# Beyond postural mechanisms: specific and fine-tuned postural strategies emerged

Prince François, Andréa Sorel, Amélie Poiré-Lehoux, Francine Pilon, Yann Sonier

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Prince François, Andréa Sorel, Amélie Poiré-Lehoux, Francine Pilon, Yann Sonier. Beyond postural mechanisms: specific and fine-tuned postural strategies emerged. Multidisciplinary Biomechanics Journal, 2025, 50th congress of the.., 10.46298/mbj.16183. hal-05191735v2

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Submitted on 23 Oct 2025

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# Beyond Postural Mechanisms: Specific and Fine-Tuned Postural Strategies Emerged

François Prince<sup>a, b, c*</sup>, Andréa Sorel<sup>a, d</sup>, Amélie Poiré-Lehoux<sup>a</sup>, Francine Pilon<sup>b</sup>, Yann Sonier<sup>a, e</sup>

a Star Lab, Centre intégré de traumatologie, Hôpital du Sacré-Coeur de Montréal, CIUSSS-NÎM, Montréal, Canada

b Olympic and Paralympic Research Team, Canada

c Département de chirurgie, Faculté de médecine, Université de Montréal, Canada

d Faculté des Sciences du Sport, Université Aix-Marseille, France

e UFR-STAPS, Université de Reims, Champagne-Ardenne, France

* Corresponding author: francois.prince@umontreal.ca

Received date: 06/04/2025

Accepted date: 27/06/2025

Publication date: 27/10/2025

**Keywords:** posture, control, center of pressure, mechanism, strategy

© 2025 The Authors

Licence CC-BY 4.0

Published by Société de Biomécanique

## 1. Introduction

Postural control during quiet stance is a fundamental component of balance regulation. Although single force platforms reliably estimate whole-body sway using the net center of pressure (COP<sub>net</sub>), they provide no insight into the interlimb coordination mechanisms involved. In contrast, the use of dual force plates enables separate tracking (Winter et al., 1993) of the center of pressure beneath the right (COP<sub>r</sub>) and left (COP<sub>l</sub>) foot, offering a more precise view of balance control and a clear understanding of the specific role of each foot during standing. This study aims to investigate the distinct COP<sub>r</sub> and COP<sub>l</sub> contributions of each foot to postural control and to characterize, across young and older adults, the specific strategies operating to manage the two postural mechanisms underlying balance during quiet standing. The first mechanism (Foot Mechanism) acts in the antero-posterior (AP) direction while the second (Load/Unload Mechanism) is taking place to maintain balance in the medio-lateral (ML) direction. These specific strategies are examined between young and elderly participants and across various sensory conditions that alter proprioceptive and visual input.

With two force plates, the COP<sub>net</sub> can be calculated, and a Foot Mechanism was proposed representing the contribution of the COP<sub>r</sub> and COP<sub>l</sub> movement in AP and a Load/Unload Mechanism was also shown in ML direction (Winter et al., 1993; Rougier & Genthon, 2009).

The Load/Unload Mechanism acts to transfer weight from one leg to the other to control the ML position of COP<sub>net</sub>. This can only be managed by:

* activating the invertor from one side and the evertor muscles of the foot on the other side

* contracting hip adductors from one side and the abductor muscles on the other side.

## 2. Methods

### 2.1 Subjects

COP time series were extracted from an open-access data set (dos Santos, Fukuchi, Fukuchi & Duarte, 2017) of a total of 27 Young (15 Male/12 Female, 28.1yo, 1.71m, 70.3kg) and a group of 22 elderly (11 Male/11 Female, 67.8yo, 1.61m, 68.7kg).

### 2.2 Data Acquisition and Sensory conditions

Data were recorded at 100Hz from 2 AMTI force plates and trial duration was 60s under Eye Open (EO), Eyes Closed (EC), and standing barefoot on a Rigid (R) or on a 6cm Foam (F) surface. Feet were positioned in a standardized configuration (dos Santos, Fukuchi, Fukuchi & Duarte, 2017) with heels 10 cm apart and toes pointing at 20° outward.

### 2.3 Parameters Selected and Data Processing

To highlight postural strategies, times series of both COP<sub>r</sub> and COP<sub>l</sub> were extracted for both young and elderly group and for all sensory conditions. Statistical
analyses include a 2x2x2 ANOVA with repeated measures on both visual and surface conditions. Significant level was set at 0.05.

## 3. Results and discussion

Table 1 showed correlations for both COP<sub>r</sub> and COP<sub>l</sub> in AP and ML directions. A repeated-measure ANOVA revealed a significant main effect of age groups (p=.003), indicating reduced left-right coordination of ML COP's in elderly participants. A significant vision × age interaction (p=.017) further showed that removing visual input led to a greater deterioration in bilateral COP's correlation in older adults compared to young adults. These findings suggest age-related decline in interlimb postural coordination, particularly under challenging sensory conditions. Positive COP's correlations are seen in AP.

<table><caption>**Table 1.** COP<sub>r</sub> & COP<sub>l</sub> correlations. Credit: F. Prince.</caption><thead><tr><th></th><th>Young ML*</th><th>Elderly ML</th><th>Young AP‡</th><th>Elderly AP</th></tr></thead><tbody><tr><td>EO-R</td><td>-0,85†</td><td>-0,77†</td><td>0,81#</td><td>0,75#</td></tr><tr><td>EC-R</td><td>-0,79</td><td>-0,66</td><td>0,80#</td><td>0,78#</td></tr><tr><td>EO-F EC-F</td><td>-0,81†<br>-0,75</td><td>-0,70†<br>-0,61</td><td>0,92<br>0,91</td><td>0,94<br>0,94</td></tr></tbody></table>

Eyes Open (EO), Eyes Closed (EC), Rigid (R) and Foam (F) surface.
* Significant effect for Age and †Vision x Age effect in ML.
# Significant Surface effect and significant. ‡ Surface x Age effect in AP. p<0.05.

In the ML direction, the COP<sub>r</sub> & COP<sub>l</sub> correlations are in antiphase (negative correlation) and they can't assist the COP<sub>net</sub> displacement because they cancelled each other. Figure 1 showed time series of COP<sub>net</sub> in ML and the Vertical Force under one limb during quiet standing. High correlations (≈ -0,98) were shown in both young and elderly and in all conditions supporting the use of weight transfer as the major contributor to control ML COP<sub>net</sub> displacement.

In the AP direction, the correlations between COP<sub>r</sub> and COP<sub>l</sub> were high (.75 to .94) across all conditions. A significant main effect of surface (p<.001) indicated that postural coordination increased on foam compared to rigid surface. A significant surface × age interaction in the AP direction revealed that elderly participants showed a greater increase in left-right COP coordination

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Vertical Force under the right foot and COP<sub>net</sub> in ML direction of a young subject standing with Eyes Open on a Rigid surface.</figcaption></figure>

Credit: F. Prince

when standing on foam compared to rigid surface. In AP, young adults show a moderate increase in AP COP correlation when moving from a rigid surface to a foam surface. Elderly adults, however, show a more pronounced increase, reaching near-perfect bilateral coordination (r≈0.94) on foam.

This suggests that on stable ground, elderly adults already have slightly lower AP COP coordination than young adults. On foam, however, they increase their inter-foot coordination significantly, possibly reflecting: 1) a compensatory strategy to stabilize posture in response to unstable somatosensory input and 2) a greater reliance on symmetrical control between limbs to maintain balance under challenging conditions.

This paper is the first that explicitly explained how the Load/Unload postural mechanism does not involve only the hip abductor and adductor muscles but also required simultaneously, a very precise strategy delivered by the invertors and evertors muscles to move COP<sub>r</sub> and COP<sub>l</sub> in an out of phase pattern.

These results support the promotion of rehabilitation and prevention of fall programs in the elderly to train the postural mechanisms but also to restore the specific postural strategy associated with invertor and evertor muscles at the ankle.

## 4. Conclusions

Aging is associated with reduced interlimb postural coordination, particularly in the ML direction. The moderate to strong negative COP<sub>l</sub> and COP<sub>r</sub> correlations
in ML is a specific postural strategy adopted by all
participants and under challenging sensorial condi-
tions. Therefore, these challenging conditions affect
the strategy by assisting the COP<sub>net</sub> control driven by
the Load/Unload mechanism in ML. Specific out of
phase COP<sub>r</sub> and COP<sub>l</sub> training should be integrated
systematically in fall-prone population programs.

Contributor Roles

FP,AS,APL: Conceptualization, Methodology, Data curation and Validation; FP:Writing original draft; FP,AS,APL,YS,FPi: Writing review & editing; FP: Supervision.

References

dos Santos, D. A., Fukuchi, C. A., Fukuchi, R. K., & Duarte, M. (2017). A data set with kinematic and ground reaction forces of human balance. *PeerJ*, *5*, e3626. https://doi.org/10.7717/peerj.3626

Rougier, P., & Genthon, N. (2009). Dynamical assessment of weight-bearing asymmetry during upright quiet stance in humans. *Gait & Posture, 29*(3):437-43. https://doi.org/10.1016/j.gaitpost.2008.11.001

Winter, D. A., Prince, F., Stergiou, P., & Powell, C. (1993). Partitioning of medio-lateral and anterior-posterior motor responses associated with center of pressure changes in quiet standing. *Neurosc. Res. Comm, 12*, 141-148.