Electrical and contractile parameters of muscle in man: effects of 7-day "dry" water immersion

The purpose of this experiment was to test the hypothesis that the changes in single muscle twitch tension (Pt) in a cosmonaut following exposure to long-duration microgravity may have been due to the sarcoplasmic reticulum, while the decrease in maximal voluntary contraction (MVC) relative to the electrically evoked maximal tetanic tension (Po) may reflect a reduction in control neural drive resulting from the voluntary muscle inactivity.

The effects of 7-d "dry" water immersion were investigated in six subjects. Changes of contraction properties were studied in the triceps surae muscle.

After immersion, the MVC was reduced by 33.8% (p < 0.01), and the Po was reduced by 8.2% (p > 0.05). The difference between Po and MVC expressed as a percentage of Po and referred to as force deficiency, has also been calculated. The force deficiency increased by a mean of 44.1% (p < 0.01) after immersion. The decrease in Po was associated with increased maximal rate of tension development (7.2%). The twitch time-to-peak was not significantly changed, and half-relaxation time was decreased by 5.3%. However, the Pt was not significantly changed and the Pt/Po ratio was decreased by 8.7% (p < 0.01) after immersion. The muscle surface action potential presented an increase of its duration (18.8%) and a decrease of the amplitude and the total area (14.6% and 2.8%; p < 0.05-0.01, respectively).

The comparison of the electrical and mechanical alterations recorded during voluntary contractions, and in contractions evoked by electrical stimulation of the motor nerve, suggests that immersion not only modifies the peripheral processes associated with contraction, but also changes central and/or neural command of the contraction. At peripheral sites, it is proposed that the intracellular processes of contraction play the role in a contractile impairment recorded during immersion.