Enhanced Hoffman-reflex in human soleus muscle during exposure to microgravity environment

Responses of Hoffman-reflex in the soleus muscle to changes of gravity leve ls created by parabolic flight of a jet airplane were investigated in four healthy male subjects. The subjects maintained a sitting position with seat belts fastened, keeping the anterior ankle and posterior knee angles at si milar to 135 degrees. The gravity levels were altered from 1- to 2- G, and then microgravity was created for similar to 20 s. The levels were recovere d from 1.5-to 1-G during the descencling phase. The time interval between t he stimulation and either M- or H-wave was not influenced by the changes in gravity levels. The amplitude of the M-wave during hyper-and microgravity was identical to that obtained at 1-G. However,the H-wave amplitude was inc reased when the subjects were exposed to microgravity (similar to four time s vs. 1-G level). The H/M ratio was also elevated during microgravity. Furt her, such a phenomenon was maintained throughout the 20 s of microgravity e xposure. Hypergravity at 1.5- or 2-G had no effect on the H-wave amplitude. It is suggested that an acute exposure to microgravity increases the excit ability of the soleus motor pool and the increased excitability is restored immediately when the gravity level is elevated. (C) 2001 Elsevier Science Ltd. All rights reserved.