Scaling of muscle performance during escape responses in the fish myoxocephalus scorpius L

Fast-starts associated with escape responses were studied in short-horn sculpin (Myoxocephalus scorpius L.), ranging from 5.5 to 32 cm in total length (L). Electromyography and sonomicrometry were used simultaneously to measure muscle activation and length changes, respectively, in the superficial layers of fast muscle in rostral myotomes. Escape responses consisted of a half tailbeat to bend the body into a C-shape (C-bend), another half tailbeat (contralateral contraction), followed by one or two more tailbeats and/or a gliding phase. The scaling relationships for both muscle strain and shortening duration differed between the C-bend and the contralateral contraction. As a result, relative muscle shortening velocity (V/V0) scaled as -1.18L1.06 for the C-bend and as 1.23L-0. 66 for the contralateral contraction. Therefore, the scaling relationships for muscle shortening velocity varied throughout the time course of the escape response. Muscle power output was determined by using the work-loop technique to subject isolated muscle fibres to in vivo strain and stimulation patterns. Plots of the instantaneous muscle forces and velocities achieved during the contralateral contraction were found to deviate from the steady-state force-velocity relationship. Maximum instantaneous muscle power output was independent of body size, with mean maximum values of 307 and 222 W kg-1 wet muscle mass for the C-bend and the contralateral contraction, respectively.