Meeting Abstract
The diverse function of muscle during locomotion is now well appreciated, but determinants of functional changes remain elusive. While characterization of muscle function has moved from quasi-static to periodic conditions via workloops, function is still characterized primarily in unperturbed conditions. The Hill muscle model uses strain, velocity, and activation to predict force, but fails to predict history-dependent effects like shortening deactivation, which may play a larger role in functionality during perturbed or unsteady locomotion. To examine how nonlinearity and history-dependence could affect muscle function under perturbed conditions we modified the workloop approach to incorporate perturbations and replicated unsteady conditions in a fast-running cockroach, Blaberus discoidalis. We imposed cyclic oscillation consistent with running kinematics across the natural speed range (1-15 Hz) and added sinusoidal perturbations, either by superposition or by removing the underlying cycle so that the parameters of the Hill model were exactly matched during the perturbation even at different cycle frequencies. We found a non-linear effect of the underlying locomotor frequency on force and mechanical work production in the muscle whose effects could not be accounted for by passive material properties of the intact joint. In the Hill-type conditions we discovered a systematic increase in dissipation with frequency during swing phase perturbation and decrease in dissipation during stance phases. These results are consistent with limb perturbation studies that showed a more rapid perturbation recovery during swing at higher limb frequencies. Frequency dependent perturbation responses in muscle may be an integral part of adjusting neuromechanical control to changing speed.
