The Powered Simplest Walking Model Explains the Different Vertical Ground Reaction Force Amplitudes at Elevated Walking Speeds

The vertical component of walking ground reaction force (GRF) has attracted much attention in analyzing human gait. Its typical trajectory shows two peaks at the beginning and end of the stance phase. The first peak signifies step load acceptance, while the second peak is to accelerate the center of mass (COM) for the next step. As walking speed increases, these peaks grow at different rates, with the first peak’s amplitude becoming larger. No mechanistic explanation has been provided for this change. Here, we used a simple walking model and the COM work rate trajectory to investigate the vertical GRF profile change. A step is usually energized by positive work (push-off) at its start, followed by energy dissipation. It is suggested that push-off is exerted by the ankle joint. At elevated walking speeds, the ankle’s biomechanical power generation capacity may not be sufficient to energize the entire gait. Since push-off also limits the magnitude of step energy dissipation (collision), any shortfall in push-off is followed by increased collision, reflected in the first peak’s rise in vertical GRF. This is supported by the observation that collision work exceeds push-off beyond a certain walking speed threshold (even walking v = 1.35 m · s ^-1 ). Therefore, we also expect to observe different vertical GRF peak amplitudes when terrain challenges limit or disrupt the ankle’s mechanical energy output. The vertical GRF unequal peaks may indicate that lower limbs are subjected to higher dissipations that might overstress them, and the required compensation is provided by over actuating of the other joint that might yield to their faster deterioration.