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Effect of head-out water immersion on response to exercise training.

During spaceflight and head-out water immersion (WI) there is a cephalad shift in blood volume. We have recently shown that left ventricular end-diastolic dimension is significantly greater during moderate cycling exercise with WI compared with on land. The purpose of this study was to determine whether the cephalad shift in blood volume and accompanying increase in cardiac preload with WI alters the normal cardiovascular adaptations to aerobic exercise training. Nine middle-aged healthy men trained on cycle ergometers in water, nine trained on land, and four served as controls for 12 wk. Following training, both training groups showed similar increase (P less than 0.05) in stroke volume and similar decreases in heart rate (P less than 0.01) and blood pressure (P less than 0.05) at a given submaximal exercise O2 consumption (VO2). Maximal VO2 increased (P less than 0.01) similarly for both training groups. The control group did not demonstrate any significant changes in submaximal or maximal exercise responses. We conclude that the cephalad shift in blood volume with WI does not alter the normal cardiovascular adaptation to aerobic exercise training.