Ontogeny of human locomotor control. I. Infant stepping, supported locomotion and transition to independent locomotion.
Locomotor patterns of human infants were studied during stepping in the newborn period (first two months of life), during supported locomotion (6-12 months of age) and during independent locomotion in children who just were able to walk by themselves without external support (10-18 months of age). Leg movements, pattern of muscular activity and reaction forces were studied by a computerized system. The locomotor pattern during the newborn period lacked the specific functions that are unique for human plantigrade locomotion. There was no heel strike in front of the body; the foot was placed instead on its forepart straight under the body. Hip and knee joints were hyperflexed during the whole step cycle and flexed synchronously during swing. The specific knee-ankle coordination of human adults was missing. The ankle extensors were activated prior to touch down together with other extensor muscles. There was no propulsive force. A similar immature non-plantigrade pattern recurred after an inactive period. During the subsequent period of supported locomotion there was a gradual transformation of the infantile pattern towards the plantigrade pattern continuing after establishment of independent locomotion. It is suggested that innate pattern generators in the spinal cord produce the infant stepping and also generate the basic locomotor rhythm in adults, but that neural circuits specific for humans develop late in ontogeny and transform the original, non-plantigrade motor activity to a plantigrade locomotor pattern.