# Evaluation of the Margin of Stability during Gait Initiation in Young Healthy Adults, Elderly Healthy Adults, and Patients with Parkinson's Disease: A Comparison of Force-Plate and Markerless Motion Capture Systems

Arnaud Simonet, Arnaud Delafontaine, Paul Fourcade, Florent Loete, Eric Yiou

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Arnaud Simonet, Arnaud Delafontaine, Paul Fourcade, Florent Loete, Eric Yiou. Evaluation of the Margin of Stability during Gait Initiation in Young Healthy Adults, Elderly Healthy Adults, and Patients with Parkinson's Disease: A Comparison of Force-Plate and Markerless Motion Capture Systems. Multidisciplinary Biomechanics Journal, 2025, 49th congress of the Société de Biomécanique, 10.46298/mbj.14578. hal-04738071v2

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# Evaluation of the Margin of Stability during Gait Initiation in Young Healthy Adults, Elderly Healthy Adults, and Patients with Parkinson's Disease:

## A Comparison of Force-Plate and Markerless Motion Capture Systems

Arnaud Simonet*<sup>a</sup>, Arnaud Delafontaine<sup>b</sup>, Paul Fourcade<sup>c</sup>, Florent Loete<sup>d</sup>, Eric Yiou<sup>c</sup>

<sup>a</sup> LADAPT Loiret, Centre de soins de suite et de réadaptation, Amilly, France

<sup>b</sup> Laboratoire d'anatomie fonctionnelle, Faculté des Sciences de la Motricité ; Laboratoire d'anatomie, de biomécanique et d'organogenèse, Faculté de Médecine, Université Libre de Bruxelles, Bruxelles, Belgique

<sup>c</sup> CIAMS, Université Paris-Saclay, Orsay, France

<sup>d</sup> Laboratoire GeePS-CENTRALESUPELEC, Gif-sur-Yvette, France

* Corresponding author: simonet.arnaud@ladapt.net

Received date: 03/04/2024

Accepted date: 28/06/2024

Publication date: 31/01/2025

**Keywords:** gait initiation, stability, markerless motion capture, force plate

© 2025 The Authors

Licence CC-BY 4.0

Published by Société de Biomécanique

# 1. Introduction

Postural stability is a prerequisite to the performance of most of our daily motor tasks. Gait initiation, the transient period between quiet standing and steady-state walking, is a functional task classically used in the literature to evaluate the capacity of healthy individuals and neurological patients to maintain stability under dynamic conditions (Yiou *et al.* 2017). Postural stability during gait initiation is commonly evaluated through the “margin-of-stability”, a quantity which takes into account both the position and velocity of the centre-of-mass within the base-of-support (Hof *et al.* 2005). This quantity is often computed from force plate system recordings (e.g. Yiou *et al.*, 2016). This study tested the agreement between the force plate system (considered here as the “gold standard”) and markerless motion capture system, an innovative and easy-to-use technology, to estimate the MoS during gait initiation in young and elderly healthy adults as well as in patients with Parkinson’s disease. Markerless motion capture system uses standard video and deep learning-based software to detect body segment landmarks directly from digital images (Kanko *et al.* 2021). In the present study, the focus was put on the margin-of-stability along the mediolateral direction, because controlling stability along this direction is known to be particularly challenging during locomotor tasks.

# 2. Methods

## 2.1 Population

Thirty-three participants divided into three groups (young healthy, elderly healthy, and patients with Parkinson's disease) took part in the experiment which was conducted in accordance with the Declaration of Helsinki.

## 2.2 Experimental Set-up and procedure

Participants initiated gait from the standing posture and continued walking to the end of a six-meter track. Gait was initiated at a spontaneous velocity (“spontaneous velocity condition”) and at a maximum velocity (“maximum velocity condition”). The force plate system was composed of two force plates (400x600mm, AMTI, USA) placed in series and embedded at the beginning of the walking track. The markerless motion capture system was composed of twelve Qualisys Hybrid cameras (Qualisys, Göteborg, Sweden) which recorded the body motion during the entire gait initiation process and the subsequent steps.

## 2.3 Margin of stability

The margin-of-stability was computed as the difference between the mediolateral base of support boundary (BoSymax) and the mediolateral position of the “extrapolated centre-of-mass” at the time of foot-contact (yCoM<sub>FC</sub>), i.e MoS=BoSymax-yCoM<sub>FC</sub>. yCoM<sub>FC</sub> was calculated as follows: yCoM<sub>FC</sub>=yCoM<sub>FC</sub>+y'CoM<sub>FC</sub>/
ω₀, where yCoM<sub>FC</sub> and y'CoM<sub>FC</sub> are respectively the
mediolateral centre-of-mass position and the velocity
at foot-contact; ω₀ is the eigenfrequency of the body
modelled as an inverted pendulum, calculated as:

ω₀ = ∫g dt / √l, where g is the gravitational acceleration and l is the length of the inverted pendulum (57.5% of body height (Winter, 2009)).

## 2.4 Statistics

For each trial, the margin-of-stability was computed with
both systems. To investigate the agreement between force
plate system and markerless motion capture system, a
Bland-Altman analysis was performed. Bland-Altman
plots were generated for each velocity condition and
group, with the horizontal axis representing the average
MoS obtained using the two systems, and the vertical
axis representing the difference between the two systems.
The lack of agreement between the two systems was
summarised by calculating the bias, estimated by the
mean difference (d) and the standard deviation of the
differences (SD). Repeated-measures (RM) ANOVA
with the Velocity (spontaneous velocity condition vs.
maximum velocity condition) and System (force plate
system vs. markerless motion capture system) as with-
in-subject factors, and the Group (young healthy, elderly
healthy adults, patients with Parkinson's disease) as
between-subject factor were also carried out.

**3. Results and discussion**

In spontaneous velocity condition (Figure 1), Bland-
Altman analysis showed that 95% of the absolute dif-
ferences between the two measurement systems ranged
between -0.01 and 0.02 m for young healthy (which
corresponded to a [-17 and 20%] range of relative
difference), -0.02 and 0.02 m for elderly healthy adults
([-23 and 26%]) and -0.02 and 0.03 m for patients
with Parkinson's disease([-28 and 30%]). In maximum
velocity condition (Figure 1), 95% of the absolute dif-
ferences between the two systems ranged between -0.02
and 0.03 m for young healthy (-21 and 25%), -0.02
and 0.03 m for elderly healthy adults (-27 and 36%]
and -0.04 and 0.04 m for patients with Parkinson's
disease [-41 and 37%]). Absolute bias was very close
to zero and the deviation was considered neglectable
for each velocity condition and group, indicating that
the accuracy of the markerless motion capture system
was good. RM ANOVA further showed that there was
a significant main effect of the Group (F[2,476]=5.7,

p<0.001) and the Velocity (F[1,476]=28.1, p<0.001)
on the margin-of-stability. In contrast, there was no
significant main effect of the System, nor any significant
interaction between factors. In other words, although
relatively high differences in the margin-of-stability do
occur between markerless motion capture system and
force plate system on a trial-by-trial basis, these two
systems are similarly effective in detecting an effect of
the group and velocity on the margin-of-stability.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Bland-Altman plots showing the margin-of-stability</figcaption></figure>

Legend: Obtained with the two systems (mean values, in abscissa), against the difference between these two systems (in ordinate), for the young healthy adults (YH), elderly healthy adults (EH) and PD groups. Each point represents one trial for one participant. Reported are the absolute values obtained in the spontaneous (left) and maximum (right) velocity conditions (SVC and MVC, respectively). For each plot, the 95% limits of agreement (red dotted lines), standard deviation (green dotted line) and bias (blue full line) are shown.

4. Conclusions

Results of the present study add to the growing body
of evidence in the literature that markerless motion
capture system is a sufficiently reliable tool to investigate
gait initiation biomechanical features in healthy par-
ticipants and patients with Parkinson's disease patients
(Simonet *et al.* 2024). This statement may have important
implications in both clinical and laboratory settings due
to the ease of use of markerless motion capture system
compared to force plate system.

Conflicts of Interest

All authors state that they have no financial or personal
relationships with other people or organizations that
could inappropriately influence (bias) their work.

References

Hof, A. L., Gazendam, M. G. J., & Sinke, W. E. (2005). The condition for dynamic stability. *Journal of Biomechanics*, 38(1), 1–8. https://doi.org/10.1016/j.jbiomech.2004.03.025

Kanko, R. M., Laende, E. K., Davis, E. M., Selbie, W. S., & Deluzio, K. J. (2021). Concurrent assessment of gait kinematics using marker-based and markerless motion capture. *Journal of Biomechanics*, 127, 110665. https://doi.org/10.1016/j.jbiomech.2021.110665

Simonet, A., Delafontaine, A., Fourcade, P., & Yiou, E. (2024). Vertical Center-of-Mass Braking and Motor Performance during Gait Initiation in Young Healthy Adults, Elderly Healthy Adults, and Patients with Parkinson's Disease: A Comparison of Force-Plate and Markerless Motion Capture Systems. *Sensors* (Basel, Switzerland), 24(4), 1302. https://doi.org/10.3390/s24041302

Winter, D. A. (2009). Biomechanics and Motor Control of Human Movement. John Wiley & Sons.

Yiou, E., Artico, R., Teyssedre, C. A., Labaune, O., & Fourcade, P. (2016). Anticipatory Postural Control of Stability during Gait Initiation Over Obstacles of Different Height and Distance Made Under Reaction-Time and Self-Initiated Instructions. *Frontiers in Human Neuroscience*, *10*, 449. https://doi.org/10.3389/fnhum.2016.00449

Yiou, E., Caderby, T., Delafontaine, A., Fourcade, P., & Honeine, J.-L. (2017). Balance control during gait initiation: State-of-the-art and research perspectives. *World Journal of Orthopedics*, 8(11), 815–828. https://doi.org/10.5312/wjo.v8.i11.815