Humans dynamically optimize walking speed to save energy and time

Humans make a number of choices when they walk, such as how fast and for how long. The preferred steady walking speed seems chosen to minimize energy expenditure per distance traveled. But the speed of actual walking bouts is not only steady, but rather a time-varying trajectory, which can also be modulated by task urgency or an individual’s movement vigor. Here we show that speed trajectories and durations of human walking bouts are explained better by an objective to minimize Energy and Time, meaning the total work or energy to reach destination, plus a cost proportional to bout duration. Applied to a computational model of walking dynamics, this objective predicts speed vs. time trajectories with inverted U shapes. Model and human experiment ( N = 10) show that shorter bouts are unsteady and dominated by the time and effort of accelerating, and longer ones are steadier and faster due to energy-per-distance. Individual-dependent vigor is characterized by the energy one is willing to spend to save a unit of time, which explains why some may walk faster than others, but everyone has similar-shaped trajectories due to similar walking dynamics. Tradeoffs between energy and time costs predict transient, steady, and vigor-related aspects of walking. ### Competing Interest Statement The authors have declared no competing interest.