3-DIMENSIONAL KINEMATIC ANALYSIS OF FROG HINDLIMB MOVEMENT IN REFLEX WIPING

The three-dimensional kinematics of the hindlimb back-wipe were examin ed in spinal frogs. The component movements were identified and the re lationship between stimulus position and hindlimb configuration was as sessed. 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 p laced in a position near to the stimulus; a pre-whisk phase (II), in w hich the endpoint of the foot moves away from the stimulus; and a whis k/extension phase (III), in which the stimulus is removed. The pre-whi sk phase contributes to force production for the whisk/extension (III) . In the placing phase a systematic relationship was found between lim b endpoint position and stimulus position in the rostro-caudal directi on. The hip, knee and metatarsal joint angles were related to the posi tion of the endpoint in the rostro-caudal direction. However, differen t frogs tended to adopt different strategies to remove the stimulus. I n 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 adju stment 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 functi on of medial-lateral stimulus placement. Thus, in order to remove the stimulus in different medial-lateral positions, the frog used a differ ent part of the foot rather than moving the foot in the direction of t he stimulus. In two frogs a relationship was observed between the elev ation of the femur and the medial-lateral stimulus position. The motio n planes of the hindlimb were studied by examining the instantaneous p lane of motion of the endpoint and the planes of motion of adjacent li mb 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 tib ia as observed for all phases. Systematic changes in the orientation o f these planes characterized the different phases. The position of the hindlimb was found to be variable prior to the placing phase. This va riability was not related to stimulus position. However, in trials wit h 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 en dpoint 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 endp oint position at the end of the placing phase.