The effects of the level of activation and shortening velocity on energy output in type 3 muscle fibres from Xenopus laevis

We investigated the effect of shortening velocity on the efficiency of single intact slow-twitch muscle fibres (type 3) of Xenopus laevis, at different levels of activation (10, 15, 20 and 40 Hz). Fused contractions were obtained at 40 Hz stimulation. When maximal isometric force had been reached, the fibres were shortened by 10% of the fibre length (L(0)) at 0.4, 1 and 2 L(0)/s. To investigate whether the oscillating force at low stimulation frequencies influenced power output and the rate of heat production, we also performed these experiments with the fibre bathed in dantrolene. The results with fused contractions in the presence of dantrolene were the same as with unfused contractions. At 40 Hz stimulation during shortening the rate of heat production increased above that measured during isometric contractions, while at the lower stimulation frequencies the rate of heat production was less than that during isometric contractions. Mechanical efficiency was highest at low activation, and increased more with shortening velocity than at high activation. The actomyosin efficiency (i.e. the efficiency corrected for ''activation heat'') was also highest at 10 Hz stimulation. We conclude that in slow-twitch muscle fibres of X. laevis, near the optimum shortening velocity, cross-bridge efficiency is highest for partially activated muscle.