We have examined the efficiency of vertebrate striated muscle at two different organizational levels: whole animals and isolated muscles. Terrestrial locomotion is used as a model of ‘normal’ muscular contraction; animal size and running speed are used as independent variables in order to change either the metabolic requirements of the muscles or the mechanical power production by the muscles over a wide range of values. The weight-specific metabolic power input to an animal increases nearly linearly with speed and increases with decreasing body size, while the weight-specific mechanical power output increases curvilinearly with speed and is independent of size. Consequently, the efficiency of the muscles in producing positive work increases with speed and the peak efficiency increases with increasing body size, attaining values of over 70% in large animals, but only 7% in small ones. The isolated muscle experiments were performed on frog muscle, and rat ‘fast’ and ‘slow’ muscles. We measured the work done, the oxygen consumed during recovery from the stimulation, and calculated the efficiency and the ‘economy’ (the cost of maintaining tension). The muscles were made to: (i) emulate the contractions seen during locomotion, i.e. shorten after a pre-stretch; or (ii) shorten at the same velocity and from the same muscle length as in (i), but without the pre-stretch. It was found that in mammalian muscles the peak efficiency with a pre-stretch attained high values, approaching the peak efficiencies in large animals. The maximum efficiency (attained at 1 length s-1 in fast muscle and at 0.5 lengths s-1 in slow muscle) did not differ much in the two muscles, whereas economy was greater in the slow muscle than in the fast muscle.

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