Electromyograms (EMGs) recorded during head-out immersion have demonstrated reduced signal amplitudes and decreased EMG/force ratios for both maximal and submaximal isometric contractions compared with measurements on dry land. Similar EMG changes have been found in spaceflight.
This study was designed to examine neuromuscular function in the legs during immersion with special reference to reflex sensitivity.
Recordings were made during plantarflexion in air and water. EMGs were recorded from surface and/or internal electrodes on the soleus (SOL) and medial gastrocnemius (MG) muscles during maximal voluntary contraction (MVC). Hoffman and Achilles tendon reflexes were measured during submaximal plantarflexion (50% MVC) in air and water.
During immersion, MVC decreased 13% while EMG amplitude of the plantarflexor muscles decreased by 29% and 35% for SOL and MG, respectively. A similar trend was observed in measurements of Hofman and Achilles tendon reflexes.
Head-out immersion induced a deterioration of neuromuscular function, perhaps by triggering inhibitory mechanisms. The origin of these mechanisms seems to be related mainly to effects of partial weightlessness, but hydrostatic pressure should also be considered.