Exercise capacity during and after spaceflight

Our aim was to elucidate why astronaut's exercise capacity after spaceflight is reduced. Therefore, the kinetics of oxygen uptake (VO2) as a measure for muscular aerobic capacity, as well as maximal oxygen uptake (VO2peak), and anaerobic threshold (PAT) as overall measures for exercise capacity were determined. Measurements of VO2peak and AT were restricted to pre- and postflight sessions.

Four crew-members of the D-2 mission (10 d) were cycling with steady state phases at 20 W and 80 W, followed by 450s of pseudo random binary sequence (PRBS) changes between 20 W and 80 W, and an incremental exercise test (10 W every 30 s) up to subjective exhaustion. Breath-by-breath VO2, VCO2, ventilation, HR, and blood pressure were continuously recorded. Blood lactate samples were drawn only during the incremental phase. The VO2 kinetics were determined by evaluation of the relationship between the workload and the instantaneous oxygen uptake of each subject. The cross-correlation function between both variables showed two characteristic items, the maximum as a measure for the muscular aerobic capacity, and its lag representing time consuming processes.

Each subject showed an individual characteristic of VO2 kinetics. In flight, no significant changes were detected compared to preflight data. Decreases in lags of cross-correlation function maxima and decreases in mean blood pressure during exercise indicate lowered blood volumes 2 d after the flight. Lowered blood volumes can explain the losses in exercise capacity. The maximum of the cross-correlation function did not change significantly which indicates unchanged muscular oxidative capacity.