Maximal oxygen uptake (V_O2,max) decreases as a consequence of bed rest (Fortney et al. 1996; Convertino, 1997). However, the rate at which this decrease occurs has not been established yet. The opportunity of performing V_O2,max determinations before and after bed rest periods of varying duration with the same protocols allowed us to make a step forward along this direction.

A total of 17 healthy young men were investigated in three programmes: four (age 22 ± 1.3 years) after a 15-day bed rest, seven (age 28 ± 1.0 years) after a 42-day bed rest and six (age 33 ± 1.8 years) after a 90-day bed rest. All these programmes were organised by the European Space Agency (ESA) either in Köln, Germany (2001) or in Toulouse, France (1994 and 2001). The data obtained in 1994 after a 42-day bed rest were published in a different context (Ferretti et al. 1997).

Individual V_O2,max was determined during graded exercise on a bicycle ergometer. The oxygen uptake (V_O2) at the metabolic steady state was measured at rest and during exercises of increasing intensities. Starting from 50 W, power was progressively augmented by steps of 50 W, reduced to 25 W as the expected individual maximum power was approached. The duration of each work load was at least 5 min. Successive work loads were separated by 5 min recovery intervals, during which time blood samples (20 µl) were obtained from an ear lobe at 1, 3 and 5 min for determination of blood lactate concentration ([La]) by means of an electro-enzymatic method. Heart rate (HR) was determined continuously. V_O2 was measured either by the open circuit method (1994) or on a breath-by-breath basis (2001). Individual V_O2,max was established from the plateau attained by the relationship between V_O2 and power above a given power.

The results are shown in Fig. 1 where V_O2,max, expressed relative to the values observed before bed rest set equal to 100 %, are plotted as a function of the bed rest duration. The same decrease was observed for maximal power. Maximal HR and maximal [La] were the same after as before bed rest in the three studies. The average rate of the decrease in V_O2,max was 1.08 % day^-1 for 15-day duration bed rest, 0.39 % day^-1 for the 42-day duration bed rest and 0.32 % day^-1 for the 90-day duration bed rest.

These results suggest that the decrease in V_O2,max as a consequence of bed rest may tend to an asymptote. We postulate that this tendency may reflect the coupling of a more rapid cardiovascular deterioration with a slower reduction in muscle mass.

This study was supported by the Swiss National Science Foundation grant number 32-61780.00 to Guido Ferretti and by the Italian Space Agency to Carlo Capelli. The bed rest programmes were organised by the European Space Agency (ESA) in collaboration with the Centre National d’Etudes Spatiales (CNES) and the Institut de Médecine Spatiale (MEDES), France, the German Institut for Aerospace Medicine and the Japanese Space Agency (NASDA).

All procedures accord with current local guidelines and the Declaration of Helsinki.