Kinematics and control of frog hindlimb movements.

1. The determinants of the motion path of the hindlimb were explored in both intact and spinal frogs. In the spinal preparations the kinematic properties of withdrawal and crossed-extension reflexes were studied. In the intact frog the kinematics of withdrawal and swimming movements were examined. Frog hindlimb paths were described in joint angle (intrinsic) coordinates rather than limb endpoint (extrinsic) coordinates. 2. To study withdrawal and crossed-extension reflexes, the initial angles at the hip, knee, and ankle were varied. Withdrawal and crossed extension were recorded in three dimension (3-D) with the use of an infra-red spatial imaging system. Swimming movements against currents of different speeds were obtained with high-speed film. 3. Three strategies were considered related to the form of the hypothesized equilibrium paths specified by the nervous system: all trajectories lie on a single line in angular coordinates; all trajectories are directed toward a common final position; and all trajectories have the same direction independent of initial joint configuration. 4. Joint space paths in withdrawal were found to be straight and parallel independent of the initial joint configuration. The hip and knee were found to start simultaneously and in 75% of the conditions tested to reach maximum velocity simultaneously. Hip-knee maximum velocity ratios were similar in magnitude over differences in initial joint angles. This is consistent with the observation of parallel paths and supports the view that the nervous system specifies a single direction for equilibrium trajectories. 5. Straight line paths with slopes similar to those observed in withdrawal in the spinal preparation were found in swimming movements in the intact frog. Straight line paths in joint space are consistent with the idea that swimming and withdrawal are organized and controlled in a joint-level coordinate system. The similarities observed between spinal and intact preparations suggest that a common set of constructive elements underlies these behaviors. 6. Path curvature was introduced when joint limits were approached toward the end of the movement. Depending on the initial joint angles, the joint movements ended at different times. When initial joint angles were unequal, joints moving from smaller initial angles reached their functional limits earlier and stopped first. 7. In withdrawal and crossed extension in the spinal frog, velocity profiles at a given joint were similar over the initial portion of the curve for movements of different amplitude. This is consistent with the idea that withdrawal and crossed-extension movements of different amplitude are produced by a constant rate of shift of the equilibrium position.