The independent effects of speed and propulsive force on joint power generation in walking.

Walking speed is modulated using propulsive forces (F) during push-off and both preferred speed and F decrease with aging. However, even prior to walking slower, reduced F may be accompanied by potentially unfavorable changes in joint power generation. For example, compared to young adults, older adults exhibit a redistribution of mechanical power generation from the propulsive plantarflexor muscles to more proximal muscles acting across the knee and hip. Here, we used visual biofeedback based on real-time F measurements to decouple and investigate the interaction between joint-level coordination, whole-body F, and walking speed. 12 healthy young subjects walked on a dual-belt instrumented treadmill at a range of speeds (0.9-1.3m/s). We immediately calculated the average F from each speed. Subjects then walked at 1.3m/s while completing a series of biofeedback trials with instructions to match their instantaneous F to their averaged F from slower speeds. Walking slower decreased F and total positive joint work with little effect on relative joint-level contributions. Conversely, subjects walked at a constant speed with reduced F, not by reducing total positive joint work, but by redistributing the mechanical demands of each step from the plantarflexor muscles during push-off to more proximal leg muscles during single support. Interestingly, these naturally emergent joint- and limb-level biomechanical changes, in the absence of neuromuscular constraints, resemble those due to aging. Our findings provide important reference data to understand the presumably complex interactions between joint power generation, whole-body F, and walking speed in our aging population.