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 pos tural adjustments occurs via a coupling of simple muscle responses, su ch as stretch reflexes, or via selection from an innate repertoire of centrally generated response patterns. 2. Postural responses during si tting on a moveable platform were assessed in eleven healthy infants a t 5-6, 7-8 and 9-10 months of age. Multiple surface EMGs and kinematic s were recorded while the infants were exposed to slow and fast horizo ntal 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 resu lted predominantly in an activation of the neck flexor, the rectus abd ominis and rectus femoris muscle, while the neck-, thoracal- and lumba r extensor muscles (NE, TE, LE) and the hamstrings (Ham) showed varyin g amounts of inhibition. During Bw translations NE, TE, LE and Ham wer e preferably activated. The muscle activity could not be explained by simple stretch reflex mechanisms, but is likely to reflect centrally g enerated motor activity maturing in a predetermined way. However, indi cations for a contribution of str etch reflex mechanisms were also pre sent. 4. With increasing age the variation in muscle activation patter ns decreased, resulting in a selection of tile 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.