Influence of 120-days 6 degrees head-down tilt bed rest on the functional properties of the neuromuscular system in man.

The effect of a 120-d 6 degrees head-down tilt bed rest on the functional properties of the human triceps surae muscle have been studied in a group of male volunteers (n = 6), mean age 38 +/- 2 yr. According to tendometry measurements, maximum voluntary contraction and maximum strength in response to a nerve stimulation at a frequency of 150 impulses s(-1) after head-down tilt decreased by 45.5% and 33.7%, respectively (p < 0.05-0.01), and maximal twitch declined by 36.7% (p < 0.05). The magnitude of force deficiency increased by 60.2% (p < 0.001). Time to peak tension, half-relaxation time and total contraction time of twitch increased (p < 0.05-0.001); the maximum rate of rise of isometric tetanic tension as absolute tension exerted by the triceps surae muscle in response to an electrical stimulation of the motor nerve and during voluntary contraction performed on command to contract quickly and strongly decreased. With force expressed as a percentage of the maximum developed, the force-time curves did not alter significantly. A post head-down tilt fatigability index of the triceps surae muscle was to 0.61 +/- 0.02 as opposed to 0.80 +/- 0.03 prior to an exposure (p < 0.05). It is postulated that changes of contractile properties of the triceps surae muscle head-down tilt are linked to the atrophic processes, alterations of muscle length and the state of sarcoplasmic reticulum on the one hand, and to the altered state of the central nervous system, on the other.