Abstract
Centre of mass (CoM) motion during human balance recovery is largely influenced by the ground reaction force (GRF) and the centre of pressure (CoP). During gait, foot placement creates a region of possible CoP locations in the following double support (DS). This study aims to increase insight into how humans modulate the CoP during DS, and which CoP modulations are theoretically possible to maintain balance in the sagittal plane. Three variables sufficient to describe the shape, length and duration of the DS CoP trajectory of the total GRF, were assessed in perturbed human walking. To counteract the forward perturbations, braking was required and all CoP variables showed modulations correlated to the observed change in CoM velocity over the DS phase. These correlations were absent after backward perturbations, when only little propulsion was needed to counteract the perturbation. Using a linearized inverted pendulum model we could explore how the observed parameter modulations are effective in controlling the CoM. The distance the CoP travels forward and the instant the leading leg was loaded largely affected the CoM velocity, while the duration mainly affected the CoM position. The simulations also showed that various combinations of CoP parameters can reach a desired CoM position and velocity at the end of DS, and that even a full recovery in the sagittal plane within DS would theoretically have been possible. However, the human subjects did not exploit the therefore required CoP modulations. Overall, modulating the CoP trajectory in DS does effectively contributes to balance recovery.
Highlights
Healthy humans have excellent capabilities to maintain balance and avoid falling during walking
Together with the length and width of each foot it sets the base of support (BoS), determining the possible future centre of pressure (CoP) locations for the upcoming gait phases (Hof et al, 2005)
When the CoP is located posterior of the centre of mass (CoM), the ground reaction force (GRF) can accelerate the CoM forward, whereas the CoM can be decelerated when the CoP is located anterior of the CoM (Jian et al, 1993; Winter, 1995). Though this model is often used to describe single support (SS), we investigated its application during double support (DS), to better understand of the effects of CoP modulations on the CoM motion
Summary
Healthy humans have excellent capabilities to maintain balance and avoid falling during walking. Various studies emphasize the use of the ankle strategy in the sagittal plane to modulate the CoP within the BoS, following foot placement (Gruben and Boehm, 2014; Hof et al, 2005; Matjačić et al, 2017; Millard et al, 2009; Vlutters et al, 2016). These studies clearly demonstrate CoP modulations after balance perturbations. How these modulations relate to changes in the CoM position and velocity was not investigated
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