Aerobic fitness: II. Orthostasis and vo2peakfollowing head-down tilt

J. W. Williamson, X. Shi, J. J. Chen, C. G. Crandall, W. G. Squires, L. P. Krock, P. B. Raven

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


To determine whether endurance exercise trained (ET) subjects would experience greater reductions in peak oxygen delivery and orthostatic tolerance (OT) than untrained (UT) subjects, both peak oxygen uptake (VO2peak) during upright bicycle ergometry and tolerance time during 70° head-up tilt (HUT) were compared within and between groups before and after 4 h of −6° head-down tilt (HDT). Eight ET subjects with a mean VO2pcak of 61.7 ± 1.6 ml•kg−1-mn−1 were matched for age, height, and weight with eight UT subjects (VO2pcak = 38.4 ± 1.7 rnl•kg−1• min−1). Following HDT, decreases in plasma volume (PV) were larger for ET subjects (−3.7 ± 0.5 ml-kg−1) than for UT subjects (−2.3 ± 0.3 ml-kg−1), P < 0.03. Reductions in VO2pcak for ET subjects (−5.4 ± 1.1 ml • kg−1 • min−1) were also greater than for UT subjects (−2.4 ± 0.8−1-min-’), P < 0.05. The ET (N = 6) subjects also had a significant decrease in OT time (−13 0 ± 4.2 min) during post-HDT HUT, which was not observed for the UT group (N= 6). A significant inverse correlation was found pre-HDT VO2pcak and the change in OT time, r = −0.74, P < 0.01. The decrease in OT was also significantly correlated to the PV decrease, r = 0.59, P < 0.04. The UT subjects had significantly augmented pressor responses to HUT manifested by the increases in both HR and MAP following HDT. However, after HDT, the ET subjects failed to demonstrate an immediate increase in HR with the initiation of HUT. These data suggest that ET individuals may experience a greater PV loss than UT individuals following acute simulation of microgravity. The greater reduction in PV may also explain the greater reductions in both VO2pcak and OT experienced by the ET subjects following 4 h of HDT.

Original languageEnglish (US)
Pages (from-to)999-1006
Number of pages8
JournalMedicine and science in sports and exercise
Issue number9
StatePublished - Feb 1992


  • Deconditioning
  • Endurance
  • Exercise training
  • Head-down tilt
  • Orthostasis
  • Peak oxygen uptake

ASJC Scopus subject areas

  • Orthopedics and Sports Medicine
  • Physical Therapy, Sports Therapy and Rehabilitation


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