Effect of standing or walking on physiological changes induced by head down bed rest: Implications for spaceflight

Background/Hypothesis: To simulate exposure to microgravity and to determine the effectiveness of intermittent exposure to passive and active +1 Gz force (head-to-foot) in preventing head-down bed rest (HDBR) deconditioning, 4 d of 6° HDBR were used. Methods: Volunteers were 9 males, 30-50 yr, who performed periodic standing or controlled walking for 2 or 4 h · d^-1 in 15-min bouts, one bout per hour, or remained in a continuous HDBR control condition (0 Gz). Results: Standing 4 h (S4) completely prevented, and standing 2 h (S2) partially prevented, decreases in post-HDBR orthostatic tolerance (survival rates with 30 min of upright tilt at 60°). Walking, both 2 h (W2) and 4 h (W4), and S4 attenuated decreases in peak oxygen uptake compared to 0 Gz. Compared to 0 Gz, both S4 and W4 attenuated plasma volume loss during HDBR. Urinary Ca²⁺ excretion increased over time with HDBR; the quadratic trend for urinary Ca²⁺, however, was attenuated with W2 and W4. Conclusions: We concluded that various physiological systems benefit differentially from passive +1 Gz or activity in +1 Gz and, in addition to the duration of the stimulus, the number of exposures to postural stimuli may be important moderating factor.