Step-by-Step Modulation of Braking and Propulsive Forces During Treadmill-Imposed Velocity Transitions Versus Continuous Velocity Change

Background: Adjusting walking speed on demand is fundamental to daily mobility and is achieved by modulating the anterior–posterior ground reaction force. However, kinetic control at the onset of a velocity change has not been isolated from control during an ongoing change or from the confounding influence of instantaneous speed. This study characterized step-by-step modulation of peak braking force (BF) and peak propulsive force (PF) during externally imposed velocity transitions and continuous velocity change. Methods: Nineteen healthy young adults walked on an instrumented dual-belt treadmill while belt speed was modulated sinusoidally. Transitions from constant speed to acceleration (CtoA) or deceleration (CtoD) were compared with continuous acceleration and deceleration. Each outcome was analyzed with a linear mixed-effects model with stepwise belt speed as a covariate. Results: The condition × step interaction was significant for both BF and PF in each condition pair (all PFDR < 0.001; η2p = 0.042–0.223). PF changed at the transition step in both CtoA and CtoD, whereas BF changed at the transition step in CtoA but one step later in CtoD. These changes differed from the continuous conditions and were not explained by instantaneous belt speed. Vertical ground reaction force and spatiotemporal parameters did not parallel these changes. Conclusions: Treadmill-imposed velocity transitions elicited the step-by-step modulation of braking and propulsive forces corresponding to transition onset. However, these findings cannot be directly generalized to self-initiated speed changes during overground walking.