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

Humans do not walk at exactly constant speed, except perhaps on a treadmill. Normal walking involves starting, stopping, and changing speeds. Here, we measure the metabolic energy cost of human walking when changing speed. Subjects walked with oscillating speeds on a constant-speed treadmill, alternating between walking faster and slower than the treadmill belt, moving back and forth in the laboratory frame. Metabolic rate of walking with oscillating speeds was statistically significantly higher (6 to 20%) than walking at constant speed. The metabolic rate increase was correlated with two simple models: a model based on the subject's kinetic energy fluctuations and a point-mass inverted pendulum walking model, optimized for the oscillating-speed experimental constraints. The cost of changing speeds may have significant behavioral 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 lower walking speeds for shorter distances as predicted. Further, analyzing published daily walking-bout distributions, we estimate that the cost of changing speeds is about 4-8% of daily walking energy.