Responses of Hoffman-reflex in human soleus to gravity and/or fluid shift

Responses of Hoffman (H)-reflex in human soleus to changes in the levels of gravity, activities of skeletal muscles, and/or fluid distribution of lower limbs during the parabolic flight of a jet airplane and/or using a tilting table were studied. The time interval between the electrical stimulation and the appearance of either M- or H-wave and the amplitude of M-wave were not influenced by the changes in gravity and fluid distribution levels. However, the H-wave amplitude was increased when the subjects were exposed to microgravity (microgravity-G). Hypergravity at 1.5- or 2-G had no effect on the H-wave amplitude. The H-reflex had no relation with the changes of electromyogram activities of skeletal muscles and fluid volume in lower limbs. Further, the H-wave amplitude was even decreased insignificantly when the distribution of lower limb fluid was reduced at head-down position on the table. It is suggested that an acute exposure to microgravity-G increases the excitability of soleus motor pool, but the mechanism is still unclear.