because we felt it might\ndistract from our primary outcome related to the hypothesis.\nResults: Exoskeleton assistance\n“Many participants preferred similar exoskeleton tuning on each side, resulting in similar\ngenerated torque profiles on each side. Notably, participant 6 received greater peak flexion torque\non their affected side compared to their unaffected side. Generally, the peak flexion torque provided\nby the exoskeleton was greater than the peak extension torque, with greater extension torque\nsupplied to the unaffected side (Extended Data Fig. 1). Each peak of exoskeleton torque resulted in\na positive exoskeleton power peak, with flexion torque resulting in a greater peak power than\nextension torque. The peak power generated on the unaffected side was greater than that generated\non the affected side for both flexion and extension assistance. Specifically, the peak power generated\non the unaffected side (0.17 + 0.02 W kg-1 flexion, 0.09 + 0.01 W kg-1 extension) was greater than\nthat generated on the affected side (0.13 + 0.02 W kg-1 flexion, 0.08 + 0.01 W kg-1extension) for\nboth flexion and extension assistance.”\nThere seems to be a mix of statistical model-driven results and qualitative results. If this is the case, the\nauthors should clarify which comparisons were statistically compared and which were not by distinguishing\ntheir language between these sets of comparisons. If all comparisons were statistically driven, then these\ncomparisons should also include relevant statistical output (method, p value).\nThank you; this is an important point to clarify. We only conducted statistical tests on our three primary\noutcomes: the metabolic cost of walking, the total positive biological work performed by the affected\nand unaffected hip, knee, and ankle joints, and the positive biological hip work by the affected and\nunaffected hip joints. Due to the smaller sample size (N=7) and the associated statistical power, we\ndecided to analyze only these primary outcomes and to correct for multiple comparisons. In the revised\npaper, we included the relevant statistical output after reporting the main results. We also reworded\nseveral sections to clearly indicate which outcomes were compared statistically.\nIntroduction\n“We hypothesize that the exoskeleton assistance will decrease the positive biological work done by\nboth hip joints, thereby reducing the metabolic cost of walking. Thus, our primary outcome\nmeasures will be the metabolic rate, the total positive biological work calculated as the sum of the\naffected and unaffected hip, knee, and ankle joints, and the positive biological hip work, calculated\nas the sum of the affected and unaffected hip joints.”\nFig. 2 caption\n“... c) represent the standard error. Brackets indicate that statistical tests were performed to\ncompare the two conditions, and asterisks indicate significant differences.”\nResults\n“Data were recorded during the last 2 minutes of each treadmill walking trial. Pairwise t-tests\nwere performed to assess significant differences in our three primary outcome measures:\nmetabolic cost of walking, total positive biological work, and positive biological hip work, with\nand without the exoskeleton. See Materials and Methods for more details.”\nResults: Metabolic cost\n“Thus, assistance from the powered exoskeleton reduced the metabolic cost of walking by 18 +\n2.0% on average across the seven participants (two-tailed paired t-test, P = 0.0002).”\nResults: Positive joint work\n“This exoskeleton assistance significantly reduced the total positive biological work by 14 + 4% on\naverage across the seven participants (two-tailed paired t-test, P = 0.0167). This exoskeleton\nassistance significantly reduced the positive biological hip work by 29 + 6% on average across the\nseven participants (two-tailed paired t-test, P = 0.0052).”\nMethods: Data processing and statistical analysis\n“Two-tailed paired t-tests evaluated effects of exoskeleton assistance on our primary outcome\nmeasures: metabolic power, total positive biological work (affected and unaffected hip, knee, and\nankle joints, summed), and positive biological hip work (affected and unaffected hip joints,\nsummed). We set α = 0.05 for these analyses and applied a Holm-Bonferroni correction to adjust\nfor multiple comparisons. We did not statistically compare any secondary outcome measures (hip\nwork during stance, foot clearance, kinematics). We used a two-tailed t-test because we did not have\na prior assumption that the difference between conditions (walking with or without the exoskeleton)\nwould be positive or negative. Finally, the data were normally distributed and there were no\noutliers, meeting the requirements for a two-tailed paired t-test.”\nOn a similar note as the previous comment, it is unclear whether the authors compared any hip work during\nstance, foot clearance, and kinematic variables with and without exo assistance using statistical methods.\nAs noted above, we only ran statistics on our primary outcome measures (metabolic power, total positive\nbiological work (affected and unaffected hip, knee, and ankle joints, summed), and positive biological\nhip work (affected and unaffected hip joints, summed), so we did not statistically compare hip work\nduring stance. This secondary biomechanical analysis was to provide more context and\nunderstanding of when the hip exoskeleton assistance was most effective, which was different on each\nside. As a secondary outcome, we did not run a statistical analysis. We hope this is clarified in the\nrevised manuscript sections (as above).\nReporting percentage of change alongside actual numbers is helpful in sensing whether the change in\noutcome due to assistance is a big or a small change. The authors should include this information throughout\n- currently not included in hip work during stance, foot clearance, and kinematics.\nWe agree that adding the percent change can be helpful. Thank you for the suggestion. We have\nensured that percentage changes are included in all major outcomes. The revised results are below:\nResults: Hip work during stance\n“When the unaffected side is in initial double support, the affected side is in terminal double support\n(Fig. 3). During this critical step-to-step transition, the exoskeleton provides extension assistance to\nthe unaffected side and flexion assistance to the affected side, decreasing the positive biological hip\nwork by 0.027 + 0.009 J kg-1 (41% + 15%) and 0.022 + 0.013 J kg-1 (28% + 19%), respectively, per\nstride. The positive biological hip work also decreased by 0.033 + 0.013 J kg-1(63% + 19%) and 0.014\n+ 0.014 J kg-1 (129% + 27%) on the unaffected and affected side, respectively, during the next phase\nof gait, when the unaffected side was in single support and the affe cted side was in swing. During the\nfollowing step-to-step transition (unaffected side trailing), the positive biological hip work decreased\nslightly by 0.005 + 0.009 J kg-1 (83% + 25%) on the unaffected side but increased by 0.014 + 0.009 J\nkg-1 (118% + 22%) on the affected side.\nIn the final phase, when the unaffected side is in swing and the affected side is in single support, the\npositive biological hip work decreased by 0.017 + 0.004 J kg-1 (71% + 17%) on the unaffected side\nand increased slightly by 0.004 + 0.003 J kg-1 (168% + 22%) on the affected side.”\nResults: Foot clearance\n“Minimum foot clearance trajectories were calculated by taking the minimum trajectory of the toe and\nheel during swing (Extended Data Fig. 4). Hip exoskeleton assistance increased foot clearance on both\nthe affected and unaffected sides. On the affected side, peak foot clearance increased by 4 + 2 mm\n(12% + 7%) with the exoskeleton. On the unaffected side, peak foot clearance increased by 2 + 1 mm\n(6% + 3%) with the exoskeleton.”\nResults: Kinematics\n“Hip exoskeleton assistance slightly increased the affected hip joint’s average range of motion,\nprimarily by increasing peak hip flexion by 2° + 1° (8% + 3%). The unaffected hip joint’s average\nrange of motion decreased by 2° + 1° (8% + 2%), decreasing in both peak flexion and extension.\nExoskeleton extension assistance slightly increased the average peak knee extension angle by 2° + 1°\n(31% + 18%) on both sides. Finally, with the exoskeleton, the affected ankle joint was slightly more\nplantarflexed throughout the gait cycle, while the unaffected ankle joint was slightly more dorsiflexed\nthroughout the gait cycle. These differences were generally less than 2° and did not affe ct the ankle\njoint range of motion. Thus, hip exoskeleton assistance resulted in only minor, non-clinically\nsignificant kinematic changes (Extended Data Fig. 5).”\nIn “exoskeleton assistance,” please provide quantifiable results that lead to authors’ claim that “peak power\ngenerated on the unaffected side was greater than that generated on the affected side” for both hip motion.\nThank you for pointing this out. We have revised the results to include the peak power generated on\neach side for each direction of motion:\nResults: Exoskeleton assistance\n“Many participants preferred similar exoskeleton tuning on each side, resulting in similar\ngenerated torque profiles on each side. Notably, participant 6 received greater peak flexion torque\non their affected side compared to their unaffected side. Generally, the peak flexion torque provided\nby the exoskeleton was greater than the peak extension torque, with greater extension torque\nsupplied to the unaffected side (Extended Data Fig. 1). Each peak of exoskeleton torque resulted in\na positive exoskeleton power peak, with flexion torque resulting in a greater peak power than\nextension torque. The peak power generated on the unaffected side was greater than that generated\non the affected side for both flexion and extension assistance. Specifically, the peak power generated\non the unaffected side (0.17 + 0.02 W kg-1 flexion, 0.09 + 0.01 W kg-1 extension) was greater than\nthat generated on the affected side (0.13 + 0.02 W kg-1 flexion, 0.08 + 0.01 W kg-1extension) for\nboth flexion and extension assistance.”\nDiscussion\nIn the discussion of symmetry restoration through exo, consider linking this back to metabolic cost reduction\n(i.e. relationship between asymmetry and increased metabolic cost)\nThank you for your feedback. As suggested by the reviewer, we revised the results to include analysis\non two gait symmetry outcomes, one spatial (step length) and one temporal (stance time), and included\na figure in the extended data (Extended Data Fig. 6). We also revised the discussion to connect these\nsymmetry results back to metabolic cost.\nResults: Kinematics\n“Symmetry was evaluated by calculating a symmetry index (SI) where 0% indicates perfect\nsymmetry34. Exoskeleton assistance slightly improved symmetry in stance time (13% + 3%\nasymmetry without the exoskeleton vs. 12% + 4% asymmetry with the exoskeleton), but increased\nasymmetry in step length (-8.2% + 6% without the exoskeleton vs. -14% + 5% with the exoskeleton)\n(Extended Data Fig. 6). Generally during walking with and without the exoskeleton, participants spent\nlonger in stance on their unaffected side and took longer steps onto their affected side.”\nDiscussion\n“Hip exoskeleton assistance had a small effect on the hip joint range of motion for both sides (Extended\nData Fig. 5). Specifically, the exoskeleton assistance increased the affected hip joint’s range of motion\nby 2° and decreased the unaffected hip joint’s range of motion by 2°, slightly improving symmetry in\nthe hip joint range of motion. Hip exoskeleton assistance slightly improved symmetry in stance time but\ndecreased symmetry in step length (Extended Data Fig. 6). The observed increase in step length\nasymmetry indicates that the participants take longer steps onto their affected side with the exoskeleton\nassistance. This result is likely due to the assisted hip flexion during swing. Overall, we did not observe\nany consistent trends in spatiotemporal variables across participants while walking with the exoskeleton\ncompared to walking without the exoskeleton. Although passive80 and powered24 ankle-foot\northoses/exosuit have shown consistent improvements in symmetry for stance time and step length,\nprevious powered hip exoskeleton studies have not\n56–\n. Our results seem to confirm that portable powered hip exoskeletons do not consistently increase\n58\nsymmetry. This result may be due to the assistive controller being primarily tuned to provide positive\npower as needed to reduce metabolic rate.”\nMethods: Data processing and statistical analyses\n“We evaluated interlimb asymmetry during both walking trials using a symmetry index (SI). The SI is\ncalculated by taking the difference between the spatiotemporal data of the affected and unaffected sides,\ndivided by one-half the sum of the spatiotemporal data from both sides, multiplied by 100%34. A positive\nSI indicates asymmetry towards the subject’s unaffected side, while a negative SI indicates asymmetry\ntowards the subject’s affected side. Zero indicates perfect symmetry. “\nExtended Data Fig. 7 | Spatiotemporal symmetry. Symmetry in stance time (left) and step length (right) without the exoskeleton (gray) and\nwith the exoskeleton (red). The bars represent the mean, and the error bars represent the standard error. The blue dots represent values for the\nindividual participants. 0 is perfect symmetry. The symmetry index is calculated by taking the difference between the affected and unaffected\nsides, divided by one-half the sum of the affected and unaffected sides, multiplied by 100%34. A positive symmetry index in stance time\nindicates the participant spends longer in stance on their unaffected side. A positive symmetry index in step length indicates the participant\ntakes a longer step onto their unaffected side.\nindividuals with hemiparesis using a wearable unilateral hip-assisting robot in the discussion of prior work.\nTherefore, statement “this result has never been replicated in individuals with hemiparesis” is untrue.\nHowever, the solution is unique since this device provides bilateral assistance. Authors should consider\nreframing this statement to highlight the advantages and rationale for choosing a bilateral solution with\nactive assistance bidirectionally compared to other approaches.\nThank you for bringing this to our attention. As suggested by the reviewer, we revised our manuscript to\ndiscuss this case series, including considerations related to the unilateral and unidirectional assistive\napproach, as well as the lack of statistical analysis. Also, as suggested by the reviewer, we removed the\nstatement “this result has never been replicated in individuals with hemiparesis” in the discussion.\nIntroduction\n“Although many studies have demonstrated the metabolic benefits of powered hip exoskeleton assistance\nin healthy individuals37,38,44–54, only a handful of studies have explored hip exoskeletons to assist\nindividuals with hemiparesis55–58. Unilateral hip exoskeleton assistance to the hemiparetic side has\nshown improved walking speed55,56. Moreover, bilateral assistance to both hips has shown greater\nimprovements in walking speed than unilateral assistance to the affected hip joint only56,57. A case series\nwith three individuals with hemiparesis subjects suggested that unilateral hip flexion assistance could\nimprove the metabolic cost of walking59. However, a subsequent study with nine hemiparetic subjects\nshowed no significant improvement in the metabolic cost of walking with bilateral hip assistance58.\nUnfortunately, these studies did not assess the impact of hip assistance on hemiparetic gait\nbiomechanics. As a result, it remains unknown whether concurrent hip flexion and extension assistance\nprovided by a powered hip exoskeleton can effectively compensate for impaired ankle propulsion and\nreduce metabolic cost.”\nDiscussion\n“The reduction in the metabolic cost of walking obtained in this study using a portable hip exoskeleton\n(18%) is substantially greater than the metabolic reduction previously achieved using an ankle exosuit\ntethered to a benchtop actuator (10%)25. A case series with three individuals with\nhemiparesis suggested potential for unilateral hip flexion assistance to reduce the metabolic cost of\nwalking59. Still, they found a much lower reduction compared to this study (8.6%), and no follow-\nup studies confirmed its statistical significance. Thus, our analysis suggests that bilateral,\nbidirectional hip assistance may be more effective than unilateral, unidirectional hip assistance\nand ankle assistance in improving the metabolic cost of walking with a portable exoskeleton or\nexosuit.”\nDiscussion paragraph acknowledging the smaller sample size and lack of difference between different\ngroupings could be better polished by synthesizing these results more. While further studies with increased\nsample are certainly warranted, consider the possibility that these results suggest this exo could be a viable\nsolution for people with wide range of impairment, size or sex, and use of other assistive devices.\nThank you for emphasizing a key value of this study. We revised the discussion section to include a\nparagraph on the limitations, specifically emphasizing that the exoskeleton can assist a diverse range of\nindividuals with hemiparesis.\nAlthough this study includes only a small sample size (N=7), the sample was diverse. The two female\nparticipants (S1 and S2) had an average metabolic cost reduction similar to that of the male\nparticipants (18% women vs 18% men). Two participants (S2, S6) used ankle-foot orthoses (AFO). The\naverage metabolic cost reduction for these two participants was slightly less than for the others (17%\nvs 19%). This result suggests that individuals with ankle-foot orthoses may experience slightly less\nmetabolic benefit from hip exoskeleton assistance, and it could be due to the AFOs restricting ankle\npush-off and limiting gait adaptability77,78. We did not observe any other trends in the age, chronicity,\nweight, or other biological factors of the participants presented in Table 1. Our results indicate that\nhip exoskeletons can assist a wide range of individuals with hemiparesis based\non level of impairment, size, sex, and use of other assistive devices. Further studies with a larger\nsample size are needed to assess important clinical and biological factors that may impact the\nefficacy of hip exoskeleton assistance.\nBroadly speaking, the organization of the discussion could be improved, similar to the reviewer comment\nwith the introduction. Some suggestions include: 1) Inclusion of a limitation paragraph towards the end of\nthe discussion so they appear together (small sample size, weight, tuning method), 2) more transitions\nbetween paragraphs, and 3) grouping ideas together.\nThank you for this suggestion. Following your feedback, we revised the discussion to group ideas in the\nfollowing order: main paper contribution/significance, metabolic rate/cost of transport, total/joint positive\nwork, assistance power/torque, range of motion, and foot clearance. We also included a limitation section\nat the end of the discussion and improve transitions between paragraphs.\nMethods\nPlease provide more detail on the training sessions individuals received – what was the average number of\nsessions and average duration over which time period? For individuals who received more than one training,\ndid the authors tune the parameters fresh? Or start at the converged parameter from the previous training? How\ndid the authors accommodate if user preference and comfort has changed between training?\nThis is an important point to clarify. Thank you for bringing it to our attention. We include additional\ninformation on the training and tuning process in the revised methods section:\nMethods: Exoskeleton training and assistance tuning\n“The amount of exoskeleton training was not standardized across participants in this study to\naccommodate differences in walking ability between participants. Some participants exhibited less\nconsistent walking patterns, which made exoskeleton tuning less consistent and necessitated more\ntuning and training. Furthermore, some participants needed more rest and walked less each\ntraining day, so they had more training days than participants who could walk longer in one session.\nOn average, participants completed 2+] additional training sessions (not including the fit/tune\nsession) before data collection. Participants generally completed a session once a week, completing\ntraining over 4+] weeks on average. The experimenter started with the converged parameters from\nthe prior training session and tuned as necessary from there. Parameters were retuned in each\ntraining session based on any changes in user preference or comfort, and any gait changes observed\nby the experimenter. The exoskeleton parameters were kept the same as the final tuned parameters\nfrom the final training session during the metabolic cost evaluation and motion capture\nexperiments.”\nPlease provide more detail on how the authors used “feedback from participants and the experimenter’s\njudgment” to tune the assistance parameters.\nThank you, this is another important point for clarification. We revised the methods section as follows\nto include more detail on the tuning process:\nMethods: Exoskeleton training and assistance tuning:\n“This tuning procedure was done independently for the affected and unaffected sides. Exoskeleton\nassistance parameters were tuned based on the experimenter’s training and experience via visual\ninspection of gait during the walking sessions89. The experimenters also watched a live estimate of\nexoskeleton power based on the exoskeleton torque and velocity, and adjusted timing and peak torque\nparameters to maximize positive power generation, which is critical for reducing the metabolic cost\nof healthy subjects45,79. After each adjustment, the participant was asked if the assistance felt better\nor worse. If it felt better, the experimenter would continue adjusting that parameter in that direction\nuntil it felt worse. If the change felt worse, the parameter was reverted to its previous value, and a\ndifferent change was made. Comfort was assessed by asking the participants for their subjective\nfeedback and visually inspecting the exoskeleton interfaces for excessive bending or movements.\nParticipants walked for approximately half an hour, less than 10 minutes at a time, with rest between\nwalking trials.”\nIn the last paragraph of methods (discussion of stats), please include additional information regarding how\nthe authors determined the assumptions of a two-tailed paired t-test were evaluated and met for the data.\nWe added this information to the revised methods section.\nMethods: Data processing and statistical analysis\n“Two-tailed paired t-tests evaluated effects of exoskeleton assistance on our primary outcome\nmeasures: metabolic power, total positive biological work (affected and unaffected hip, knee, and\nankle joints, summed), and positive biological hip work (affected and unaffected hip joints,\nsummed). We set α = 0.05 for these analyses and applied a Holm-Bonferroni correction to adjust\nfor multiple comparisons. We did not statistically compare any secondary outcome measures (hip\nwork during stance, foot clearance, kinematics). We used a two-tailed t-test because we did not have\na prior assumption that the difference between conditions (walking with or without the exoskeleton)\nwould be positive or negative. Finally, the data were normally distributed and there were no\noutliers, meeting the requirements for a two-tailed paired t-test.”\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThis study demonstrates that applying a hip exoskeleton to stroke patients effectively reduces the metabolic\ncost of walking. While this metabolic improvement is a valuable outcome, it is of paramount importance\nfor clinical populations to ensure that the assistive torque does not introduce additional burdens or potential\nharms. The current evidence—specifically, the absence of EMG data, the observed increase in ground\nreaction forces (Extended Data Fig. 1), and the lack of significant improvement in gait symmetry (Extended\nData Fig. 2)—makes it difficult to conclusively rule out such adverse effects. Therefore, I would\nrecommend a more comprehensive analysis of the existing data to evaluate changes in other critical patient-\noriented metrics, such as gait stability, joint trajectory symmetry, and other clinically relevant indicators,\nfollowing the application of exoskeleton assistance.\nThank you for your recommendation. Following your suggestion, we performed additional analysis to\nevaluate critical patient-oriented metrics and clinically relevant indicators. Specifically, we evaluated\nstability by calculating the extrapolated center of mass and stride width with and without the\nexoskeleton. This analysis shows no changes in stability. In addition, we evaluated joint trajectory\nsymmetry by calculating the symmetry index for the joint range of motion and by performing 1-\ndimensional spatial parametric mapping with and without the exoskeleton (Extended Data Figs 6-7).\nAlthough there were no statistically significant differences in joint kinematics, we observed an\nimprovement in hip joint range of motion symmetry. Based on this additional analysis, we can\nconclusively rule out adverse effects. We present the results of this additional analysis in the results\nsection of the revised manuscript and extended data.\nResults: Kinematics\nKinematic trajectories during walking with and without the exoskeleton are presented in\nExtended Data Fig. 5. Hip exoskeleton assistance slightly increased the affected hip joints’\naverage range of motion (Extended Data Fig. 6), primarily by increasing peak hip flexion by\n2° + 1° (7% + 3%). The unaffected hip joints’ average range of motion decreased by 2° + 1°\n(5% + 3%), decreasing in both peak flexion and extension. Exoskeleton extension assistance\nslightly increased the average peak knee extension angle by 2° + 1° (33% + 18%) on both\nsides. Finally, with the exoskeleton, the affected ankle joint was slightly more plantarflexed\nthroughout the gait cycle, while the unaffected ankle joint was slightly more dorsiflexed\nthroughout the gait cycle. These differences were generally less than 2° and resulted in a\nslightly decreased range of motion at the affected ankle joint (Extended Data Fig. 6). Spatial\nParametric Map (SPM) analysis demonstrated that there were no statistically significant\ndifferences (two-tailed paired t-test, α= 0.05) in the kinematic trajectories of each joint during\nwalking with the exoskeleton compared to without the exoskeleton (Extended Data Fig. 7).\nThus, hip exoskeleton assistance resulted in only minor, non-clinically significant and non-\nstatistically significant kinematic changes.\nSymmetry was evaluated by calculating a symmetry index (SI) where 0% indicates perfect\nsymmetry34. Hip exoskeleton assistance improved symmetry in the hip joint range of motion\n(-30% + 10% without the exoskeleton vs. -18% + 9% with the exoskeleton) but had only\nminor effects on the knee and ankle range of motion symmetry (Extended data Fig. 8) ....\nTo evaluate gait stability, we evaluated the extrapolated center of mass and step width\nduring walking with and without the exoskeleton (Extended Data Fig. 10). The minimum\nmediolateral distance from the extrapolated center of mass to the average center of pressure\nof the affected and unaffected sides was relatively unchanged, decreasing by 0.2 + 0.3 cm\n(5% + 8%) on the affected side and by 0.06 + 0.34 cm (4% + 11%) on the unaffected side\nwith the exoskeleton. Furthermore, the step width was 20 + 2 cm during walking with and\nwithout the exoskeleton.\nDiscussion\nHip exoskeleton assistance had a small effect on the hip joint range of motion for both sides\n(Extended Data Fig. 5-6). Specifically, the exoskeleton assistance increased the affected hip\njoints’ range of motion by 2° and decreased the unaffected hip joints’ range of motion by 2°,\nimproving symmetry in hip joint range of motion by 12% (Extended Data Fig. 7). The\nobserved kinematic changes are small and below the thresholds for clinical and statistical\nsignificance (Extended Data Fig. 8). Furthermore, we did not observe any meaningful\nchanges in the extrapolated center of mass and step width, indicating that hip exoskeleton\nassistance did not adversely affect stability (Extended Data Fig. 10).\nMethods\nTo evaluate changes in kinematic trajectories during walking with the exoskeleton compared to\nwalking without the exoskeleton, we performed a 1-dimensional Statistical Parametric Map\n(SPM) analysis, using the spm1d 0.4.10 package in Matlab 2024b103. SPM was run on the\nkinematic trajectories of each joint (affected and unaffected hip, knee, and ankle) during walking\nwith the exoskeleton compared to walking without the exoskeleton to identify statistically\nsignificant changes (two-tailed paired t-tests with α = 0.05).\nNo exo With exo\nS1 S2 S 3 S4 S5 S6 S7\nAffected side Unaffected side\ng ]edn\n[ootimof\ne gnapr\nHig]edn[ootimof\ne gnaer\neK\ng ]edn[oomtiof\ne g naerk\nnlA\nExtended Data Fig. 6 | Joint range of motion. The range of motion of the\naffected (left) and unaffected (right) side hip (top), knee (middle), and ankle\n(bottom) joints during walking without the exoskeleton (gray) and with the\nexoskeleton (red). The bars represent the mean, and the error bars represent the\nstandard error. The blue dots represent values for the individual participants.\nNo exo With exo\nS1 S2 S 3 S4 S5 S6 S7\nHip Knee Ankle\n]%[\nyrtemmys\nnotoim\nfo egnaR\nExtended Data Fig. 7| Joint range of motion symmetry index. The symmetry index for the range of motion of the\nhip (left), knee (middle), and ankle (right) joints during walking without the exoskeleton (gray) and with the exoskeleton\n(red). The bars represent the mean, and the error bars represent the standard error. The blue dots represent values for\nthe individual participants. The symmetry index is calculated by taking the difference between the affected and\nunaffected sides, divided by one-half the sum of the affected and unaffected sides, multiplied by 100%36. At perfect\nsymmetry, the index would equal 0%. A negative symmetry index indicates the participant has a larger range of motion\non their unaffected side.\nTrajectory difference Significance threshold\nAffected side Unaffected side\n5\n]ged[\nelgna\npiH]ged[\nelgna\neenK\n]ged[\nelgna\nelknA\n0\n-5\n5\n0\n-5\n5\n0\n-5\nGait cycle [%] Gait cycle [%]\nExtended Data Fig. 7 Spatial parametric map analysis. The difference in the\nkinematic trajectories of the affected (left) and unaffected (right) hip (top),\nknee (middle), and ankle (bottom) joints during walking with the exoskeleton compared\nto walking without the exoskeleton (red, solid lines). The threshold for a statistically\nsignificant difference at each joint is indicated by the dashed lines (gray). No\nkinematic differences exceed the respective threshold for statistical significance\n(two-tailed paired t-test, α= 0.05).\nNo exo Wth exo\nS1 S2 S 3 S4 S5 S6 S7\nAffected side Unaffected side\nExtended Data Fig 10 | Stability analysis. The minimum mediolateral distance from the extrapolated center\nof mass to the center of pressure for the affected and unaffected sides (left) and the step width (right) during\nwalking without the exoskeleton (gray) and with the exoskeleton (red). The bars represent the mean, and the\nerror bars represent the standard error. The blue dots represent values for the individual participants.\n]mc[\nhtdiw\npetS\necnatsid\nlaretaloidem\nmuminiM\n]mc[\nPOC\not\nMOC\ndetalopartxe\nAdditionally, a specific clarification is needed regarding Extended Data Fig. 1 (Vertical GRF). After the\ntoe-off event, the ground reaction force remains non-zero for a period of time. Could the authors explain\nthe reason for this?\nThank you for pointing this out. To address this discrepancy, we conducted additional analysis by\nexamining the results of each participant separately (see Extended Data Fig. 12 below). This analysis\nshows that the visual discrepancy is an artifact resulting from averaging the toe off timing across\nparticipants. As you can see from the figure below, the participants had different toe off timing with\ngreater variability on the unaffected side compared to the affected side. As a result, the average GRF\nvalue at the average toe off timing is greater than that observed in individual participants. This analysis\nexplains why the ground reaction force remains non-zero for a period of time after toe off.\nResults: Kinetics\nThe peak vertical GRF increased by 0.04 + 0.006 N kg-1 (4.4% + 0.6%) on both the affected\nand unaffected sides (Extended Data Fig. 12).\nAverage Toe off\nS1 S2 S3 S4 S5 S6 S7 Toe off\n1.2\n1\n0.8\n0.6\n0.4\nNo exo With exo\n0 5 0 1 0 0 0 5 0 1 0 0\nGait cycle [%] Gait cycle [%]\nExtended Data Fig. 12 | Vertical ground reaction force (GRF) trajectories. The vertical GRF trajectories\nof each participant's unaffected (top) and affected (bottom) sides during walking without the exoskeleton\n(left) and with the exoskeleton (right). Each participant's average toe off location is marked with a square in\nthe corresponding color. The average trajectory across subjects is shown in the dashed black line, with the\naverage toe off location across participants represented as a solid circle.\nedis\ndetceffanU\n-gk\nN[\nFRG\nlacitreV\n0.2\n0\n1.2\n1\n0.8\n0.6\n0.4\n0.2\n0\nedis\ndetceffA\n]1-gk\nN[\nFRG\nlacitreV\nReviewer #2 (Remarks to the Author):\nThe authors have done an excellent job of addressing my comments/feedback from the original review. I\nhave no further feedback. This is an excellent paper and I look forward to citing it.\nThank you for your previous feedback, which has improved the quality of our paper. We are pleased\nto hear that you are satisfied with our revisions.\nReviewer #3\nThe revision is improved with complete analyses are more complete and clear writing. The authors have\naddressed prior reviews thoroughly. A few minor comments should still be addressed before the work can\nbe accepted for publication.\nThank you for your suggestions. You can find a point-by-point answer below.\nIntroduction\n Consolidate the hypothesis. The last paragraph states “we hypothesize that ...” twice. Please merge into a\nsingle, stand-alone hypothesis that cleanly motivates the study.\n Relocate the outcomes sentence. The paragraph also ends with outcome definitions. The sentence, “Thus,\nour primary outcome measures will be ...”, reads like Methods/Results rather than a concluding line of the\nIntroduction. Reword it to close the rationale succinctly or move it to the Results.\nAs recommended by the reviewer, we consolidated the two hypotheses into one and removed the\nfinal sentence in the paragraph. The revised paragraph is copied below:\n“Based on these findings, we hypothesize that a hip exoskeleton could offset the impaired\nankle propulsion in individuals with hemiparesis by simultaneously assisting the affected and\nunaffected hip joints, leading to reduced metabolic cost and positive biological work on both\nhip joints. To test this hypothesis, we assessed the metabolic cost of treadmill walking at a\nconstant self-selected speed, with and without a lightweight, portable, bilateral hip\nexoskeleton designed to supplement the users’ hip flexion and extension, while measuring the\npositive biological work performed by both hip joints.”\nResults\n Make the gait-phase reporting fully parallel. In the “Hip work during stance” subsection, use a symmetric\nstructure across all four phases and begin each with “During ...” for immediate readability. In addition, present\nall four phases in one paragraph rather than splitting three in one paragraph and the last in a separate paragraph\n– unless the authors intentionally isolate the final phase, in which case add a brief interpretation.\nThank you for this suggestion. We merged the paragraphs and used parallel structure as\nrecommended.\n“When the unaffected side is in initial double support, the affected side is in terminal double\nsupport (Fig. 3). During this critical step-to-step transition, the exoskeleton provides extension\nassistance to the unaffected side and flexion assistance to the affected side, decreasing the\npositive biological hip work by 0.026 + 0.008 J kg-1 (60% + 18%) and 0.022 + 0.013 J kg-1\n(49% + 26%), respectively, per stride. During the next phase of gait, when the unaffected side\nwas in single support and the affected side was in swing, the positive biological hip work also\ndecreased by 0.035 + 0.013 J kg-1 (64% + 19%) and 0.014 + 0.014 J kg-1 (55% + 27%) on\nthe unaffected and affected side, respectively. During the following step-to-step transition\n(unaffected side trailing), the positive biological hip\nwork decreased slightly by 0.005 + 0.009 J kg-1 (83% + 25%) on the unaffected side but\nincreased by 0.014 + 0.009 J kg-1 (118% + 22%) on the affected side. In the final phase, when\nthe unaffected side is in swing and the affected side is in single support, the positive biological\nhip work decreased by 0.017 + 0.004 J kg-1 (71% + 17%) on the unaffected side and increased\nslightly by 0.004 + 0.003 J kg-1 (168% + 22%) on the affected side.”\nDiscussion\n Streamline the opening. The first four sentences largely repeat the Introduction. Condense or remove them\nso the first paragraph directly summarizes the main findings and their magnitude.\nWe condensed the opening paragraph as recommended.\n“Individuals with hemiparesis have a significantly higher metabolic cost of walking than healthy\nindividuals2,22,24, limiting physical activity8, community participation, and reducing overall health\nand quality of life1,3,9. Portable powered exoskeletons have successfully improved the metabolic\ncost of walking in young, healthy individuals39,40,46–51,53–56, but have not yet done so in individuals\nwith hemiparesis25,60. Here we show that assistance from a lightweight, portable, bilateral hip\nexoskeleton decreased the metabolic cost of walking in individuals with hemiparesis (n=7) by\n18%, equivalent to removing a 13.5-kg backpack from a healthy individual32 (Fig. 1c). The\nobserved metabolic cost reduction is substantial and has the potential to improve mobility and\nquality of life after stroke.”\n Move speculative long-term implications. The paragraph on potential long-term benefits is speculative; it\nbelongs later, after core findings and comparisons to prior work.\nWe moved the paragraph to the end of the discussion as recommended.\n Ensure no new data is presented in discussion. Reference Extended Data Table 1 in the Results, not only\nin the Discussion\nThanks for catching that. We moved that reference to the results section, where it should have\nbeen. We also double-checked that no new data is presented in the discussion.\n“Thus, if we account for the metabolic penalty of the exoskeleton mass, the average\nmodified exoskeleton performance index65,66 was 0.515, which is about half the theoretical\nmaximum of 1 (Extended Data Table 1).”\n Clarify the comparison of performance index. The last two sentences currently hedge between two\nexplanations for the lower performance index (device efficiency vs. pathology-related demands). Please\ntake a position and support it: (1) If the authors attribute it to exoskeleton design, indicate which design\nfeatures plausibly reduce energy transfer (e.g., mass distribution, timing, peak torque limits, interface\ncompliance, etc.) and why. (2) If the authors attribute it to hemiparetic gait, cite literature indicating higher\npositive mechanical work requirements or altered muscle efficiency in hemiparesis and explain how that\nmaps to your findings.\nThanks for this comment. Upon further reflection, we decided that the most likely cause for the\ndifference in performance index is not the exoskeleton but the hemiparetic gait cycle. We revised\nthe paragraph accordingly. We clarify this gap in the subsequent paragraph.\n“The metabolic penalty of wearing our portable hip exoskeleton was estimated at 11.76 W,\nsimilar to the average metabolic penalty of other advanced hip exoskeletons (6.93 – 17.7 W,\nExtended Data Table 1). When accounting for this metabolic penalty, the performance index\nof our exoskeleton was 0.515, well below the theoretical maximum of 1, which would\nindicate perfect energy transfer between the exoskeleton and the user. The apparent\nefficiency of our device is notably lower than that of portable power hip exoskeletons\nmeasured in previous studies with healthy subjects (0.515 versus 0.74-0.92, Extended Data\nTable 1). This difference may indicate that greater positive mechanical energy is necessary\nto achieve the same metabolic benefits in individuals with hemiparesis compared to healthy\nsubjects.”\nFurthermore, the subsequent paragraph questions the performance index’s validity in pathological gait.\nReporting it and then questioning it reads as internally inconsistent. Since this metric is still informative,\nkeep it succinctly, acknowledge the key assumptions, and fold this limitation into the prior paragraph that\ndiscusses the performance index rather than a stand-alone paragraph.\nWe respectfully disagree that questioning the validity of the performance index is an internal\ninconsistency.\n Reposition the sample-size paragraph. The paragraph expanding on applicability and addressing sample\nsize is useful but not tied to specific results. Move it to the paragraph immediately preceding Limitations.\nWe moved the paragraph as recommended.\nMethods\n No comment\nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the\nNature Communications initiative to facilitate training in peer review and to provide appropriate recognition\nfor Early Career Researchers who co-review manuscripts.\nThank you for reviewing our work. Your comments helped us improve our manuscript."}, {"round": 3, "reviewer_comments": {"reviewer_1": "All my concerns have been addressed. I think this paper is ready for publication."}, "author_response": "Thank you for your previous feedback, which has improved the quality of our paper. We are pleased\nto hear that you are satisfied with our revisions."}]

Decision letter