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 ~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 ~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(P) and walking speed, building confidence that interventions designed to increase F(P) 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.