HOW SWIMMING FISH USE SLOW AND FAST MUSCLE-FIBERS - IMPLICATIONS FOR MODELS OF VERTEBRATE MUSCLE RECRUITMENT

We quantified the intensity and duration of electromyograms (emgs) fro m the red and white axial muscles in five bluegill sunfish (Lepomis ma crochirus) which performed three categories of behavior including stea dy swimming and burst and glide swimming at moderate and rapid speeds. Steady swimming (at 2 lengths/s) involved exclusively red muscle acti vity (mean posterior emg duration = 95 ms), whereas unsteady swimming utilized red and white fibers with two features of fiber type recruitm ent previously undescribed for any ectothermic vertebrate locomotor mu scle. First, for moderate speed swimming, the timing of red and white activity differed significantly with the average onset time of white l agging behind that of red by approximately 40 ms. The durations of the se white emgs were shorter than those of the red emgs (posterior mean = 82 ms) because offset times were effectively synchronous. Second, co mpared to steady and moderate speed unsteady swimming, the intensity o f red activity during rapid unsteady swimming decreased while the inte nsity of white muscle activity (mean white emg duration = 33 ms) incre ased. Decreased red activity associated with increased white activity differs from the general pattern of vertebrate muscle recruitment in w hich faster fiber types are recruited in addition to, but not to the e xclusion of, slower fiber types.