TRANSIENT DISTURBANCES TO ONE LIMB PRODUCE COORDINATED, BILATERAL RESPONSES DURING INFANT STEPPING

Transient disturbances were applied to the lower limbs of infants (3-1 0 mo of age) while they were supported to stepped on a treadmill. The aim was to determine how stepping infants respond to novel disturbance s that would disrupt equilibrium during independent walking. Their res ponses were also compared with those from lower mammals and adult huma ns. In the first series of experiments, the motion of the limb in the swing phase was transiently stopped by the experimenter grasping the l imb for a short time (0.1-1.7 s). During such disturbances, the stance phase was prolonged in the contralateral limb, and the onset of the s wing phase was delayed. The degree to which the stepping was modified in the contralateral limb depended on the amount of load experienced b y that limb. If the contralateral limb was bearing very little weight at the time of the disturbance, its rhythm did not change appreciably. In the second series of experiments, load was added to the infant by pushing down on the pelvis during the stance phase. This greatly prolo nged the stance phase and delayed the swing phase. It did not increase the amplitude of the extensor electromyogram (EMG) of the loaded limb . In conclusion, the neural circuitry controlling stepping in the infa nts responds to disturbances in an organized fashion that is conducive to maintaining equilibrium and forward progression.