This study examined stumbling corrective (tripping) responses to mechanical disturbances applied to the foot during stepping in healthy human infants, in whom independent walking had not yet developed. During treadmill-elicited stepping, a foam-padded baton instrumented with a force transducer was used to deliver light touches to either the dorsum or the side of the foot at various times of the step cycle. Disturbances to the dorsum of the foot during the swing phase resulted in a general enhancement of flexor activity, including a significant facilitation in the tibialis anterior muscle and an increase in knee flexion during swing. There was also an increase in step cycle duration. Stance phase disturbances resulted in a significant inhibitory effect in the quadriceps muscle and after early stance disturbances, a significant prolongation of the stance phase duration. Disturbances applied to the side of the foot during the swing phase of forward stepping did not result in any effect on the kinematic pattern. If the infant was stepping sideways, however, disturbances to the side of the foot resulted in an increase in flexion during the swing phase. The results show that infants have a phase-dependent reflex response to light touches applied to the foot. Furthermore, the reflex response is location specific and task specific. Thus, much of the sophistication necessary for controlling sensory inputs in walking is present well before the onset of independent walking.
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