Recruitment Patterns and Contractile Properties of Fast Muscle Fibres Isolated from Rostral and Caudal Myotomes of the Short-horned Sculpin

Muscle action during swimming and the contractile properties of isolated muscle fibres were studied in the short-horned sculpin Myoxocephalus scorpius at 5 ̊C. Semi-steady swimming, startle responses and prey-capture events were filmed with a high-speed video at 200frames s21, using fish 22–26cm in total length (L). Electromyographical (EMG) recordings, synchronised with the video, were made from fast muscle in rostral and caudal myotomes at points 0.40L and 0.80L along the body. Fast muscle fibres were first recruited at tail-beat frequencies of 3.7–4.2Hz, corresponding to a swimming speed of 1.7 L s21. Electrical activity in the muscles occurred during 16–38% of each tail-beat cycle regardless of frequency. Muscle fibres were activated during the lengthening phase of the cycle. In caudal myotomes, the onset of the muscle activity occurred at a phase of 75–105 ̊ at 3.7Hz, decreasing to approximately 50 ̊ at frequencies greater than 4.5Hz (0 ̊ phase was defined as the point at which muscle fibres passed through their resting lengths in the stretch phase of the cycle; a full cycle is 360 ̊). Prey capture was a stereotyped behaviour consisting of a preparatory movement, a powerstroke at 7–9Hz and a glide of variable duration. The delay between the activation of muscle fibres in rostral and caudal myotomes during prey capture and startle responses was approximately 10ms. Fast muscle fibres isolated from rostral and caudal myotomes were found to have similar isometric contractile properties. Maximum tetanic stress was 220kN m22, and half-times for force development and relaxation were approximately 50ms and 135ms respectively. Power output was measured by the ‘work loop’ technique in muscle fibres subjected to sinusoidal length changes at the range of frequencies found during swimming. Under optimal conditions of strain and stimulation, muscle fibres from rostral and caudal myotomes produced similar levels of work (3.5 J kg21) and generated their maximum power output of 25–30 W kg21 at the tail-beat frequencies used in swimming (4–8Hz). Progressively delaying the onset of stimulation relative to the start of the strain