Muscle activity in steady swimming scup, Stenotomus chrysops, varies with fiber type and body position

The red and pink aerobic muscle fibers are used to power steady swimming in fishes. We examined red and pink muscle recruitment and function during sw imming in scup, Stenotomus chrysops, through electromyography and high-spee d cine. Computer analysis of electromyograms (EMGs) allowed determination o f initial speed of muscle recruitment and duty cycle and phase of muscle el ectromyographic activity for both fiber types. This analysis was carried ou t for three longitudinal positions over a range of swimming speeds. Fiber t ype and longitudinal position both affected swimming speed of initial recru itment. Posterior muscle is recruited at the lowest swimming speed, whereas more anterior muscle is not initially recruited until higher speeds. At mo re anterior positions, the initial recruitment of pink muscle occurs at a h igher swimming speed than the recruitment of red muscle. The duty cycle of pink muscle EMG activity is significantly shorter than that of red muscle, reflecting a difference in the onset time of activation during each cycle o f length change: pink muscle onset time follows that of red. The different patterns of usage of red and pink muscle reflect differences in their contr action kinetics. Because pink muscle generates force more rapidly than red muscle, it can be activated later in each tailbeat cycle. Pink muscle is us ed to augment red muscle power production at higher swimming speeds, allowi ng a higher aerobically based steady swimming speed than that possible by r ed muscle alone.