[Neuromuscular responses of the triceps surae muscle to prolonged passive stretch of the foot extensor muscles under conditions of simulated microgravity].
The effect of a 60-day 6 degrees head-down tilt (HDT) of bed rest with and without countermeasures on the mechanical properties of human the triceps surae muscle was studied in 12 healthy young men subjects. One group (n=6; mean age 30.8 +/- 3.1 years) underwent a 60-day HDT and a second group (n=6; mean age 30.5 +/- 1.9 years) underwent HDT with countermeasures (long-term passive stretching). The results showed that the contractile properties of skeletal muscle change considerably. After HDT without countermeasures the maximal voluntary contraction (MVC) declined by 33.5% (P < 0.05), the electrically evoked tetanic tension at 150 Hz (Po) and isometric twitch contraction (Pt) reduced by 18.0% (P < 0.02) and 17.3% (P < 0.05), respectively. Time-to peak tension (TPT) of the twitch increased by 3.4%, but half-relaxation time (1/2 RT) decreased by 7.2%, and total contraction time (TCT) increased not significantly. The difference between Po and MVC expressed as a percentage of Po and referred to as force deficiency (Pd), has also been calculated. The Pd increased by 61% (P < 0.001). The rate of rise of voluntary contractions calculated according to an relative scale significantly reduced, but the electrically evoked contraction no substantial changes were observed. After HDT with long-term passive stretching caused decrease by 43.0% (P < 0.05) in MVC, and Pt and in Po by 17.0%, and by 19.4% (P < 0.05), respectively. The Pd increased significantly by 100% (P < 0.001). TPT, and 1/2 RT not change significantly, and TCT of the twitch increased by 6.5%, respectively in relation to the control condition. The rate of rise of electrically evoked tetanic tension did not change significantly during HDT with countermeasures but the rate of rise in on isometric voluntary tension development were decreased. These data indicate that prolonged passive stretching of a single muscle decreases voluntary strength. Thus, the received data specify, that the long passive stretching of a muscle does not find out preventive facilitate contractile properties of the muscular device from negative influences of mechanical unloading. This study presents evidence that prolonged passive muscle stretching can lead to some modification of contraction material behavior. In addition, altered material properties seem to affect proprioceptive feedback and, therefore, the motor unit activation in proportion to the contractile failure.
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D000328:Adult; D001696:Biomechanical Phenomena; D005528:Foot / Q000502:physiology*; D006801:Humans; D007103:Immobilization / Q000502:physiology*; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D052580:Muscle Stretching Exercises; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology*; D013997:Time Factors; D018474:Weightlessness Simulation