Sensory gating for the initiation of the swing phase in different directions of human infant stepping.

Humans can make smooth, continuous transitions in walking direction from forward to backward. Thus, the processing of sensory input must allow a similar continuum of possibilities. Hip extension and reduced load are two important conditions that control the transition from the stance to swing phase during forward stepping in human infants. The purpose of this study was to determine whether the same factors also regulate the initiation of the swing phase in other directions of stepping. Thirty-seven infants between the ages of 5 and 13 months were studied during supported forward and sideways stepping on a treadmill. Disturbances were elicited by placing a piece of cardboard under the foot and pulling the cardboard in different directions. In this way, the leg was displaced in a particular direction and simultaneously unloaded. We observed whether the swing phase was immediately initiated after the application of disturbances in various directions. Electromyography, vertical ground reaction forces, and hip motion in frontal and sagittal planes were recorded. The results showed that the most potent sensory input to initiate the swing phase depends on the direction of stepping. Although low load was always necessary to initiate swing for all directions of walking, the preferred hip position was always one directly opposite the direction of walking. The results indicated the presence of selective gating of sensory input from the legs as a function of the direction of stepping.