Electromyographic study of newborn stepping in neonates and young infants.

The purpose of this study was to examine the developmental changes in the functional mechanisms of leg muscles in newborn stepping over the first four months in ten normal neonates. Neonatal stepping in the first month showed excessive coactivation, that is, co-contraction patterns of mutual antagonists appeared especially during stance phase. The discharge patterns of co-contraction in neonatal stepping began to change to reciprocal patterns in young infant stepping (after the first month), but excessive muscular activities associated with a slightly squatted posture and forward lean still remained. Strong muscle activities of leg extensors due to a parachute reaction of the legs before floor contact, not seen in the neonatal period, began to appear in the young infant period from one month of age to three months. We suggest that these gradual changes of leg muscular activity in newborn stepping are evoked by development of balance, postural control, and strength, thereby modulating the neonatal stepping reflex.