Proprioceptive disturbances in weightlessness revisited.

The senses of limb position and movement become degraded in low gravity. One explanation is a gravity-dependent loss of fusimotor activity. In low gravity, position and movement sense accuracy can be recovered if elastic bands are stretched across the joint. Recent studies using instrumented joysticks have confirmed that aiming and tracking accuracy can be recovered in weightlessness by changing viscous and elastic characteristics of the joystick. It has been proposed that the muscle spindle signal, responsible for generating position sense in the mid-range of joint movement, is combined with input from joint receptors near the limits of joint movement to generate a position signal that covers the full working range of the joint. Here it is hypothesised that in low gravity joint receptors become unresponsive because of the loss of forces acting on the joint capsule. This leads to a loss of position and movement sense which can be recovered by imposing elastic forces across the joint.