Conduction velocity of nerve and muscle fiber action potentials after a space mission or a bed rest

Objective: The purpose of the present investigation was to study the readaptation of a human muscle after a prolonged stay in microgravity and after a bed rest of several months. Methods: The surface electromyogram of the soleus muscle was recorded in 6 cosmonauts and 6 bed rest subjects at 5 different torque levels and, in addition, the direct muscle responses (M responses) to supramaximal stimulation of the posterior tibial nerve were also recorded in the bed rest subjects. In a supplementary experiment in normal subjects, M response was recorded with an array of electrodes. Results: The median frequency (MF) of the power spectrum of the surface electromyogram was reduced, at all torque levels, immediately after the test period. In the bed rest subjects, the latency of the M response peaks and the inter-peak interval increased during the test period. Recovery to normal occurred within about 10 days. In the normal subjects, the peaks of the M response were conducted along the muscle with a velocity between 21 and 30 m/s. Conclusions: All these results point to a reduction of the conduction velocity in the branching axon terminals and in the muscle fibers during space missions and bed rest.