10. Work, Efficiency and Elastic Recovery

This chapter describes how speed, age, body mass and gravity affect work and efficiency during locomotion. In adult humans and in children the efficiency increases with running speed up to values well above the maximal efficiency of muscle contractile machinery suggesting elastic recovery. According to the spring-mass model of running, a reciprocal relationship is found between power spent against gravity and step frequency resulting in a lower external power in children; their higher step frequency however involves a greater internal power with the result that mass-specific power and efficiency are about the same as in adults. Similarly, in old subjects, a reduction of the vertical push during the running step causes, as in the youngest, a lower power spent against gravity, but a greater step frequency and internal power. The well known increase in efficiency of animal locomotion with increasing body mass is found to coincide with a decrease of elastic hysteresis in the stretch-shorten cycle during the rebound of running, trotting and hopping animals of different size. An increase in gravity causes a proportional increase of external work done by running humans and expands the range of speeds where the rebound is on-off ground symmetric. In sprint running the average power appears to be sustained by the contractile component at low speeds and, for an appreciable fraction, by elastic recovery at high speeds. The role of contractile component and elastic recoil is described during vertical jumps off both feet of different amplitude and under different simulated gravity values.