Effect of standing or walking on physiological changes induced by head down bed rest: implications for spaceflight.
OBJECTIVE
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 degrees 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 degrees). 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 an important moderating factor.
Vernikos
J
J
Life Science Division, NASA Ames Research Center, Moffett Field, CA, USA.
Ludwig
D A
DA
Ertl
A C
AC
Wade
C E
CE
Keil
L
L
O'Hara
D
D
D000328:Adult; D000704:Analysis of Variance; D001794:Blood Pressure / Q000502:physiology; D002118:Calcium / Q000652:urine; D018493:Cardiovascular Deconditioning / Q000502:physiology*; D016009:Chi-Square Distribution; D006112:Gravitation; D018475:Head-Down Tilt / Q000009:adverse effects*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology; D010953:Plasma Volume / Q000502:physiology; D011187:Posture / Q000502:physiology; D012044:Regression Analysis; D016019:Survival Analysis; D013575:Syncope / Q000209:etiology; D018667:Tilt-Table Test / Q000009:adverse effects; D016138:Walking / Q000502:physiology; D018544:Weightlessness Countermeasures
D002118:Calcium