The effect of body immersion on postural adjustments was studied in ten healthy subjects. Reaction times, for pushing or pulling a rigid handle, in response to a visual stimulus were measured. In addition EMG recordings were taken from upper arm and lower leg muscles during three levels of body immersion while standing on a platform (immersed to spinal levels: lumbar nerve root 2 (L2); thoracic nerve root 4 (T4); and cervical nerve root 7 (C7)), while floating and while standing or sitting out of water. 2. With increasing levels of body immersion there was a near linear reduction in the amplitude of the gastrocnemius (GM) EMG activity before (200 ms) the onset of a force signal from pulling, but immersion had a significantly weaker effect on the amplitude of the tibialis anterior (TA) EMG during pushing movements. There was no significant difference in the effect of body immersion on biceps femoris (BF) and rectus femoris (RF). Under free-floating conditions postural adjustments did not occur in response to pull or push movements. There were no adaptational changes of EMG adjustments during successive trials at a given immersion level. 3. Under non-immersed conditions reaction times were significantly shorter during sitting than during standing. This difference is assumed to be due to the postural adjustments required while standing before the onset of a voluntary arm movement. While standing, reaction times were significantly longer for pull compared with push movements. Under all conditions of body immersion the reaction times remained longer compared with the sitting condition, even when no leg muscle EMG adjustments were present. 4. It is assumed that the differential effect of body immersion on the antagonistic leg muscles is due to the differential neuronal control of antagonistic leg muscles with a strong influence from proprioceptive input (most probably from load receptors) on the leg extensors. The longer reaction times seen during body immersion, where no postural adjustments were evident, suggests that a supraspinal command to the leg muscles precedes the voluntary arm movement. However, because of the changed/decreased afferent input no postural adjustments are generated.
