The effect of a 120-day 5° head-down tilt (HDT) bed rest on the mechanical properties and electromechanical delay (EMD) of the human triceps surae (TS) muscle was studied in four (mean age 31.5 ± 1.7 yr) healthy young women subjects. The TS mechanical properties were evaluated based on the following indicators: maximal voluntary contraction (MVC), maximal strength (P ; frequency 150 Hz), peak twitch force (P ), time-to-peak tension (TPT), half-relaxation time (1/2 RT) and tension development time to reach 25, 50, 75 and 90 % of maximal tension. Force deficit (P) were estimated. In response to a light signal, the subject was supposed to make a voluntary foot flexion, with the instruction «to exert the fastest and greatest tension». EMD measurements were recorded from each subject during voluntary contraction. Surface electrodes sensed electromyographic (EMG) activity in the soleus muscle. A separate timer was used to determine total reaction time (TRT). Premotor time (PMT) was taken to be the time inter­val from the delivery of the signal to change in EMG. EMD was the time interval between the change in EMG and movement i.e. the time interval between EMG and the onset of muscle tension. After HDT P , MVC and P decreased by 24.4, 36.1 and 11.5 %, respectively, while Pd° increased by 38.8 %. TPT increased by 13.6 %, while 1/2RT decreased by 19.2 %. The rate of increase of voluntary contractions calculated according to a relative scale significantly increased, while the rate of development of electrically evoked contrac­tion did not show any significant differences. The voluntary contraction EMD increased by 27.4 %; PMT by 8.7 %, and TRT by 13.6 %. Thus, the mechanical changes suggest that weightlessness changes not only the peripheral processes as­sociated with contractions but also the central and neural command. EMD is a simple and quick method for evaluation of muscle stiffness changes. Moreover, EMD can serve as an indicator of the functional condition of the neuromuscular sys­tem.