Ontogeny of postural adjustments during sitting in infancy: variation, selection and modulation.

1. The aim of the study was to find out whether the development of postural adjustments occurs via a coupling of simple muscle responses, such as stretch reflexes, or via selection from an innate repertoire of centrally generated response patterns. 2. Postural responses during sitting on a moveable platform were assessed in eleven healthy infants at 5-6, 7-8 and 9-10 months of age. Multiple surface EMGs and kinematics were recorded while the infants were exposed to slow and fast horizontal forward (Fw) and backward (Bw) displacements of the platform. 3. From the youngest testing age onwards, largely variable but direction-specific muscle activation patterns were present. Fw translations resulted predominantly in an activation of the neck flexor, the rectus abdominis and rectus femoris muscle, while the neck-, thoracal- and lumbar extensor muscles (NE, TE, LE) and the hamstrings (Ham) showed varying amounts of inhibition. During Bw translations NE, TE, LE and Ham were preferably activated. The muscle activity could not be explained by simple stretch reflex mechanisms, but is likely to reflect centrally generated motor activity maturing in a predetermined way. However, indications for a contribution of stretch reflex mechanisms were also present. 4. With increasing age the variation in muscle activation patterns decreased, resulting in a selection of the most complete patterns. The ability to modulate the amplitude of the selected, most complete patterns during Fw translations, with respect to platform velocity and initial pelvis position, emerged at 9-10 months.