MYOTOMAL MUSCLE FUNCTION AT DIFFERENT LOCATIONS IN THE BODY OF A SWIMMING FISH

We describe experiments on isolated, live muscle fibres which simulate their in vivo activity in a swimming saithe (Pollachius virens). Supe rficial fast muscle fibres isolated from points 0.35, 0.5 and 0.65 bod y lengths (BL) from the anterior tip had different contractile propert ies. Twitch contraction time increased from rostral to caudal myotomes and power output (measured by the work loop technique) decreased. Pow er versus cycle frequency curves of rostral fibres were shifted to hig her frequencies relative to those of caudal fibres. In the fish, phase differences between caudally travelling waves of muscle activation an d fish bending suggest a change in muscle function, along the body. In vitro experiments indicate that in vivo superficial fast fibres of ro stral myotomes are operating under conditions that yield maximum power output. Caudal myotomes are active as they are lengthened in vivo and initially operate under conditions which maximise their stiffness, be fore entering a positive power-generating phase. A description is pres ented for the generation of thrust at the tail blade by the superficia l, fast, lateral muscle. Power generated rostrally is transmitted to t he tail by stiffened muscle placed more caudally. A transition zone be tween power generation and stiffening travels caudally, and all but th e most caudal myotomes generate power at some phase of the tailbeat. R ostral power output, caudal force, bending moment and force at the tai l blade are all maximal at essentially the same moment in the tailbeat cycle, as the tail blade crosses the swimming track.