Factors determining the time course of \({\dot{V}}\hbox{O}_{2\max}\) decay during bedrest: implications for \({\dot{V}}\hbox{O}_{2\max}\) limitation

The aim of this study was to characterize the time course of maximal oxygen consumption \(({\dot{V}}\hbox{O}_{2\max})\) changes during bedrests longer than 30 days, on the hypothesis that the decrease in \({\dot{V}}\hbox{O}_{2\max}\) 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, \({\dot{V}}\hbox{O}_{2\max}\) maximal cardiac output \((\dot{Q}_{\rm max})\) and maximal systemic O₂ delivery \(({\dot{Q}}\hbox{aO}_{2{\rm max}})\) were measured. After all periods of HDT, \({\dot{V}}\hbox{O}_{2\max},\;{\dot{Q}}_{\rm max}\) and \({\dot{Q}}\hbox{aO}_{2{\rm max}}\) were significantly lower than before. The \({\dot{V}}\hbox{O}_{2\max}\) decreased less than \({\dot{Q}}_{\rm max}\) after the two shortest bedrests, but its per cent decay was about 10% larger than that of \({\dot{Q}}_{\rm max}\) after 90-day bedrest. The \({\dot{V}}\hbox{O}_{2\max}\) decrease after 90-day bedrest was larger than after 42- and 14-day bedrests, where it was similar. The \({\dot{Q}}_{\rm max}\) and \({\dot{Q}}\hbox{aO}_{2{\rm max}}\) declines after 90-day bedrest was equal to those after 14- and 42-day bedrest. The average daily rates of the \({\dot{V}}\hbox{O}_{2\max}, \;{\dot{Q}}_{\rm max}\) and \({\dot{Q}}\hbox{aO}_{2{\rm max}}\) decay during bedrest were less if the bedrest duration were longer, with the exception of that of \({\dot{V}}\hbox{O}_{2\max}\) in the longest bedrest. The asymptotic \({\dot{V}}\hbox{O}_{2\max}\) decay demonstrates the possibility that humans could keep working effectively even after an extremely long time in microgravity. Two components in the \({\dot{V}}\hbox{O}_{2\max}\) 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.