Effects of short term bed – rest on exercise response in humans

Exposure to real (space flight) or simulated microgravity (e.g., head–down tilt bed rest) induces remarkable alteration of maximal aerobic exercise capacity in humans. Maximal oxygen uptake (V'O2max) drops rapidly during the first 3-6 days to decrease more gradually up to 22 – 17 % of the control values after 30 – 42 days of bed rest (Convertino DA, 1996, Ferretti G et al, 1997, Fortney SM et al, 1996, Saltin B et al, 1968). The prompt decay of V'O2max is mainly the consequence of the cardiovascular deconditioning ensuing during real and artificial microgravity. The shift of blood and body fluids occurring toward the head and thorax at the onset of microgravity is sensed as a volume overload signal and triggers an integrated cardiovascular-neuro-endocrine response leading to an immediate reduction of total peripheral resistance and to an increased water excretion by the kidney. The consequence is a rapid reduction in circulating volume, which may range from 8.4 % to 16 % in comparison with pre – flight conditions during 4-84 days missions (Convertino DA, 1996). The observed reduction of circulating blood volume has a detrimental impact on cardiac haemodynamics. Indeed, the decay of and SV seem to be mainly caused by the following cascade of events: the increased elimination of plasma water brings about the reduction in circulating blood volume. This leads, through its effect on venous return, to the reduction of the heart chambers volume and, on longer periods of microgravity exposure, to cardiac atrophy and re–modelling (Levine BD et al, 1997, Perhonen MA et al, 2001), which, in turn, may alter the mechanical functions of cardiac pump in vivo. As a consequence of this chain of events, the volume of blood pumped by the heart at each cardiac cycle drops. Bed rest campaigns commonly aimed to investigate, in a single group of subjects, the effect of this sort of intervention without considering the possible cross-linked effects of bed-rest and inactivity and/or confinement. In the Short Term Bed Rest – Integrative Physiology (STBR-IP) study, organised at the DLR Institute of Aerospace Medicine in Cologne (D), an experimental protocol was planned to take into considerations the weakness characterising the other similar studies carried out in the past. To this aim, the same group of subjects was studied before and after confined ambulatory and bed rest periods of identical duration. This allowed us to disentangle the effects of restricted physical activity in a confined environment from those due to bed-rest. In this paper, the results obtained during sub maximal and maximal exercise in the study at stake are reported. Data deal with oxygen uptake and cardiac output at maximal exercise assessed, in the very same subjects, before and after 14-days of head down bed-rest and ambulatory periods.