Energetics and optimization of human walking and running: the 2000 Raymond Pearl memorial lecture.

Humans seem to adjust their walking and running gaits to minimise the metabolic energy cost of locomotion. The walking speed that we tend to prefer is the one that minimises energy cost per unit distance, though faster speeds might seem preferable when time is valuable. At speeds up to 2 m/s, walking requires less energy than running, and we walk. At higher speeds, running is more economical, and we run. At each speed we use the stride length that minimises energy costs. A computer model that predicts metabolic rates for all conceivable gaits of a simple biped helps to understand these and other features of human gait. The energy cost of walking is increased on uphill slopes and also on soft ground. Consequently, zigzag paths should be preferred to straight ones, up hills of more than a critical gradient. Also, it may be more economical to divert a path around a hill than to travel along a straight line. Simple theories of optimum diversions are presented, both for hilly ground and for ground interrupted by marshy patches, on which costs of walking are increased. Energy costs are also increased by heavy loads, though it seems possible in some circumstances to carry moderate loads without measurable extra cost.