CONVERTINO, V. A. Endurance exercise training: conditions of enhanced hemodynamic responses and tolerance to LBNP. Med. Sci. Sports Exert., Vol. 25, No. 6, pp. 705-712, 1993. In cross-sectional comparisons, several investigators have reported highly trained endurance athletes to have a prevalence toward orthostatic hypotension and intolerance compared with average fit individuals. These observations have raised concern that regular exercise designed to increase aerobic capacity may impair regulatory mechanisms of blood pressure control and that perhaps certain populations of individuals with a predisposition for fainting exhibit an inability to elevate heart rate, vasoactive hormones, and peripheral resistance during an orthostatic challenge. In longitudinal experiments, when exercise training was performed by subjects who increased their aerobic capacity by 20% but maintained VO2max below 50 ml.kg-1.min-1, tolerance to lower body negative pressure (LBNP) was increased by 28%. Exercise training did not compromise baroreflex functions despite evidence of increased resting vagal cardiac tone and reduced sympathetic tone. In contrast to fainters, increased orthostatic tolerance in the exercised-trained subjects was associated with no alteration in their ability to elevate heart rate, vasoactive hormones, and peripheral resistance at peak LBNP. However, cardiac output and mean arterial blood pressure were maintained during higher submaximal LBNP levels by a 20% increase in stroke volume. The elevation in stroke volume during LBNP after training was associated with blood volume expansion. Although orthostatic tolerance may be reduced in some highly trained endurance athletes, data from this and other longitudinal experiments argue for the notion that orthostatic stability is not compromised by exercise training that does not increase VO2max above a critical threshold (-55-60 ml.kg-1.min-1) and provoke associated cardiovascular adaptations. Since average VO2max of astronauts and pilots fall below this threshold, there is no compelling evidence to suggest that these individuals should refrain from such regular exercise activity.
(C)1993The American College of Sports Medicine