Changed joint position sense and muscle activity in simulated weightlessness by water immersion.

Background: Previous studies suggested that proprioceptive feedback for passive arm positioning and isometric forces deteriorates under water. Here we investigate whether a similar deficit exists for active arm positioning. Since deficits were attributed to a reduced muscle tone but findings about muscle tone in water are ambiguous, we re-evaluated this issue.

Methods: With their right forearm, 24 subjects reproduced visual templates which showed a forearm at 45 degrees, 90 degrees, and 135 degrees orientations in the sagittal plane on land (Dry) and during water immersion (Wet). Mean reproduction error and its standard deviation were calculated in allocentric (space-referenced) and egocentric (body-referenced) coordinates. Additionally, 12 of the 24 subjects also participated in an experiment where relaxed left arm EMG was registered in Wet and Dry.

Results: Mean error was comparable in Wet (7.72 degrees) and Dry (6.79 degrees), but error variability was significantly smaller in Wet (7.52 degrees) than in Dry (9.58 degrees). Errors in allocentric (3.42 degrees) differed from egocentric coordinates (11.08 degrees), independent of Wet and Dry. Resting EMG was significantly lower in Wet (3.02 microV) than in Dry (3.73 microV).

Discussion: Proprioceptive feedback for active arm movements is enhanced under water, probably due to high water viscosity, which increases spindle afferents during active but not passive arm movements or isometric responses. We found no evidence that the reference frame for orientation judgments differ between Wet and Dry. Muscle tone of the relaxed arm was reduced under water, corroborating that water immersion degrades proprioception during isometric tasks and passive arm positioning. This is probably not relevant for active arm movements, which seem to increase rather than decrease muscle force to overcome water's viscosity.