Factors determining the kinetics of VO2max decay during bed-rest: implications for VO2max limitation

The aim of this study was to characterize the time course of maximal oxygen consumption (V'O2 max) changes during bedrests longer than 30 days, on the hypothesis that the decrease in V'O2 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 (Qmax) and maximal systemic O2 delivery (QaO2max) were measured. After all periods of HDT, V'O2 max; Qmax and QaO2max were significantly lower than before. The V'O2 max decreased less than Qmax after the two shortest bedrests, but its percent decay was about 10% larger than that of Qmax 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 Qmax and QaO2max decline after 90-day bedrest was equal to those after 14- and 42-day bedrest. The average daily rates of the V'O2 max; Qmax and QaO2max decay during bedrest were less if the bedrest duration was 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.