[Effect of immersion hypokinesia on the motor unit potentials of human muscles]

Ten test subjects were exposed to water immersion for 3 days. Extraterritorial potentials of low and high threshold motor units (LMU and HMU) of m. biceps brachii were measured using a specially designed multielectrode and an electromyograph. The results revealed substantial changes in LMUs. A significant decrease in the propagation rate of muscle potentials was observed in 9 out of 10 test subjects. The average propagation velocity of LMU potentials was 4.93 +/- 0.16 m/s before exposure, 4.12 +/- 0.07 m/s after a day of immersion and 4.29 +/- 0.09 m/s after three days of immersion, the mean decrease being 20% and 15%, respectively. The total duration of LMU potentials increased significantly during immersion, the increase making 17.5% after a day of immersion and 13% after three days of immersion. The total depolarized zone length diminished by 1.65 mm after a day of immersion and by 1.29 mm after three days of immersion. Conversely, this exposure produced no significant changes in HMU potentials. It can be assumed that the major cause of changes in LMU potentials is reduced muscle activity.