Unlike most manmade machines, animals move through their world using flexible appendages, which bend due to internal muscle and body forces, but also due to forces from the environment. Fishes, in particular, must cope with fluid dynamic forces that not only resist their overall swimming movements but also may have unsteady flow patterns, vortices, and turbulence. We have been characterizing how the muscle tissue itself, due to its own intrinsic properties, is able to respond to perturbations. We have developed a modified work loop protocol to determine how muscle in the silver lamprey, Ichthyomyzon unicuspis, responds to perturbations during the swimming cycle. A small section of axial musculature, ~2 myomeres in length, was dissected and used to perform standard in vitro work loops. A 1Hz sinusoidal length change of 6% of the optimal length was imposed on the muscle and both active and passive force were measured. Then, small sinusoidal perturbations at different frequencies are added to the baseline length change. We find that the effective stiffness and damping of muscle varies during the swimming cycle, and that the timing of activation can alter both the magnitude and timing of peak stiffness and damping. The results are analyzed using a new system identification technique based on harmonic transfer functions, which allow us to use these data to predict the muscle function under other conditions. In particular, we are investigating how muscle behaves as part of a feedback loop, when coupled to other muscles and to the body and fluid. Together, these results are starting to produce an integrative understanding of how fish swim effectively in their complex, turbulent environment.