Production of finely graded forces in humans: effects of simulated weightlessness by water immersion.

We have shown before that subjects exposed to a changed gravitoinertial environment produce exaggerated manual forces. From the observed pattern of findings, we argued that initial forces were exaggerated because of abnormal vestibular activity and peak forces because of degraded proprioceptive feedback. If so, only peak but not initial forces should be affected by water immersion, an environment that influences proprioceptive feedback but not vestibular activity. The present study was undertaken to scrutinize this prediction. Twelve subjects sat in a chair once immersed in water and once on dry land, while producing pre-trained isometric forces with a joystick. In a control experiment, subjects performed a four-choice reaction-time task. During the joystick task, produced initial forces were comparable in water and on land, while peak (+24%) and end forces (+22%) were significantly higher in water, as was their reaction time (+6%). During the control task, reaction time was comparable in water and on land. Our findings corroborate the above notion that initial forces increase when the vestibular system is stimulated (gravitoinertial change, visual field motion, but not water immersion), while peak forces increase when proprioceptive feedback is degraded (probably all three scenarios) and are not corrected until response end. Our findings further confirm the absence of cognitive slowing in simple-choice reaction tasks under shallow-water immersion conditions.