Endurance fitness and orthostatic tolerance.

Background: Several lower body negative pressure studies conducted in recent years have suggested that aerobically fit individuals have poo-orthostatic tolerance (OT).

Hypothesis: There will be significant differences between OT of highly fit endurance runners and unfit individuals.

Methods: Subjects were 17 men and 17 women aged 29-42 yr who were administered two orthostatic tests (20 min standing) before, and 5 min following a VO2max test. There were 6 men and 6 women who were highly trained endurance runners with VO2max of 63 and 50 ml.kg-1.min-1 for the men and women, respectively; 6 men and 6 women had average fitness (VO2max of 40 and 32 ml.kg-1.min-1, respectively), and the remaining 5 men and 6 women had poor fitness (VO2max of 32 and 26 ml.kg-1.min-1, respectively).

Results: The trained men and women showed good OT before and after exercise, which was characterized by very low orthostatic heart rates (54 and 64 bpm before, 70 and 75 bpm after exercise, for the men and women, respectively); relatively large pulse pressures (PP); and a lack of any adverse subjective reactions. The untrained subjects' orthostatic heart rates were 30-40 bpm higher than in the trained subjects, (18 bpm higher after, vs. before exercise) their PP was narrower, and there were 7 cases of orthostatic weakness (dizziness, yawning) and 8 fainting episodes. (6 cases before, 9 cases after exercise).

Conclusions: Aerobically fit individuals have good OT while unfit individuals show poor OT. Since the maintenance of good OT is important in the Space Shuttle flights, endurance training enhances crew/vehicle safety.