Power Output of Fish Muscle Fibres Performing Oscillatory Work: Effects of Acute and Seasonal Temperature Change

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Abstract
Fast muscle fibres were isolated from the abdominal myotomes of the shorthorned sculpin Myoxocephalus scorpius L. Sinusoidal length changes were imposed about resting muscle length and fibres were stimulated at a selected phase during the strain cycle. The work output per cycle was calculated from the area of the resulting force-position loops. The strain amplitude required for maximum work per cycle had a distinct optimum at ±5 % of resting length, which was independent of temperature. Maximum positive work loops were obtained by retarding the stimulus relative to the start of the length-change cycle by 30° (full cycle=360°). The maximum negative work output was obtained with a 210° stimulus phase shift. At intermediate stimulus phase shifts, work loops became complex with both positive (anticlockwise) and negative (clockwise) components. The number and timing of stimuli were adjusted, at constant strain amplitude (±5% of resting muscle length), to optimize net positive work output over a range of cycle frequencies. The cycle frequency required for maximum power output (work per cycle times cycle frequency) increased from around 5–7 Hz at 4°C to 9–13 Hz at 15°C. The maximum tension generated per cycle at 15°C was around two times higher at all cycle frequencies in summer-relative to winter-acclimatized fish. Fast muscle fibres from summer fish produced consistently higher tensions at 4°C, but the differences were only significant at 15 Hz. Acclimatization also modified the relationship between peak length and peak force at 4°C and 15°C. The maximum power output of muscle fibres showed little seasonal variation at 4°C and was in the range 20–25 W kg−1. In contrast, at 15°C, maximum muscle power output increased from 9 W kg−1 in the winter- to 30 W kg−1 in the summeracclimatized fish