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 det ermine the effectiveness of intermittent exposure to passive and activ e +1 Gz force (head-to-foot) in preventing head-down bed rest (HDBR) d econditioning, 4 d of 6 degrees HDBR were used. Methods: Volunteers we re 9 males, 30-50 yr, who performed periodic standing or controlled wa lking for 2 or 4 h . d(-1) in 15-min bouts, one bout per hour, or rema ined in a continuous HDBR control condition (0 Gz). Results: Standing 4 h (S4) completely prevented, and standing 2 h (S2) partially prevent ed, decreases in post-HDBR orthostatic tolerance (survival rates with 30 min of upright tilt at 60 degrees). Walking, both 2 h (W2) and 4 fi (W4), and S4 attenuated decreases in peak oxygen uptake compared to 0 Gz. Compared to 0 Ct, both S4 and W4 attenuated plasma volume loss du ring HDBR. Urinary Ca2+ excretion increased over time with HDBR; the q uadratic trend for urinary Ca2+, however, was attenuated with W2 and W 4. Conclusions We concluded that various physiological systems benefit differentially from passive +1 Gz or activity in +1 Gz and, in additi on to the duration of the stimulus, the number of exposures to postura l stimuli may be an important moderating factor.