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 orthostatic intolerance and a reduction in work capacity. However, maximal systemic oxygen uptake (V̇O₂) and transport, which may be viewed as a measure of the functional integrity of the cardiovascular system and its regulatory mechanisms, has not been systematically measured in space or immediately after return to Earth after spaceflight. We studied 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 Sciences flights (SLS-1 and SLS-2). Peak V̇O₂ (V̇O(2peak)) was measured with an incremental protocol on a cycle ergometer after prolonged submaximal exercise at 30 and 60% of V̇O(2peak). We measured gas fractions by mass spectrometer and ventilation via turbine flowmeter for the calculation of breath-by-breath V̇O₂, heart rate via electrocardiogram, and cardiac output (Q̇c) via carbon dioxide rebreathing. Peak power and V̇O₂ were well maintained during spaceflight and not significantly different compared with 2 wk preflight. V̇O(2peak) was reduced by 22% immediately postflight (P < 0.05), entirely because of a decrease in peak stroke volume and Q̇c. Peak heart rate, blood pressure, and systemic arteriovenous oxygen difference were unchanged. We conclude that systemic V̇O(2peak) is well maintained in the absence of gravity for 9-14 days but is significantly reduced immediately on return to Earth, most likely because of reduced intravascular blood volume, stroke volume, and Q̇c.