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OPEN

Estimation of stride‑by‑stride
spatial gait parameters usinginertial measurement unit attachedto the shank with inverted
pendulum model
Yufeng Mao1, Taiki Ogata1*, Hiroki Ora1, Naoto Tanaka2 & Yoshihiro Miyake1
Inertial measurement unit (IMU)-based gait analysis systems have become popular in clinicalenvironments because of their low cost and quantitative measurement capability. When a shank isselected as the IMU mounting position, an inverted pendulum model (IPM) can accurately estimate
its spatial gait parameters. However, the stride-by-stride estimation of gait parameters using one
IMU on each shank and the IPMs has not been validated. This study validated a spatial gait parameter
estimation method using a shank-based IMU system. Spatial parameters were estimated via the
double integration of the linear acceleration transformed by the IMU orientation information. To
reduce the integral drift error, an IPM, applied with a linear error model, was introduced at the midstance to estimate the update velocity. the gait data of 16 healthy participants that walked normallyand slowly were used. The results were validated by comparison with those extracted from an opticalmotion-capture system; the results showed strong correlation (r > 0.9) and good agreement with the
gait metrics (stride length, stride velocity, and shank vertical displacement). In addition, the biasesof the stride length and stride velocity extracted using the motion capture system were smaller in the
IPM than those in the previous method using the zero-velocity-update. The error variabilities of the
gait metrics were smaller in the IPM than those in the previous method. These results indicated thatthe reconstructed shank trajectory achieved a greater accuracy and precision than that of previous
methods. This was attributed to the IPM, which demonstrates that shank-based IMU systems with
IPMs can accurately reflect many spatial gait parameters including stride velocity.
In recent years, many studies have focused on the development of an inertial measurement unit (IMU) equippedwith an accelerometer, gyroscope sensor, and magnetometer for a gait analysis system that can provide quantitative gait parameters such as stride length, velocity, gait cycle. Compared to the golden standard for gait analysis—amotion capture system and instrumented walkways—the IMU-based system is cost effective, lightweight, andversatile, which are suitable characteristics for clinical and residential a­ pplications1. In particular, the inverted
pendulum model (IPM) is considered useful for estimating spatial gait parameters such as stride length fromacceleration and angular velocities measured by the ­IMU2,3. However, the step-by-step accuracy of the spatial
parameters estimated using the IMU data and IPM model has not been validated thus far.
Kinematics information in the gait cycle is a component of gait considered in clinical gait a­ nalyses4. For
example, variability in stride length can be used to assess the progression of Parkinson’s disease (PD)5, andstride velocity can be used to predict the risk of adverse events in the ­elderly6. To estimate spatial gait parametersfrom acceleration and angular velocities measured by the IMU, the previous studies implemented a segmentation ­algorithm7–10. Integral computations for spatial parameter estimation are reset at each segmentation point,
which reduces errors caused by the measurement noise. The implementation details of these algorithms dependon the attachment position of the IMU because different positions produce different signal characteristics anddifferent assumptions need to be considered to improve estimation accuracy. A wide selection of attachment

1

Department of Computer Science, Tokyo Institute of Technology, Kanagawa 226‑8503, Japan. 2Department
of Systems and Control Engineering, Tokyo Institute of Technology, Kanagawa 226‑8503, Japan. *email:ogata@c.titech.ac.jp

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Figure 1.  Scatter plots of (a) stride length, (b) stride velocity, and (c) shank vertical displacement extracted
from the proposed method and motion capture system. Each dot indicates the value in one stride. The black line
shows the best fit line.
positions for the IMUs involved bilateral ­shanks7,11, bilateral ­insteps12–15, and bilateral ­heels16,17. In terms of
ensuring attachment stability, the shank position seems to be a good choice because the soft tissue of the lower
shank has less movement than that at the other ­positions18. Further, when the IMU is fixed to the heel or instep
position on footwear, the moving artifact of the footwear affects the estimation accuracy. However, comparedto the heel or instep attachment positions closer to the ground, the shank position has room for improvementin terms of estimating spatial parameters such as stride length and stride velocity. The shank-based method canbe used to obtain reliable gait parameters and visualise the 3D-trajectory of each s­ tride19,20. In a previous study,
zero-velocity-update (ZUPT) was introduced, which re-initialises the integrated velocity to zero at the segmentation point (mid-stance). However, the shank possesses a small velocity at the mid-stance2, which results in theerror being included in the estimated spatial gait parameters. Instead of assuming the velocity at the mid-stanceto be zero, the inverted pendulum model (IPM)2,3, which considers the IMU movement at the mid-stance asa circular motion in the sagittal plane and estimates the velocity using angular velocity has the potential toprovide a more accurate result. In fact, Wu et al. showed that the total error of the walking distance estimatedusing shank-mounted IMUs and an IPM was smaller than that estimated using the same IMUs and a ­ZUPT19.
However, it remains unclear if stride-by-stride estimation using shank-mounted IMUs and the IPM is superior
to that using ZUPT. As mentioned above, some variability in the spatial gait parameters is important for the gait
assessment of people with gait disorders, and to investigate such variability in gait parameters, stride-by-stride
estimation of gait parameters is necessary. This study aimed to validate the stride-by-stride estimation method
for spatial gait parameters using shank-mounted IMUs and the IPM. In addition, we investigated whether the
stride-by-stride estimation of the spatial gait parameters using the IPM was superior to that using the ZUPT.
A modified IPM that considered the posture of the IMU for shank trajectory estimation was implemented. For
evaluating the proposed method, we performed a concurrent validation experiment using an optical motion
capture system with a high spatial resolution. Next, we compared the proposed method to a ZUPT method toinvestigate whether the IPM improved the accuracy of the stride-by-stride estimation. We used a previously
reported ­method20 as the ZUPT method because this method had been evaluated for stride-by-stride spatial
gait parameters.

Results

All 16 participants completed both tasks; the data for the normal speed task of one of the participants could not
be analysed because of synchronisation issues (the motion capture system did not capture the stamp event), andtherefore, it was excluded. The motion capture system suffered from loss of data because of the reflective markers; these invalid data were excluded, and a total of 695 strides were extracted from the motion capture system,with 283 strides for the normal speed task and 412 strides for the slower speed task.
First, we validated the proposed method compared to the results from the motion capture system. Figure 1 shows scatter plots for the stride length, stride velocity, and shank vertical displacement between the data
extracted from the proposed method and those obtained from the motion capture system. The stride length andvelocity decreased when the participants walked slowly. Pearson’s correlation coefficient and the error distribution
are summarised in Table 1. All parameters achieve high correlation with Pearson correlation coefficient r > 0.90.
Thus, the proposed method estimated the parameters appropriately.
Second, we investigated whether the proposed method estimated the parameters more accurately than theprevious method. The Bland–Altman plots for the parameters estimated using the proposed method and a previous method are shown in Fig. 2. The biases of stride length, stride velocity, and shank vertical displacement forthe proposed and previous methods were 0.006 m and −0.059 m, 0.007 m/s and −0.047 m/s, and −0.010 m and−0.009 m, respectively. The stride length and stride velocity estimated using the proposed method achieved avery small bias compared with those estimated using the previous method. The 95% confidence intervals of stride
length, stride velocity, and shank vertical displacement for the proposed and previous methods were 0.099 m
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Parameters

r

E

|E|

|E|%

Stride length (m)Normal

0.98

0.012 (0.026)

0.024 (0.016)

1.7% (1.1%)

Slow

0.99

0.002 (0.025)

0.020 (0.015)

1.6% (1.2%)

Overall

1.00

0.007 (0.025)

0.020 (0.016)

1.9% (1.4%)

Shank vertical displacement (m)Normal

0.93− 0.011 (0.007)

0.011 (0.006)

6.9% (3.9%)

Slow

0.91

− 0.009 (0.007)

0.009 (0.006)

6.3% (4.0%)

Overall

0.92− 0.010 (0.007)

0.010 (0.006)

6.5% (4.0%)

Stride velocity (m/s)
Normal

0.98

0.014 (0.028)

0.026 (0.018)

2.0% (1.4%)

Slow

0.99

0.002 (0.021)

0.016 (0.013)

1.8% (1.4%)

Overall

1.00

0.007 (0.025)

0.020 (0.016)

1.9% (1.4%)

Table 1.  Pearson’s correlation coefficient (r), the mean (SD) of error (E), absolute error (|E|), and relative
absolute errors (|E|%) for the gait parameters extracted from the proposed method and motion capture system.

and 0.115 m, 0.095 m/s and 0.137 m/s, and 0.026 m and 0.030 m, respectively. Thus, the error variability for allparameters estimated using the proposed method was smaller than that using the previous method.

Discussion

This study aimed to validate stride-by-stride gait estimation using IMUs attached to the shank and an IPM. The
parameters were extracted from the walking data of 16 participants, and they were compared with those obtainedusing a motion capture system as reference. The results showed high correlation and good agreement between
the two systems for stride length, stride velocity, and shank vertical displacement, which verifies the technicaleffectiveness of the proposed method.
In terms of the Bland–Altman analysis, the proposed method achieved a −0.044 to 0.057 m limit of agreement
for the stride length, which is comparable to that achieved using the heel- or instep-based m­ ethod14,15,17,21. The
relative absolute error of the shank vertical displacement is slightly worse than other parameters with an overallvalue of 6.5% (Table 1). When comparing the spatial parameters in the forward (stride length) and vertical directions (shank vertical displacement), the magnitude of the error tended to be different. The value of movement in
the vertical direction was smaller than that in the forward direction, and therefore, the shank vertical displacement would be susceptible to noise. However, the shank vertical displacement still achieved good agreementwith a mean of -0.010 m and the limit of agreement of −0.023 m and 0.003 m.
Compared to the previous ­method20 as shown in Fig. 2, the proposed method achieved a considerably smaller
bias with a stride length of 0.005 m and a stride velocity of 0.007 m/s. This is because the modified IPM used inthis study compensates the velocity estimation error at each segmentation point. Further, this result demonstrated
that the IPM achieves good performance in terms of 3D-trajectory estimation. The previous IPM required an
event assumption that the shank tilt angle in the sagittal plane is zero; however, this was difficult to ­guarantee22.
The the proposed method combines orientation estimation that computes the update velocity in three dimensions
and does not require an event assumption. The estimated shank vertical displacement from both methods still
have a close bias, which indicates that the vertical direction has a very small velocity at the segmentation pointand does not gain considerable benefit from the IPM. The bias of the shank vertical displacement that appears
in both methods can be affected by IMU calibration where the linear acceleration in the vertical direction is
calculated by subtracting the gravity c­ omponent23,24.
The above results indicates that the IPM contributes to spatial gait parameter estimation with sufficient accuracy compared to the motion-capture system. The limitation of the current study is that it does not use patient
data or elderly data. However, although measurement targets can walk such that the sole of their foot contacts
the ground and the feasibility of the pendulum model for abnormal gait assessed in a prior ­research2 indicatesthat the proposed method can be applied to patients or the elderly, this still needs to be proved. In particular,
the IPM may be strict in estimating the gait parameters of people with severe gait disorders. For example, theIPM may work well when people limp or shuffle because their gait trajectories would not fit to the pendulum
model. Future research needs to address the gait events detection problem for abnormal gait because it is theprerequisite for most segmentation-algorithm-based gait trajectory estimation methods. However, the IPM
would be useful in distinguishing people with slight gait disorders from healthy people. For example, somestudies have attempted to classify early PD patients and healthy elders using gait parameters to develop an early
diagnosis method for PD ­patients25,26. Because the gait of early PD patients is similar to that of healthy people,
it is necessary to estimate the walking trajectory as accurately as possible. Therefore, the IPM would work well
to classify early PD patients and healthy elders.
For data processing, we did not use high- or low-pass filters for the IMU data because we initially consideredthat the proposed method could almost eliminate the drift. In fact, the proposed method showed high accuracy
in trajectory estimation. However, such filters would theoretically improve the estimation accuracy. Some studies
developed a filtering method for IMU data in gait trajectory ­estimation23,24. In the future, such filters would beapplied or added to the proposed method.
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Figure 2.  Bland–Altman plot for stride length, stride velocity, and shank vertical displacement estimated by (a)the proposed method and (b) the previous ­method20 for 283 strides under the normal speed task (◦) and 412
strides under the slower speed task (+). The Bland–Altman plot provides information about the mean difference,
which indicates the bias between two systems, and the 95% confidence interval, which is known as the limit ofagreement (LOA) that shows the difference between the values measured by two systems for most i­ ndividuals27.

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Figure 3.  Configuration of the IMUs. Two IMUs are attached to the shank position right above the malleolus
at a distance of r. The axes x, y, z are the coordinate system of the IMU, where the z-axis is perpendicular to thesagittal plane formed by the y and z-axes.

Figure 4.  Samples of angular velocity, angle in the sagittal plane, and gait events. The heel-strike (HS) and toeoff (TO) events were defected from the angular velocity.

Although we could not discuss gender-related difference for the limitation number of the participants, it
is possible to increase the accuracy of trajectory estimation using gender differences in the gait. In fact, somestudies investigated gender differences in the g­ ait28–30. For example, women’s foot progression angle tends to be
more internally rotated than those of men in the stance p­ hase28. This difference can affect the trajectory andestimation accuracy of the trajectory.

Conclusions

In this study, an IMU-based stride-by-stride gait analysis method was validated with an optical motion capturesystem. The method explored the possibility of using the IPM to analyse the shank trajectory that estimates thestride length, stride velocity, and shank vertical displacement. The accuracy of each parameter estimated by the
IPM method was compared with that estimated using a previous method based on the ZUPT. Using the proposed
method, the average error of the extracted stride length achieved a small bias of 0.006 m. Overall, this study isa step toward the development of a shank-based gait analysis system that will serve as a convenient objective
measurement tool for future clinical diagnoses.

Methods

System setups. In this study, two IMUs (accelerometer and gyroscope) were mounted on the shanks on
both sides, immediately above the malleolus at a distance of r. The attachment position and coordinate system of
the IMUs are shown in Fig. 3. The x, y, and z axes represented inferior/superior, posterior/anterior, and medial/
lateral directions, respectively.
Gait event detection and data segmentation via angular velocity. Heel-strike (HS) and toe-off(TO) events were first detected based on the angular velocity in the sagittal plane ωz7 (Fig. 4). The search regionsfor HS and TO events were defined based on the shank tilt angle in the sagittal plane θz20 calculated using the
integration of ωz . Then, each HS was defined as the first peak that appears after the local maximum of θz (shank
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max forward; SMF) and each TO was defined as the minimum of ωz that appears before the local minimum of θz(shank max backward; SMB). After the HS and TO in each gait cycle were detected, the data were segmented by
the mid-stance (MS), which is defined as the maximum point of ωz between HS and ­TO11.

Orientation estimation from acceleration and angular velocity.

The quaternion system was introduced to describe orientation information in this study. We use superscripts to represent the coordinate frame
wherein a variable is located and the rotation direction of a quaternion. Superscripts S and E represent the
IMU coordinate frame and the laboratory coordinate frame, respectively. Further, we use (k) to represent thekth sample of an instantaneous variable and (i) to represent a variable in the ith gait cycle. For example, qSE (k)represents the instantaneous quaternion that can convert a vector from frame S to frame E. In addition, ms(i) is
the mid-stance event in the ith gait cycle.
At each ms(i), we assumed that the IMU is reset and the accelerometer only detects gravity. Therefore,
 the rotaTtion quaternion qSE (k) at ms(i) is computed using the accelerometer output aS (k) and gravity g E = 1 0 0 as
θ (k) , k = ms(i)qSE (k) = cos θ (k)(1)
2 n(k) sin 2
with

 


1
θ (k) aS (k)

E

cos
=
1+ 
aS (k) · g , k = ms(i)
2
2
 


1
θ (k) aS (k)

E

sin
=
1− 
aS (k) · g , k = ms(i)22aS (k)
E
n(k) = aS (k) × g , k = ms(i)

(2)

(3)

(4)

where ∥ ∥ represents the L2-norm, · is the dot product, and × is the cross product.
The remaining rotation quaternions in each segment were computed via the integration of angular velocity
ωS (k) for each sample aswithqSE (k − 1) + q̇SE (k)�t , k ∈ (ms(i), ms(i + 1))
qSE (k) =  SEq (k − 1) + q̇SE (k)�t 
q̇SE (k) =



1 SE
q (k − 1) ⊗ 0 ωS (k) , k ∈ (ms(i), ms(i + 1))
2

where ⊗ denotes the quaternion product and t represents the sampling interval.
Using qSE (k), we can transform the measured acceleration into the laboratory coordinate frame as
 E 


0 a (k) = qSE (k) ⊗ 0 aS (k) ⊗ (qSE (k))∗

(5)

(6)

(7)

where (qSE (k))∗ is the conjugate of qSE (k).
Finally, the linear acceleration ãE (k) is obtained by removing the gravitational component from aE (k).

ãE (k) = aE (k) − g E(8)Velocity estimation and drift removal via inverted pendulum model. After linear acceleration isdetermined, the velocity v E (k) is computed using the trapezoidal integral with the sampling interval asv E (k) = v E (k − 1) +ãE (k) + ãE (k − 1)
�t
2

(9)

However, this integral result contains significant drift. This drift occurs from the accumulation of sensor
errors such as bias in the gyroscope and jitter in the accelerometer. To solve this problem, a modified IPM wasintroduced in this phase. At each ms(i), we assume the movement of the shank to be rotational motion with themalleolus as a fulcrum (Fig. 5). Thus, the velocity at ms(i) can be calculated as the cross product between the
angular velocity in the laboratory coordinate frame ωE (k) and the position r(k) of the IMUv̆ E (k) = ωE (k) × r(k), k = ms(i)

(10)

with

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Figure 5.  Motion of the IMU at the mid-stance can be modelled as a rotational motion in a three-dimensional
space. The position vector r and angular velocity in laboratory coordinate frame ωE were computed from theestimated orientation of IMU. Thus, the update velocity v̆ E can be recovered as the tangential velocity associatedwith r and ωE .

ωE (k) = qSE (k) ⊗ ωS (k) ⊗ (qSE (k))∗ .

(11)

We assume that aS (k) contains the gravitational component only at ms(i), and therefore, r(k) is defined as thevector in the aS (k) direction with a magnitude of the distance between the IMU and the malleolus r asaS (k)
r(k) = r aS (k) , k = ms(i).

(12)

If the IMU sensor is attached at a certain distance from the ankle, r is known in advance and there is no needto measure it each time.
Then, we model the drift error e(k) as a linear ­variation31 that occurs over time with a constant slope andintercept in each segment ase(k) = α(i)(k − ms(i)) + β(i), k ∈ [ms(i), ms(i + 1)](13)

where α(i) and β(i) are the slope and the intercept of the error model in the ith gait cycle, respectively.
Using the velocity v̆ E (k) estimated from the IPM, α(i) and β(i) are computed asα(i) =v E (ms(i + 1)) − v̆ E (ms(i + 1)) − v E (ms(i)) + v̆ E (ms(i))
ms(i + 1) − ms(i)β(i) = v E (ms(i)) − v̆ E (ms(i))

(14)
(15)

Finally, the drift error is removed from velocity v E (k) by subtracting the modelled error, and the correctionE
result ṽ (k) is obtained asṽ E (k) = v E (k) − α(i)(k − ms(i)) − β(i), k ∈ [ms(i), ms(i + 1)](16)

Trajectory estimation and coordinate frame transformation.

First, the trajectory pE (k) is estimated by the direct integration of the corrected velocity ṽ E (k) via a trapezoidal rule first aspE (k) = pE (k − 1) +ṽ E (k) + ṽ E (k − 1)
�t
2

(17)

Then, a new coordinate frame P is introduced for visualising the stride trajectory and computing the spatial
parameters. In frame P, the y axis direction is aligned to the stride forward direction and the x axis is aligned
along the vertical direction. The rotation matrix REP (i) between the frames P and E is computed by solving theequationI 3 = REP (i) · P E (i)(18)

1I3 = 0
0

(19)

where



0
1
0


0
0
1



and each column of matrix P E (i) = x E (i) y E (i) z E (i) is defined as
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Figure 6.  Typical ankle trajectory before (in frame E) and after transformation (in frame P). Thistransformation allows visualising the trajectory and computing the spatial gait parameters. Stride length isdefined as the displacement in the anterior-posterior direction; shank vertical displacement is defined as the
maximum displacement in the superior-inferior direction.

pE (ms(i + 1)) − pE (ms(i))
y E (i) = pE (ms(i + 1)) − pE (ms(i))

T
1 0 0 × y E (i)z E (i) = T 1 0 0 × y E (i)x E (i) = y E (i) × z E (i)

(20)

(21)

(22)

The final estimate trajectory pP (k) in each gait cycle is transformed using the rotation matrix applied to pE (k):pP (k) = REP (i) · pE (k), k ∈ [ms(i), ms(i + 1))(23)

The example of the stride before and after the transformation is shown in Fig. 6.

Spatial gait parameter computation. The definitions of stride length and shank vertical displacement
are shown in Fig. 6. Stride length during one gait cycle is computed as the displacement in the y direction pyP (k)between two successive mid-stance events.
Stride length(i) = pyP (ms(i + 1)) − pyP (ms(i))(24)

Shank vertical displacement is calculated as the maximum displacement value in the x direction pxP (k).

Shank vertical displacement(i) = max pxP (k), k ∈ (ms(i), ms(i + 1))(25)

Stride velocity is defined as the division of the stride length and stride duration.

Stride length(i)Stride duration(i)(26)

Stride duration(i) = hs(i + 1) − hs(i)(27)

Stride velocity(i) =with

Evaluation experiment

In this study, six females and ten males (age: 23 ± 2; height: 164 ± 7 cm) participated in the concurrent validation experiment. This experiment was conducted in accordance with the Declaration of Helsinki and approvedby the ethics committee of the Tokyo Institute of Technology. Written informed consent was obtained from allparticipants.

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The participants completed a 4 × 10 m walking tasks at two different self-selected walking speeds: normal
and a slower speed, in a fixed order. In the future, we assume elders or patients with gait disorder such as PD
patients as the target for our system. Aging and disorders decrease people’s gait v­ elocity32,33, and therefore, we
employed the slow walking condition. The participants conducted one trial under each condition. Two IMUs
(TSND121, ATR-Promotions, Japan) with features of an accelerometer (±8 g ) and a gyroscope (±1000◦ /s) wereused to implement the proposed method. The size of the TSND121 is 37 mm × 46 mm × 12 mm and the weightis approximately 22 g . Each IMU was placed into a housing pocket with an elastic band, and it was attached tothe shank in the position 0.03 cm above the malleolus.
A motion capture system with 12 cameras (VENUS3D, NOBBYTECH, Japan) and optical motion capture
software (Motive:Tracker, NaturalPoint, Inc.) was used as the reference system. The motion capture system wascalibrated well to ensure that the overall displacement error was under 1 mm. The sampling frequencies were set
to 100 Hz for both the IMU and the motion capture system. Before the start of the experiment, participants wereasked to stamp their left and right foot once to synchronise events between the IMUs and the motion-capturesystem. We used Python (Python Software Foundation) for data processing and analysis.
For the statistical analysis, we compute the difference in the stride-by-stride gait parameter results extractedusing the IMU and the motion-capture system as the error for the proposed method. We computed the meanand standard deviation of the error, the absolute error, and the relative absolute error for each parameter. Further,
Bland–Altman ­analysis34 was introduced to assess the agreement between the IMU and the motion-capture
measurement system for the ­previous20 and proposed methods.
One participants’ data in normal speed was excluded because the motion capture data cannot be synchronized with the IMU data. Finally, 15 normal speed data and 16 slower speed data were analyzed. After removing
invalid data resulting from the reflective markers loss in the invisible area, a total of 722 strides were extracted
from the motion capture system, with 289 strides (19.27 strides per one participant) in the normal speed task
and 433 strides (27.06 strides per one participant) from the slower speed task. The number of strides analyzed
is not less than that measured in previous ­studies13,14,20. All strides can be found in the IMU estimation results
without missing detection. Thus, we analyzed these data to validate the proposed method.
Received: 21 January 2020; Accepted: 30 December 2020

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Acknowledgements

This work was supported by JST CREST Grant Number JPMJCR1433.

Author contributions

Y.M. and H.O. developed the gait measurement system. Y.M. and N.T. conducted the evaluation experiment.
Y.M., T.O., H.O., N.T., and Y.M. discussed and interpreted the results. Y.M. and T.O. drafted the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Correspondence and requests for materials should be addressed to T.O.
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