RECRUITMENT PATTERNS AND CONTRACTILE PROPERTIES OF FAST MUSCLE- FIBERS 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-homed sculpin Myoxocephalus scorpius at 5-degrees-C. Semi-steady swimming, startle responses and prey-capture events were filmed with a high-speed video at 200 frames s-1, using fish 22-26 cm 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 3.7-4.2 Hz, corresponding to a swimming speed of 1.7 L s-1. 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-degrees at 3.7 Hz, decreasing to approximately 50-degrees at frequencies greater than 4.5 Hz (0-degrees 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-degrees). Prey capture was a stereotyped behaviour consisting of a preparatory movement, a powerstroke at 7-9 Hz 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 10 ms. Fast muscle fibres isolated from rostral and caudal myotomes were found to have similar isometric contractile properties. Maximum tetanic stress was 220 kN m-2, and half-times for force development and relaxation were approximately 50 ms and 135 ms 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 kg-1) and generated their maximum power output of 25-30 W kg-1 at the tail-beat frequencies used in swimming (4-8 Hz). Progressively delaying the onset of stimulation relative to the start of the strain cycle resulted in an initial modest increase, followed by a decline, in the work per cycle. Maximum positive work and net negative work were done at stimulus phase values of 20-50-degrees and 120-140-degrees respectively. The EMG and swimming studies suggest that fast muscle fibres in both rostral and caudal myotomes do net positive work under most conditions.