Spatial variation in fast muscle function of the rainbow trout Oncorhynchus mykiss during fast-starts and sprinting

Fish fast-starts and sprints are rapid kinematic events powered by the late ral myotomal musculature. A distinction can be made between fast-starts and sprint-swimming activity. Fast-starts are kinematic events involving rapid , asymmetrical movements. Sprints involve a series of symmetrical, high-fre quency tailbeats that are kinematically similar to lower-frequency, sustain ed swimming. The patterns of muscle recruitment and strain associated with these swimming behaviours were determined using electromyography and sonomi crometry. Axial patterns of fast muscle recruitment during sprints were sim ilar to those in slow muscle in that the duration of electromyograhic (EMG) activity decreased in a rostrocaudal direction. There was also an axial sh ift in activity relative to the strain cycle so that activity occurred rela tively earlier in the caudal region. This may result in caudal muscle perfo rming a greater proportion of negative work and acting as a power transmitt er as well as a power producer. The threshold tailbeat frequency for recrui tment of fast muscle differed with location in the myotome, Superficial mus cle fibres were recruited at lower tailbeat frequencies and shortening velo cities than those deeper in the musculature, During sprints, fast muscle st rain ranged from +/-3.3% l(0) (where lo is muscle resting length) at 0.35FL (where FL is fork length) to +/-6.3% l(0) at 0.65FL, Fast-starts involved a prestretch of up to 2.5% l(0) followed by shortening of up to 11.3% l(0), Stage 1 EMG activity began simultaneously, during muscle lengthening, at a ll axial locations. Stage 2 EMG activity associated with the major contrala teral contraction also commenced during lengthening and proceeded along the body as a wave. Onset of muscle activity during lengthening may enhance mu scle power output.