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.
Life Science Division, NASA Ames Research Center, Moffett Field, CA, USA.
J
J
Vernikos
D A
DA
Ludwig
A C
AC
Ertl
C E
CE
Wade
L
L
Keil
D
D
O'Hara
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