The counter-rotating mechanism makes a substantial contribution to balance-movement coordination during the anticipatory period of gait initiation

Maintaining balance during gait initiation is essential for mobility and functional independence. The transition from a postural state to steady-state gait relies on two mechanisms to accelerate the whole-body centre of mass (WBCoM): moving centre of pressure (CoP); and counter-rotating segments to modulate internal whole-body angular momentum (H_M). While the moving CoP mechanism is well-understood and known to generate mechanical instability, the role of the counter-rotating mechanism is less clear. Therefore, the present study quantified the contribution of the counter-rotating mechanism and explored its coordination with the moving CoP mechanism during the anticipatory period of gait initiation. Based on a sample of 13 healthy participants, we computed the time evolution of CoP, H_M, the coefficient of cancellation (i.e., the extend to which segmental angular momenta counterbalance each other), and the relative contribution of each mechanism to WBCoM acceleration. We tested whether the contribution of the counter-rotating mechanism was significantly different from zero using a Statistical Parametric Mapping t-test. In the frontal plane, the counter-rotating mechanism did not appear to play a substantial role. However, in the sagittal plane, we found that: (1) the period of mechanical instability was longer than the period that have been identified based on the moving CoP mechanism alone; (2) rotational instability was reduced in anticipation of unipedal support; and (3) the counter-rotating mechanism significantly contributed to WBCoM forward acceleration. Overall, our findings emphasise the critical role of the counter-rotating mechanism in the sagittal plane for balance-movement coordination. Considering the counter-rotating mechanism appears important for understanding, assessing, and developing interventions for individuals with balance impairments.