The metabolic cost of changing walking speeds is significant, implies lower optimal speeds for shorter distances, and increases daily energy estimates

Abstract

Humans do not generallywalk at constant speed, except perhaps on a treadmill. Normalwalking involves starting, stopping and changing speeds, in addition to roughlysteady locomotion.Here,wemeasure themetabolic energy cost ofwalking when changing speed. Subjects (healthy adults) walked with oscillating speeds on a constant-speed treadmill, alternating between walking slower and faster than the treadmill belt, moving back and forth in the laboratory frame. The metabolic rate for oscillating-speed walking was significantly higher than that for constant-speed walking (6-20% cost increase for +0.13-0.27 m s21 speed fluctuations). The metabolic rate increase was correlated with two models: Amodel based on kinetic energy fluctuations and an inverted pendulum walkingmodel, optimized for oscillating-speed constraints. The cost of changing speeds may have behavioural implications: We predicted that the energy-optimal walking speed is lower for shorter distances. We measured preferred human walking speeds for different walking distances and found people preferred lower walking speeds for shorter distances as predicted. Further, analysing published daily walking-bout distributions, we estimate that the cost of changing speeds is 4-8% of daily walking energy budget.