Electromyographic study of the contractile and electrical properties of the human triceps surae muscle in a simulated microgravity environment.

1. The effects of 7 days of simulated space flight, achieved with the technique of 'dry' water immersion, on human triceps surae muscle function have been investigated. 2. The maximal voluntary contraction (MVC) was reduced by 33.8 % (P < 0.01) while the electrically evoked maximal tetanic contraction force (Po) decreased by 8.2 % (P > 0.05). This suggests that most of the force loss is due to a reduction in motor drive. 3. The decrease in Po was associated with a small increase in maximal rates of tension development (7.2 %). The twitch tension (Pt) was not significantly changed and the Pt : Po ratio was decreased by 8.7 % after immersion. 4. A standard fatigue test, consisting of sixty 1 s intermittent isometric contractions (50 impulses s-1) separated by 1 s rest decreased tetanic force to approximately 60 % of initial values, but force reduction was not significantly different before and after immersion: the fatigue index was 36.2 +/- 5.4 % before and 38.6 +/- 2.8 % after immersion (P > 0.05). Whilst there were similar changes in mechanical output between control and disused muscles, there were differences in the pattern of electrical activity.