The effect of a 60-day 6° head-down tilt of bed rest with and without prolonged passive muscle "stretching" training on the mechanical properties of the human triceps surae muscle was studied in 13 healthy male subjects. One group (n = 6; mean age 30.8 ± 3.1 years) underwent a 60-day head-down tilt, and a second group (n = 7; mean age 30.4 ± 1.2 years) underwent head-down tilt with prolonged passive muscle stretching. Head-down tilt without prolonged passive muscle "stretching" training showed maximal voluntary contraction declined by 34 % (p < 0.05) and the electrically evoked tetanic tension at 150 impulses·s-1 and isometric twitch contraction reduced by 17 % (p < 0.02) and 18 % (p < 0.05), respectively. Time-to-peak tension, and half-relaxation time of the twitch slightly decreased by 3% (p > 0.05), and 7 %, respectively, but total contraction time slightly increased. The difference between electrically evoked tetanic tension and the maximal voluntary contraction expressed as a percentage of electrically evoked tetanic tension (referred to as force deficiency), has also been calculated. The force deficiency increased by 61 % (p < 0.001). After head-down tilt with prolonged passive muscle "stretching" training, the time-to-peak tension did not change, and half-relaxation time of the twitch decreased. In addition, there was a 14 % lengthening in the total duration of the twitch. The results of prolonged passive muscle "stretching" training demonstrated a clear deterioration of voluntarily and electrically induced muscle contractions. Passive "stretching" training caused a decrease by 29 % (p < 0.05) in the maximal voluntary contraction. The isometric twitch contraction, and electrically evoked tetanic tension both showed reductions by 17 %, and by 19 % (p < 0.05), respectively. The force deficiency decreased significantly by 21 % (p < 0.02). The rate of rise of electrically evoked tetanic tension and feature of voluntary contractions significantly reduced during head-down tilt with prolonged passive muscle "stretching" training. These basic experimental findings concluded that prolonged passive "stretching" training of a single muscle did not prevent a reserve of neuromuscular function.