Three-dimensional kinematic analysis of frog hindlimb movement in reflex wiping

The three-dimensional kinematics of the hindlimb back-wipe were examined in spinal frogs. The component movements were identified and the relationship between stimulus position and hindlimb configuration was assessed. The planes of motion of the hindlimb were examined throughout the movement. The backwipe comprises three essential phases: a placing phase (I), in which the foot is drawn over the back of the frog and placed in a position near to the stimulus; a pre-whisk phase (II), in which the endpoint of the foot moves away from the stimulus; and a whisk/extension phase (III), in which the stimulus is removed. The pre-whisk phase contributes to force production for the whisk/extension (III). In the placing phase a systematic relationship was found between limb endpoint position and stimulus position in the rostro-caudal direction. The hip, knee and metatarsal joint angles were related to the position of the endpoint in the rostro-caudal direction. However, different frogs tended to adopt different strategies to remove the stimulus. In one strategy, when the knee angle was strongly related to the rostro-caudal stimulus position, the metatarsal angle was weakly related and vice versa. Other strategies were observed as well. There was no adjustment in limb endpoint position for stimulus placement in the medial-lateral direction. Consistent with this finding, the point on the foot at which stimulus contact occurred changed systematically as a function of medial-lateral stimulus placement. Thus, in order to remove the stimulus in different medial-lateral positions, the frog used a different part of the foot rather than moving the foot in the direction of the stimulus. In two frogs a relationship was observed between the elevation of the femur and the medial-lateral stimulus position. The motion planes of the hindlimb were studied by examining the instantaneous plane of motion of the endpoint and the planes of motion of adjacent limb segments. The motion of the endpoint was found not to be planar in any phase of the wipe. In contrast, planar motion of the femur and tibia was observed for all phases. Systematic changes in the orientation of these planes characterized the different phases. The position of the hindlimb was found to be variable prior to the placing phase. This variability was not related to stimulus position. However, in trials with multiple wipes, once an initial limb configuration was assumed, the limb returned to this configuration before each wipe in the sequence. Evidence for motor equivalence was sought in two ways. The pattern of hindlimb joint angles corresponding to a fixed position of the limb endpoint was examined, and the variability of the endpoint positions was examined for fixed stimulus positions. It was found that for a given endpoint position there was little variation in joint angles. However, for a fixed stimulus position there was greater variation in the endpoint position at the end of the placing phase.