Mechanical work and efficiency in level walking and running

Abstract

The mechanical power spent to accelerate the limbs relative to the trunk in level walking and running, Wint, was measured [in humans] at various constant speeds (3-33 km/h) with the cinematographic procedure used by Fenn at high speeds of running. Wint increases approximately as the square of the speed of walking and running. For a given speed Wint is greater in walking than in running. In walking above 3 km/h, Wint is greater than the power spent to accelerate and lift the center of mass of the body at each step, Wext. In running Wint < Wext up to about 20 km/h, whereas at higher speeds Wint > Wext. The efficiency of positive work was measured from the ratio Wtot/Net energy expenditure: this is greater than 0.25 indicating that both in walking and in running the muscles utilize, during shortening, some energy stored during a previous phase of negative work (stretching). In walking the efficiency reaches a maximum (0.35-0.40) at intermediate speeds, as may be expected from the properties of the contractile component of muscle. In running the efficiency increases steadily with speed (from 0.45 to 0.70-0.80) suggesting that positive work derives mainly from the passive recoil of muscle elastic elements and to a lesser extent from the active shortening of the contractile machinery. These findings are consistent with the different mechanics of the 2 exercises.