Summary

Fast muscle fibres were isolated from abdominal myotomes of Atlantic cod (Gadus morhua L.) ranging in size from 10 to 63 cm standard length (L_s). Muscle fibres were subjected to sinusoidal length changes about their resting length (L_f) and stimulated at a selected phase of the strain cycle. The work performed in each oscillatory cycle was calculated from plots of force against muscle length, the area of the resulting loop being net work. Strain and the number and timing of stimuli were adjusted to maximise positive work per cycle over a range of cycle frequencies at 8°C.

Force, and hence power output, declined with increasing cycles of oscillation until reaching a steady state around the ninth cycle. The strain required for maximum power output (W_max) was ±7-11% of L_f in fish shorter than 18 cm standard length, but decreased to ±5 % of L_f in larger fish. The cycle frequency required for W_max also declined with increasing fish length, scaling to L_s^−0.51 under steady-state conditions (cycles 9–12). At the optimum cycle frequency and strain the maximum contraction velocity scaled to L_s^−0.79. The maximum stress (P_max) produced within a cycle was highest in the second cycle, ranging from 51.3 kPa in 10 cm fish to 81.8 kPa in 60 cm fish (P_max=28.2L_s^0.25). Under steady-state conditions the maximum power output per kilogram wet muscle mass was found to range from 27.5 W in a 10 cm L_s cod to 16.4 W in a 60 cm L_s cod, scaling with L_s^−0.29 and body mass (M_b)^−0.10

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