Effect of stimulus frequency and duty cycle on force and work in fish muscle

Abstract

Stimulus trains of varying pulse number and pulse frequency were applied to the isolated pectoral adductor muscle of the sunfish Lepomis gibbosus. The number of pulses varied from 1 to 70, and the pulse frequency varied from 10 to 500 Hz. Muscle was continuously cycled at 2 Hz, and work was measured directly. Maximum force (177 kN/m2) was achieved with a train of 30 pulses at 169 Hz. Maximum work of 9.1 J/kg (power of 18.2 W/kg) was achieved with a train of 30 pulses at 169 Hz and a duty cycle (fraction of the cycle during which the muscle is activated) of 35%. Work values increased with an increase in the number of pulses per train, up to a maximum of 30 pulses. Duty cycle was the most important parameter in obtaining maximum work. The effect of varying stimulus pulse frequency on work values could not be accurately predicted from force values alone. For example, work increased 8-fold from twitch to maximum, whereas force increased only 2.6-fold.