Mechanical and electrical changes in human muscle after dry immersion.

The effects of a 7-day dry immersion, with the body protected from contact with the water, were investigated in six subjects. Changes in contraction properties were studied in the triceps surae muscle. After immersion, the maximal voluntary contraction (MVC) was reduced by 33.8% (P < 0.01), and the electrically evoked maximal tetanic contraction (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 was also 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 rates of tension development (7.2%) and of tension relaxation. The twitch time to peak was not significantly changed, and half relaxation and total contraction time were decreased by 5.3% and 2.8%, respectively, but the twitch tension (Pt) was not significantly changed and the Pt:Po ratio was decreased by 8.7% after immersion. The muscle surface action potential showed an increase in duration (18.8%) and decreases in amplitude and total area (14.6% and 2.8%; P < 0.05-0.01, respectively). Comparison of the electrical and mechanical alterations recorded during the voluntary contractions, and in the contractions evoked by electrical stimulation of the motor nerve, would suggest that immersion not only modified the peripheral processes associated with contraction, but also changed the central and/or neural command of the contraction. At peripheral sites, it is proposed that the intracellular processes of contraction played a role in the contractile impairment recorded during immersion.