Communication Dans Un Congrès Année : 2025

Mechanical instability lasts longer than the backward centre of pressure shift during gait initiation

Résumé

Introduction: Ensuring balance is vital for everyday movement. During gait initiation, a common paradigm to assess balance-movement coordination in humans, mechanical instability must be generated during the anticipatory period to initiate movement. A first phase of the anticipatory period contributing to generate mechanical instability was described, with a backward centre of pressure (CoP) shift, accelerating the whole-body centre of mass (WBCoM) forward [1]. However, this analysis does not consider the rotational contribution of the free segments. The internal whole-body angular momentum (HM) around the mediolateral axis also exhibited a first phase where HM is directed forward, followed by a second one where some segments rotated in the opposite direction to limit the mechanical instability and the increase in HM magnitude, interpreted as a stabilising mechanism [2]. The current study aims to determine whether the forward directed HM only corresponds to the mechanical instability generated by the backward CoP shift, or whether the mechanical instability lasts longer than the backward CoP shift.
Methods: Thirteen healthy participants (23.5 ± 4.2 years, 1.72 ± 0.1 m, 65.5 ± 8.8 kg) performed three gait initiation trials. Participants were asked to voluntarily initiate gait and walk for five meters from a hip-width upright standing posture. Ground reaction forces and moments were recorded by four force plates (Sensix) at 2000 Hz, lowpass filtered at 20 Hz and used to compute the CoP. The trajectories of 54 markers placed on anatomical landmarks were recorded with 15 infrared cameras (Vicon) at 200 Hz, lowpass filtered at 10 Hz. HM was computed as the sum of the segment angular momenta transferred to the WBCoM based on a 13 segments whole-body model. The coefficient of cancellation, defined as the amount of segment angular momentum compensating each other divided by the total magnitude of HM was then computed [3]. The duration of the mechanical instability phase was computed using the CoP and HM methods. For both methods, the phase began when the WBCoM acceleration was larger than the mean +3 standard deviations measured during quiet standing. The instability phase ended when the CoP was the most backward and toward the swing limb [1], and when the time derivative of HM became positive (i.e., when some segments started to rotate in the opposite direction), respectively. We compared the phase durations computed by each method using a paired-sample t-test.
Results: The duration of the mechanical instability phase based on the CoP was significantly shorter than the one based on HM (260 ± 73 vs. 368 ±101 ms, p<0.001).
[Figure 1: Average and standard deviation of internal whole-body angular momentum (HM, top) and coefficient of cancellation (bottom) during the anticipatory period, ending at foot-off. The black and red vertical lines represent the phase delimitation based on the CoP and HM methods, respectively.]
Discussion: Our results suggest that the whole-body continues to rotate forward after the end of the backward CoP shift (after the black vertical line in Figure 1), without any regulation from opposite segment rotation, as indicated by a low coefficient of cancellation until change in the time derivative of HM. This suggests that mechanical instability is not large enough to initiate gait at the end of the backward CoP shift, and that the whole-body must continue to rotate forward before any regulation appears. This phase of mechanical instability would not have been detected by analysing only the CoP. These results being obtained on young healthy participants, future studies should assess if this longer period of mechanical instability is also present in older participants and/or participants with balance deficits.
References:
1. Crenna et al, Exp Brain Res, 172 :519-532, 2006
2. Bechet et al, MBJ, 1, 2024
3. Bennet et al, Hum. Mov. Sci, 29 :114-124 : 2010

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hal-05176650 , version 1 (22-07-2025)

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Romain Bechet, Romain Tisserand, Laetitia Fradet, Floren Colloud. Mechanical instability lasts longer than the backward centre of pressure shift during gait initiation. 30th Congress of the European Society of Biomechanics, ETH Zürich, Jul 2025, Zürich, Switzerland. ⟨hal-05176650⟩
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