Muscles, Elastic Energy, and the Dynamics of Body Stiffness in Swimming Eels

SYNOPSIS. TO investigate the capacity of the myomeric muscles to actively change the stiffness of the body during bending, mid-caudal sections (spanning two to three intervertebral joints) of intact, freshly-killed American eels, Anguilla rostrata, were dynamically bent (3 Hz, ±4% maximal muscle strain) using the whole-body work- loop technique. Following unstimulated cycles, the left- and right-sides of the mus- culature were alternately stimulated at supra-maximal voltages and at eight dif- ferent phases relative to the strain cycle of the muscle. The body's flexural stiffness (Nm2) increased maximally by a factor of three relative to that when the muscles were unstimulated. The net external work (J kg"1) needed to bend the body de- creased maximally by a factor of seven relative to that when the muscles were unstimulated. Both of these mechanical features varied sinusoidally with changes in the phase of the stimulus. Stimulus phase of caudal muscle in live swimming eels, taken from other studies, leads to the prediction that the caudal muscles increase body stiffness and produce net positive mechanical work simultaneously during steady forward swimming. The association of increased body stiffness and net positive muscle work, and the occurrence of maximal net muscle power output at a stimulus phase of 325° (as the muscle segment lengthens), suggests that net positive power is produced, in part, using an elastic strain energy mechanism.