Factors determining the kinetics of V’O2max decay during bed-rest: implications for V’O2max limitation

The aim of this study was to characterize the time course of maximal oxygen consumption (V'O2 max) changes during bed rests longer than 30 days,on the hypothesis that the decrease in _VO2 max tends to asymptote. On a total of 26 subjects who participated in one of three bedrest campaigns without countermeasures, lasting 14, 42 and 90 days, respectively, V'O2 max maximal cardiac output (Q'max) and maximal systemic O2 delivery (Q'aO2max) were measured. After all periods of HDT, V'O2 max; Q'max and Q'aO2max were significantly lower than before. The V'O2 max decreased less than Q'max after the two shortest bedrests, but its per cent decay was about 10% larger than that of Q'max after 90-day bedrest. The V'O2 max decrease after 90-day bedrest was larger than after 42- and 14-day bedrests, where it was similar. The Q'max and Q'aO2max declines after 90-day bedrest was equal to those after 14- and 42-day bedrest. The average daily rates of the V'O2 max; Q'max and Q'aO2max decay during bedrest were less if the bedrest duration were longer, with the exception of that of V'O2 max in the longest bedrest. The asymptotic V'O2 max decay demonstrates the possibility that humans could keep working effectively even after an extremely long time in microgravity. Two components in the V'O2 max decrease were identified, which we postulate were related to cardiovascular deconditioning and to impairment of peripheral gas exchanges due to a possible muscle function deterioration.