Human postural reflexes and gravity--an under water simulation.

This study represents the first attempt to investigate the influence of gravity on postural adjustments. Subjects were displaced while standing under water on a movable platform, while the buoyancy of the body was adjusted by using a variety of lead vests. Under water, an approximately linear relationship was found between body weight and impulse directed electromyographic response amplitudes in the leg and thigh muscles. Loading of the subjects out of water resulted in a saturation of the response amplitude. The biomechanical signals recorded during the displacements indicated that neither vestibulospinal nor muscle proprioceptive reflex mechanisms can account for the effect observed under water. It is suggested that the EMG responses are mediated by reflexes which are activated by pressure receptors within the body in order to hold the centre of gravity over the feet.