POSTURAL ADJUSTMENTS ACCOMPANYING FAST POINTING MOVEMENTS IN STANDING, SITTING AND LYING ADULTS

The present study evaluated the effect of different positions, which v aried 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 add itional series, consisting of bilateral arm movements and unilateral a rm movements with an additional weight, were performed to assess wheth er additional task-load affected postural adjustments differently in a sitting and standing position. Two pointing strategies were used - de spite identical instructions. Seven subjects showed an elbow extension throughout the movements. They used the deltoid (DE) as the prime mov er (DE group). The other seven subjects performed the movement with a slight elbow flexion and used the biceps brachii (BB) as the prime mov er (BB group). The two strategies had a differential effect on the pos tural adjustments: postural activity was less and substantially later in the BB-group than in the DE group. Anticipatory postural muscle act ivity was only present in the DE group during stance. In all positions and task-load conditions the dorsal postural muscles were activated b efore 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 ca udally 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 effe ct on the activity of the neck and thoracic extensor muscles. This dif ference in behaviour of lower and upper postural muscles suggests that they could serve different postural tasks: the lower muscles being mo re involved in keeping the centre of mass within the limits of the sup port surface, and the upper ones in counteracting the reaction forces generated by movement onset. Increasing task-load by performing bilate ral movements and - to a minor extent - during loaded unilateral movem ents affected the temporal and quantitative characteristics of the pos tural adjustments during standing and sitting in a similar way. The ef fect was present mainly during the early part of the response (within 100 ms after prime mover onset). This suggests that feedforward or ant icipatory mechanisms play a major role in the task-specific modulation of postural adjustments.