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Istituto di Fisiologia Umana dell'Università di Milano

Centro di Studio per la Fisiologia del Lavoro Muscolare del C.N.R., 20133 Milano, Italy

1. The mechanical power spent to accelerate the limbs relative to the trunk in level walking and running, _int, has been measured at various `constant' speeds (3-33 km/hr) with the cinematographic procedure used by Fenn (1930a) at high speeds of running.

2. _int increases approximately as the square of the speed of walking and running. For a given speed _int is greater in walking than in running.

3. In walking above 3 km/hr, _int is greater than the power spent to accelerate and lift the centre of mass of the body at each step, _ext (measured by Cavagna, Thys & Zamboni, 1976b). In running _int < _ext up to about 20 km/hr, whereas at higher speeds _int > _ext.

4. The total work done by the muscles was calculated as W_tot = |W_int| + |W_ext|. Except that at the highest speeds of walking, the total work done per unit distance W_tot/km is greater in running than in walking.

5. The efficiency of positive work was measured from the ratio W_tot/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).

6. 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 two exercises.

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