Summary

Bundles of red myotomal muscle fibres isolated from dogfish were electrically stimulated at 12 sC. Peak twitch force was 54 % of that produced by a brief isometric tetanus. Relaxation was slower than in white fibres, but much faster than would be expected for the tonic fibres found in amphibian muscle. These two results indicate that the red fibres in dogfish are slow, but not tonic, in their behaviour. Net work output and heat production were measured during complete cycles of sinusoidal movement. The following variables were kept constant: peak-to-peak movement, about 7 % of the muscle fibre length; tetanus duration, 33 % of the mechanical cycle time; stimulus frequency, 40 Hz. The frequency of movement and the timing of the stimulation were varied for each preparation to find the conditions optimal for power output and those optimal for efficiency (the ratio of net work output to total energy output as heat+work). To achieve either maximum power or maximum efficiency, the tetanus must start while the muscle fibres are being stretched, before the beginning of the shortening part of the mechanical cycle. The highest power output was produced during movement at 1.02 Hz. The highest efficiency, 0.507+/−0.045 (+/−s.e.m., N=9), was at 0.61-0.95 Hz. The efficiency is higher than that previously measured during sinusoidal movement of white fibres; the difference, 0.095+/− 0.045 (+/−s.e.m. of the difference, d.f. 20), is statistically significant at the 5 % level.