The red muscles in dogfish are used during sustained swimming, and the white ones during burst swimming (Bone, 1966). The force-velocity relationship was studied in detail with the white muscle fibres (Curtin & Woledge, 1988). The present study investigates the contractile properties of bundles of isolated red muscle fibres from dogfish, Scyliorhinus canicula (L.). Donor animals were killed by a blow to the head followed by decapitation. The muscle bundle (mean cross-sectional area 0.359 ± 0.042 mm²) was stimulated tetanically at about 30 Hz every 5 min at optimal muscle length, which is 7.4 ± 0.3 mm. The sarcomere length is 2.32 ± 0.02 µm (n = 8). The slack-test method (Edman, 1979) was used to determine the shortening velocity at zero load, which was 1.70 ± 0.10 L₀ s^-1 (n = 6). (All reported values are means ± S.E.M. and the number of samples = 7, unless stated in the brackets.)

Force-velocity relationship was investigated using a step and ramp protocol. The muscle was shortened from L₀ with a step followed by a constant velocity ramp. Force was measured during the first part of the ramp shortening when the force was constant. The following equation was fitted to the force-velocity data:

((P/P₀) + (a/P₀)) (V + b) = ((P₀*/P₀) + (a/P₀)) b,

where P is force during shortening at velocity, V, P₀ is the isometric force before shortening, and a, b and P₀* are fitted constants (see Fig. 1). Our results show that the isometric force P₀ was 142.4 ± 10.3 kN m^-2; the maximum velocity of shortening V_max, equal to b P₀*/a, was 1.814 ± 0.071 L₀ s^-1; and the curvature of the relationship a/P₀* was 0.225 ± 0.024. The intercept P₀*/P₀ was 1.228 ± 0.053.

In three of the experiments, the fibre bundle was stretched at a constant velocity to investigate the negative velocity part of the force-velocity relationship. Force was measured when length reached L₀. The results are shown in Fig. 1. The mean force was 1.519 ± 0.032 P/P₀ (n = 3) during stretch and was independent of the velocity of stretch (-0.28 to -0.63 V/V_max).

The maximum power (force multiplied by velocity of shortening) was 0.107 ± 0.005 PV/P₀V_max which occurred at 0.297 ± 0.012 V/V_max.

Compared with white muscle fibres, the red muscle fibres have a lower P₀ (49 %), lower V_max (48 %), but the force-velocity curves are similar in terms of other fitting constants (P₀*/P₀ and a/P₀). This means that the red and the white muscles have equal capacities to produce power within the limits set by the isometric force and maximum velocity of shortening of each fibre type.

This work was supported by the BBSRC (UK).