Meeting Abstract
Muscle’s intrinsic properties can stabilize movement through the length- and velocity-dependence encompassed in Hill-like models, but these models fail to predict history-dependent effects which may play a larger role in function during perturbed or unsteady locomotion. We examined whether muscle function varied during identical rapid perturbations occurring at different frequencies of movement. We modified the workloop approach to incorporate perturbations and replicated unsteady conditions in a fast-running cockroach, Blaberus discoidalis. Prior to perturbation, we imposed length oscillations consistent with running kinematics across the natural speed range (1-20 Hz). We imposed rapid stretch perturbations (1/10th preferred stride period, 6% muscle length) against this varying time-history of movement. The muscle experienced the same kinematics and stimulation during all perturbations, thereby isolating the effect of history. The Hill model would predict identical perturbation responses across all conditions. By contrast, we found that the energy dissipated by the muscle during these identical perturbations varied almost 10-fold across cycle frequencies. While muscle behavior appeared viscoelastic during the stretch, we did not observe a consistent relationship between force history and muscle stiffness. Instead, we found a shift in force offset during the perturbation that correlated strongly to muscle force during the cyclic history prior to the stretch. The effects are partly explained by adding a series elastic element to the Hill model. Our results suggest that as muscle force varies across behavioral context (e.g. from slow to fast running), perturbations are met with a mechanical response that matches the motor load.
