Several studies have reported increased plasma volume and enhanced baroreflex sensitivity 24 h after an acute, maximal bout of exercise (Convertino & Adams, 1991; Gillen et al. 1991). This raises the possibility of an effective, time-efficient countermeasure to the decreased orthostatic tolerance experienced after extended spaceflight. The purpose of this study was to determine whether a single bout of maximal, aerobic exercise improves orthostatic tolerance over a 24 h period. Ten healthy subjects (mean ± S.D., age, 22.5 ± 2.2 years; height, 175 ± 7.9 cm; weight, 78.3 ± 6.8 kg) participated in the study after giving informed consent. The study was approved by the local institutional review board. Each subject completed five progressive lower body negative pressure (LBNP) tests, including a control test, to the onset of presyncopal symptoms in random order. Exercise tests consisted of a Bruce treadmill test; orthostatic tolerance was then tested at 0, 3, 6 and 24 h post-exercise on separate days. Heart rate (HR), systolic and diastolic blood pressures, calf circumference, cardiac output, and forearm blood flow (FBF) were measured during each LBNP test. An analysis of variance was used to determine statistical significance at the 0.05 level. A Tukey’s post-hoc test was then used to test for individual differences between testing times. Orthostatic tolerance as indicated by cumulative stress index (mmHg min) was significantly reduced immediately following exercise (852.3 ± 92.18 to 711.6 ± 49.3 mmHg min; P < 0.05), and returned to control level by 24 h post-exercise (849.2 ± 153.6 mmHg min). HR was higher than control immediately after exercise (98.2 ± 7.2 vs. 74.1 ± 5.4 b.p.m.; P < 0.05) and throughout the LBNP test. FBF significantly decreased during LBNP in the control (4.69 ± 2.46 to 2.84 ± 1.32 ml (100 ml)^-1 min^-1), 3 h (4.11 ± 1.95 to 2.66 ± 1.73 ml (100 ml)^-1 min^-1), 6 h (4.30 ± 1.55 to 2.69 ± 1.38 ml (100 ml)^-1 min^-1), and 24 h (5.12 ± 1.47 to 3.37 ± 1.34 ml (100 ml)^-1 min^-1) conditions (P < 0.05). FBF was higher than control values right after exercise (7.22 ± 2.12 vs. 4.74 ± 2.02 ml (100 ml)^-1 min^-1) and remained elevated throughout the test (P < 0.05). These data suggest that orthostatic tolerance is not improved 24 h after a single, maximal bout of aerobic exercise. Orthostatic tolerance decreased immediately following exercise, possibly because of an altered starting point for the cardiovascular system.

This research was supported by NASA grant NAG-8-999.