## 2 Materials and Methods  
Eight custom PD-AFOs were designed and manufactured for a population of individuals (3F, 5M; age 62.0 \(\pm\) 13.1 years, BMI 25.8 \(\pm\) 2.5 kg/m\({}^{2}\)) affected by unilateral foot drop condition due to compression of the lumbosacral spine. Ethical approval was granted for the study by the local Ethical Committee (0016384, 23 December 2019) and informed consent was signed by all participants in the study. In all participants, ankle dorsiflexor muscle strength was scored \(\leq\)3 according to the Medical Research Council (MRC) scale [25], whereas the ankle plantarflexor muscles strength was scored \(\leq\)4. With the exception of ankle dorsiflexion impairment, all participants showed good residual mobility in the affected limb (i.e., they were capable of walking without crutches or other walking aids albeit with compensations at the lower limb joints). The procedure for the design and production of the AFOs has been previously documented by the same authors [26]. In summary, the leg and foot of each participant were scanned in upright double-leg standing using a Kinect-based 3D scanner (Microsoft, Redmond, WA, US). The custom PD-AFOs were designed using Blender (version 4.4, [[https://www.blender.org/](https://www.blender.org/)]([https://www.blender.org/](https://www.blender.org/))) and aligned to the 3D model of the leg and foot in weight bearing. Anatomical congruence was pursued at the foot plantar surface and at the calf shell to ensure comfort and proper fixation of the AFO to the leg. The calf-shell was aligned to the calf midline, as to be oriented along the anatomical axis of the patient's tibia. The AFOs were manufactured via selective laser sintering of a fiberglass-reinforced polyamide powder (Windform(r) GT, CRP Technology, Modena, Italy). This 3D printing technology allows us to obtain parts with a resolution of about 0.1 mm. Since the stiffness of the present AFO is highly sensitive to the cross-section dimensions of the calf-shell, selective laser sintering is critical to the correct optimization of the AFO mechanical properties. The concave shape of the calf-shell increases the moment of inertia of the cross-section, allowing it to bend almost as a rigid body around the X-axis as in Figure 1b, which is designed to be close to the physiological ankle-joint axis [26]. The bending stiffness of the AFO calf-shell in anatomical plantarflexion (\(k_{\text{p}}\)) and dorsiflexion (\(k_{\text{d}}\)) were measured by applying a 15 deg rotation in both directions using a custom apparatus (Figure 1a) [27]. An 8-camera motion analysis system (Vicon, Oxford UK) was used to track the position of 20 reflective markers placed on relevant body landmarks of the pelvis, femur, tibia, and foot/footwear according to the IOR-gait protocol [28]. Three additional markers were placed on the posterior side of the calf-shell of each custom AFO (Figure 1b). This allowed measuring the relative plantarflexion (\(\alpha_{\text{p}}\)) and dorsiflexion (\(\alpha_{\text{d}}\)) angles of the AFO calf-shell with respect to the foot. The ISB recommended convention was used to calculate hip, knee, and ankle joint rotations and the AFO calf-shell flexion/extension in gait [29]. Ground reaction forces were sampled at 2000 Hz using two force plates embedded in the floor (Kistler, Winterthur, Switzerland). Ankle power was calculated using force plate and kinematic data and normalized to the participant's body weight (W/kg). Five walking trials at self-selected speed were recorded for each participant in three conditions: wearing the custom AFO (Figure 1c), wearing an off-the-shelf polypropylene leaf spring AFO (Codivilla Spring, Ottobock, Duderstadt, Germany), and shot (no-AFO). According to the current clinical practice at the authors' institution, the Codivilla Spring is one of the most recommended off-the-shelf PD-AFOs for individuals with foot drop, when no other significant anatomical alterations are present.