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

Abstract

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.

Isometric Force Exaggeration in Simulated Weightlessness by Water Immersion: Role of Visual Feedback

“…An additional analysis compared the present data with those registered previously without visual feedback ( 7 ). In the previous study, 12 subjects produced pretrained force magnitudes of 5, 15, and 25 N in 8 possible directions.…”

“…We used a similar setup and procedures as in our previous study ( 7 ), except that visual feedback was now available, and that a subject's head was immersed at 9.8 ft (3 m) depth, in contrast to the previous study (20 cm/0.7 ft depth). Subjects wore a commercial diving jacket that was fi xed either to an aluminum frame at the bottom of a diving pool of 16.4 ft (5 m) depth (condition WET), or to a wall on land (condition DRY).…”

“…The study was preapproved by the Ethics Committee of the German Sport University. None of the 48 subjects of the present study participated in the earlier study ( 7 ).…”

“…2C and D ). Descriptive statistics of the data from the present and previous study ( 7 ) are presented in Table I . In the previous study without visual feedback, produced peak force magnitudes in WET and DRY distinctly exceeded the required target force of 15 N (in WET +203%; in DRY +143%; right half of Fig.…”

“…When subjects were asked to produce prescribed force magnitudes without visual feedback, their responses were consistently 20 to 50% larger in those environments than under normal terrestrial control conditions. In our previous underwater study ( 7 ), differences of isometric force production between wet and dry conditions emerged only with a delay during each response: they were absent for the initial force (100 ms after response onset), i.e., before proprioception becomes effective ( 5 ), but were present for the peak force (highest force magnitude after response onset) and end force (force magnitude 30 ms before the force target disappeared) ( 7 ). In contrast to these underwater data, an increase was observed already for initial force during visual fi eld motion (+42%), in hypergravity (+140%), and in microgravity (+20%).…”