Minimum oxygen cost of human walking with geometrically similar leg movements

The mechanism by which the expenditure of oxygen to walk per unit distance at an intermediate speed is minimized, by definition optimal walking, was investigated to characterize optimal walking in humans with variations in individual walking speeds. Oxygen uptake and step rate (SR) were measured among 7 young male subjects walking at an increasing speed from 16.7 to 131.7 m min^-1 with 5 m min-1 increments every 1 min on a level treadmill. Measurements of leg length (L) were also made and step length (SL) was calculated by dividing walking speed by SR. The hip joint angle (θ) was calculated as a function of both L and SL such that θ=2sin^-1[SL/(2L)] deduced from a mathematical geometrically similar model of pendulum-like legs. The minimum oxygen cost to walk per unit distance for each subject was observed over a wide range of speeds from 60 to 100 m min^-1. However, the oxygen cost of walking for all the subjects was minimized during a step cycle through a hip-joint angle of about 46 deg in the astride position, regardless of L. The stiff-legged model demonstrated that the pathway of the trunk during optimal walking with a swing leg angle of 46 deg was approximately maintained at an even level by the counteracting effects of the leg decline and the heel rise. These results suggest that the minimum oxygen cost of transport during optimal walking was achieved by the mechanism underlying the maximum interchange between the gravitational-potential and kinetic energy for the body with an even level of the trunk that reduces extra muscular work needed against internal and external resistance, as well as against gravity.