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-P) and braking (F-B) forces generated during step-to-step transitions and is simultaneously optimized to minimize metabolic cost. Of those forces, F-P 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-P and walking speed and then to quantify their effects on metabolic cost in young adults. To specifically address any link between F-P and walking speed, we used a self-paced treadmill controller and real-time biofeedback to independently prescribe walking speed or F-P across a range of condition intensities. Walking with larger and smaller F-P led to instinctively faster and slower walking speeds, respectively, with similar to 80% of variance in walking speed explained by F-P. We also found that comparable changes in either F-P or walking speed elicited predictable and relatively uniform changes in metabolic cost, together explaining similar to 53% of the variance in net metabolic power and similar to 14% of the variance in cost of transport. These results provide empirical data in support of an interdependent relation between F-P and walking speed, building confidence that interventions designed to increase F-P wilt 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.