Quantifying mechanical and metabolic interdependence between speed and propulsive force during walking.

Walking speed is a useful surrogate for health status across the population. Walking speed appears to be governed in part by interlimb coordination between propulsive (F) and braking (F) forces generated during step-to-step transitions and is simultaneously optimized to minimize metabolic cost. Of those forces, F generated during push-off has received significantly more attention as a contributor to walking performance. Our goal was to first establish empirical relations between F and walking speed and then to quantify their effects on metabolic cost in young adults. To specifically address any link between F and walking speed, we used a self-paced treadmill controller and real-time biofeedback to independently prescribe walking speed or F across a range of condition intensities. Walking with larger and smaller F led to instinctively faster and slower walking speeds, respectively, with ~80% of variance in walking speed explained by F. We also found that comparable changes in either F or walking speed elicited predictable and relatively uniform changes in metabolic cost, together explaining ~53% of the variance in net metabolic power and ~14% of the variance in cost of transport. These results provide empirical data in support of an interdependent relation between F and walking speed, building confidence that interventions designed to increase F will translate to improved walking speed. Repeating this protocol in other populations may identify other relations that could inform the time course of gait decline due to age and disease.