This study examined whether human infants under the age of 12 mo learn to modify their stepping pattern after repeated trip-inducing stimuli. Thirty three infants aged from 5 to 11 mo were studied. The infants were held over a moving treadmill belt to induce stepping. Occasionally, a mechanical tap was applied to the dorsum of the left foot during the early swing phase to elicit a high step. In some trials, the stimulus was applied for only one step. In other trials, the foot was stimulated for a few consecutive steps. We determined whether the infants continued to show high stepping immediately after the removal of the stimuli. The results showed that after the foot was touched for two or more consecutive steps, some infants continued to demonstrate high stepping for a few steps after the removal of the stimuli (i.e., aftereffect). Such adaptation was achieved by an increase in hip and knee flexor muscle torque, which led to greater hip and knee flexion during the early swing phase. Aftereffects were more commonly seen in older infants (9 mo or older). The results indicated that before the onset of independent walking, the locomotor circuitry in human infants is capable of adaptive locomotor plasticity. The increased incidence of aftereffect in older infants also suggests that the ability to adapt to repeated trip-inducing stimuli may be related to other factors such as experience in stepping and maturation of the nervous system.
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