Coupled and uncoupled limb oscillations during paw-shake response

Abstract

Intersegmental limb dynamics and muscle activities were analyzed for consecutive cycles of paw-shake responses from chronic-spinalized cats to investigate how hindlimb trajectories organize into a pattern with regular oscillations, a steady-state response, or alternatively, into a pattern with irregular oscillations, a nonsteady-state response. In the spinalized preparation, steady-state and nonsteady-state responses have an equal likelihood of emerging from the initial cycles of a paw-shake response, suggesting that regular coupling of joint oscillations is not planned by pattern-generating networks within lumbosacral segments. To examine the characteristics of coupled and uncoupled limb oscillations during paw-shake responses, we assessed patterns of muscle activity and hindlimb kinematics of six adult chronic-spinalized cats. Additionally, we used inverse-dynamics techniques to quantify the intersegmental dynamics of the paw, leg, and thigh. Our data indicate that by the second cycle of both steady-state and nonsteady-state responses, the basic pattern of interaction between muscle and motion-dependent torques at the ankle and knee joints was established. During subsequent cycles of steady-state responses, a consistent sequence of timing changes occurred, such that, just prior to steady-state oscillations, torque maximums peaked simultaneously at each joint and joint reversals occurred simultaneously. Although nonsteady-state responses showed a similar sequence during beginning cycles, increased ankle muscle and net torques during middle cycles created larger inertial torques at the knee joint that were not counteracted and resulted in irregular and uncoupled knee oscillations. It is likely that neither steady-state nor nonsteady-state oscillations are planned by pattern-generating networks within lumbosacral segments, but that patterns of interjoint coordination emerge from the coupling among oscillators. For paw-shake responses in the spinalized preparation, coupling may depend on interactions between central circuits and motion-dependent feedback that is necessary to stabilize inertial effects due to large ankle joint accelerations.