Electrically evoked and voluntary properties of the human triceps surae muscle: effects of long-term spaceflights.

The influence of microgravity on voluntary and electrically evoked contractions of the triceps surae (TS) of seven crewmembers was studied before and after spaceflights (space MIR flights: MIR-18, -22, -25, -26, and -27). Measurements were made pre-flight on days 30 and post-flight on days 3, respectively, included examination of the properties of isometric maximal voluntary contraction (MVC), isometric twitch contractions (Pt) and tetanic contractions (Po). Force deficit (Fd, delta% = Po - MVC) has also been calculated. After space mission, the MVC decreased by means of 42%, Po by means of 26% and Pt increased by means of 15%. The Fd increased by means of 50%. Time-to-peak tension of the TS and total contraction time increased by means of 8%, but half-relaxation time decreased by means of 21%. Total contraction time increased by a mean of 8%. Force-velocity of properties of the TS calculated according to a relative scale of voluntary contraction development significantly decreased. The calculations of the same properties of electrically evoked contraction development did not differ substantially from the initial physiological state. The comparison of the mechanical alterations recorded during voluntary contractions and in contractions evoked by electrical stimulation of the motor nerve, suggests that weightlessness not only modifies the peripheral processes associated with contractions, but also changes central and/or neural command of the contraction.