MAXIMAL EXERCISE PERFORMANCE AFTER ADAPTATION TO MICROGRAVITY

The cardiovascular system appears to adapt well to microgravity but is compromised on reestablishment of gravitational forces leading to ort hostatic intolerance and a reduction in work capacity. However, maxima l systemic oxygen uptake (Vo(2)) and transport, which may be viewed as a measure of the functional integrity of the cardiovascular system an d its regulatory mechanisms, has not been systematically measured in s pace or immediately after return to Earth after spaceflight. We studie d six astronauts (4 men and 2 women, age 35-50 yr) before, during, and immediately after 9 or 14 days of microgravity on two Spacelab Life S ciences flights (SLS-1 and SLS-2). Peak Vo(2) (Vo(2peak)) was measured with an incremental protocol on a cycle ergometer after prolonged sub maximal exercise at 30 and 60% of Vo(2peak). We measured gas fractions by mass spectrometer and ventilation via turbine flowmeter for the ca lculation of breath-by-breath Vo(2), heart rate via electrocardiogram, and cardiac output (Qc) via carbon dioxide rebreathing. Peak power an d Vo(2) were well maintained during spaceflight and not significantly different compared with 2 wk preflight. Vo(2peak) was reduced by 22% i mmediately postflight (P < 0.05), entirely because of a decrease in pe ak stroke volume and Qc. Peak heart rate, blood pressure, and systemic arteriovenous oxygen difference were unchanged. We conclude that syst emic Vo(2peak) is well maintained in the absence of gravity for 9-14 d ays but is significantly reduced immediately on return to Earth, most likely because of reduced intravascular blood volume, stroke volume, a nd Qc.