It is known that inactivity results in deconditioning and physiological deconditioning induced by inactivity affects important system of the body including musculoskelatal. These physiological changes may result in altered muscle function and motor control [Kozlovskaya et al., 1991; Jaweed et al., 1995]. Decrements in motor performance could have negative implications for effective completion of mission-critical operational tasks. Up to now, owing to methodological difficulties, the property contractile properties of human skeletal muscles in a true weightless environment [Bachl et al., 1992; Tschan et al., 1994; Day et al., 1995] or during its simulation [Grigorieva, Kozlovskaya, 1987; Adams et al., 1994; Sugajima et al., 1996] were beyond the field of vision of the scientists who in the main have concentrated on examining the mechanical features of the voluntary muscular contractions. This is the first study to make quantitative measurement of the functional properties of a single muscle in a man exposed to the long-term spaceflight (SF). The investigation was concerned with the parameters of mechanical responses of the triceps surae muscle (TS), which has been shown to be a postural antigravity muscle [Campbell et al., 1973]. The purpose of study were to analyze the effects of SF the MIR-18 and MIR-22 mission on the mechanical changes of the TS. The mechanical responses of the TS were recorded by tendometry [Kots et al., 1976], which made it possible to measure single muscle contraction force by the degree of tension change in muscle distal tendon [Koryak, 1985]. Maximal voluntary contraction (MVC), maximal twitch (Pt), tetanic forces (Po) of isometric contraction elicited by electrical stimulation of tibialis nerve [Koryak, 1978], time-to-peak tension (TPT), a half-relaxation (1/2HR), and time of force development both during voluntary and evoked contractions to 25%, 50%, 75% of the MVC and Po, respectively, were evaluated as well. The difference between Po and MVC expressed as a percentage of Po and referred to as force deficiency (Fd) has also been calculated. EMG integral (IEMG) was calculated for gastrocnemius and soleus, but were than averaged. The IEMG/MVC ratio was also determined. After SF, the TPT increased by 9%, but 1/2HR and Pt decreased by 18% and 14%, respectively. MVC and Po decreased by 23% and 11%, respectively. Force deficiency increased by 36%. The value Po/Pt ratio increased by 6%. The rate of rise a voluntary tension development decreased by 19%, 45%, and 20%, respectively. However, electrical evoked tetanic development not differ substantially from the initial data. The value EMG and IEMF/MVC ratio increased by 55%, and 71%, respectively. These findings indicate thus the alterations of contractile properties were found to be booth of peripheral and central nature but more central. Relative less functional alterations of the TS compared to those observed after a 120-days bed-rest [Koryak, Kozlovskaya, 1992; Koryak, 1995] that may be related to countermeasure compliance.