Postural adjustments during spontaneous and goal-directed arm movements in the first half year of life.

We studied the development of postural control during goal-directed reaching and spontaneous arm movements in early infancy. Two groups of infants participated. The first group consisted of 10 healthy infants, who were assessed four times at the ages of 3, 4, 5 and 6 months. Each assessment consisted of simultaneous recording of video-data and surface EMGs of arm, neck, trunk, and leg muscles in various lying and sitting positions. Additionally, postural adjustments during spontaneous arm movements were studied in a second group of five infants aged 1-3 months. Already before the onset of successful reaching, which occurred at 4 5 months, both spontaneous and goal-directed arm movements were accompanied by a high amount of postural activity. During the goal-directed arm movements a preference for neck muscle activation and a direction specific organisation (dorsal postural muscles activated before the ventral antagonists) prevailed, whereas during spontaneous arm movements such a specific postural organisation was absent. With increasing age and concurrent with successful reaching, the amount of postural activity decreased. Still, the persisting postural activity continued to become more organised with increasing age. Position affected the postural adjustments accompanying goal-directed arm movements at all ages.

The development of postural adjustments during reaching in 6- to 18-month-old infants. Evidence for two transitions.

The present study focused on the developmental changes of postural adjustments accompanying reaching movements in healthy infants. We made a longitudinal study of ten infants between 6 and 18 months of age. During each session multiple surface electromyograms of arm, neck, trunk and leg muscles at the right side of the body were recorded during right-handed reaching movements in two positions ("upright sitting" in an infant chair and "long-leg" sitting without support). Simultaneously the whole session was recorded on video. Comparable data were present from the same infants at 3-5 months. Additionally, 18 infants (8-15 months) were assessed once during similar reaching tasks, but in these infants electromyographic activity of the trunk and neck muscles at both sides of the body were recorded. Our data revealed two transitions in the development of postural adjustments. The first transition was present around 6 months of age. At this age the postural muscles were infrequently activated during reaching movements. At 8 months ample postural activity reappeared and the infants developed the ability to adapt the postural adjustments to task-specific constraints such as arm movement velocity or the sitting position at the onset of the reaching movement. The second transition occurred between 12 and 15 months. Before 15 months the infants did not show consistent anticipatory postural activity, but from 15 months onwards they did, particularly in the neck muscles.

Postural adjustments accompanying fast pointing movements in standing, sitting and lying adults.

The present study evaluated the effect of different positions, which varied in the amount of bodily support, on postural control during fast pointing movements. Fourteen adult subjects were studied in standing, various sitting and lying positions. Multiple surface electromyograms (EMGs) of arm, neck, trunk and upper leg muscles and kinematics were recorded during a standard series of unilateral arm movements. Two additional series, consisting of bilateral arm movements and unilateral arm movements with an additional weight, were performed to assess whether additional task-load affected postural adjustments differently in a sitting and standing position. Two pointing strategies were used--despite identical instructions. Seven subjects showed an elbow extension throughout the movements. They used the deltoid (DE) as the prime mover (DE group). The other seven subjects performed the movement with a slight elbow flexion and used the biceps brachii (BB) as the prime mover (BB group). The two strategies had a differential effect on the postural adjustments: postural activity was less and substantially later in the BB-group than in the DE group. Anticipatory postural muscle activity was only present in the DE group during stance. In all positions and task-load conditions the dorsal postural muscles were activated before their ventral antagonists. The activation rate, the timing and--to a lesser extent the amplitude of the dorsal muscle activity was position dependent. The position dependency was mainly found in the caudally located lumbar extensor (LE) and hamstrings (HAM) muscles. The EMG amplitude of LE and HAM was also affected by body geometry (trunk and pelvis position). Position and body geometry had only a minor effect on the activity of the neck and thoracic extensor muscles. This difference in behaviour of lower and upper postural muscles suggests that they could serve different postural tasks: the lower muscles being more involved in keeping the centre of mass within the limits of the support surface, and the upper ones in counteracting the reaction forces generated by movement onset. Increasing task-load by performing bilateral movements and--to a minor extent--during loaded unilateral movements affected the temporal and quantitative characteristics of the postural adjustments during standing and sitting in a similar way. The effect was present mainly during the early part of the response (within 100 ms after prime mover onset). This suggests that feedforward or anticipatory mechanisms play a major role in the task-specific modulation of postural adjustments.