SYME, D.A.*; SHADWICK, R.E.: Mechanical Power Production by Internal Red Muscle at Different Longitudinal Body Positions in Skipjack Tuna (Katsuwonus pelamis) in Relation to Swimming
The work loop technique was used to study power output by internal red muscle from skipjack tuna, to understandthe primary function of this muscle in swimming, how different strain amplitudes at different muscle locations affect performance, and how work and power output are affected by swim speed. Red muscle was isolated from anterior (0.4L) and posterior (0.7L) positions (L=total body length) and subjected to sinusoidal length changes and stimulus parameters determined previously from swimming fish or those producing maximal net work, at 25oC. The optimum activation phase and duration decreased with increasing frequency, but were the same for anterior and posterior muscle. At test frequencies corresponding to tail beat frequencies used in steady swimming (3-5Hz) near maximal positive work was produced when the strain and activation conditions measured in swimming fish were used. Net work per cycle peaked at 2-3Hz and dropped to zero above 15 Hz. Maximum power occurred at 5Hz, but power production remained substantial beyond 10 Hz. Compared to anterior muscle, posterior fibers had higher power output (133 vs 79 W/kg), significantly shorter twitch rise times (60 vs 80 ms), but similar relaxation times. The close agreement between strain amplitude and activation parameters used in vivo and those producing maximum work per cycle in vitro, and the match between the frequency for peak power and tail beat frequencies used for cruising suggest that tunas use red muscle all along the body for maximal power generation during steady swimming. The maintained power production at higher frequencies suggests that red muscle may also contribute power during fast swimming.
