Physiology of physical inactivity, sedentary behaviours and non-exercise activity: insights from the space bedrest model

Abstract

Physical inactivity, i.e. not reaching the recommended level of physical activity (PA), and sedentary behaviours (SB), i.e. sitting time, have been associated with increased risk for common metabolic diseases. Recent epidemiological data suggest that high volumes of SB are detrimental to metabolic health, even in the presence of regular exercise, i.e. moderate/vigorous PA. This suggests that the health effects of SB are independent from those of exercise. However, experimentally testing this hypothesis is complicated because of the difficulty in disassociating SB from PA. Bedrest studies, a traditional space science model, can offer new insights. In some bedrest studies, an exercise training protocol has been used to counteract the harmful effects of inactivity. While bedrest induces an inactive and sedentary state, exercise with bedrest represents a unique model of sedentary yet physically active people. Here, we review bedrest studies with and without exercise training. Although exercise training prevents the loss of muscle mass and function, even large volumes of exercise are not sufficient to fully counteract the negative metabolic adaptations triggered by inactivity. This observation supports the existence of independent adverse health effects of SB, but also the potential benefits of non-exercise activity, i.e. daily living light PA. We gathered available data to examine the complex relationships between exercise, non-exercise activity, SB and health outcomes. Given the large amount of SB in modern societies, the sole promotion of exercise, i.e. moderate/vigorous PA may be insufficient, and promotion of light PA may be a complimentary approach to improve health.

Keywords: bedrest; exercise; light physical activity; metabolism; moderate to vigorous physical activity; non-exercise activity; physical inactivity.

Figure 1:. Schematic representation of the components of total energy expenditure during bedrest, conducted with or without exercise training.

Based on total energy expenditure, participants enrolled in bedrest protocols can be compared to the general population. Strict bedrest suppresses both components of physical activity energy expenditure: exercise activity energy expenditure and non-exercise activity energy expenditure. Exercise activity energy expenditure refers to the energy spent in MVPA and/or structured exercise. Non-exercise activity energy expenditure corresponds to any activity of daily life, which are essentially LPA. Participants who are subjected to moderate to vigorous exercise training along with bedrest maintain high exercise activity energy expenditure mainly due to MVPA. However, they are sedentary with very low levels of non-exercise activity energy expenditure and are lacking LPA. These individuals represent an extreme but unique model of “sedentary exercisers”, i.e. physically active yet sedentary people. Strict bedrest leads to a decrease of both MVPA and LPA while increasing SB. These bedrest individuals represent a model of the modern physically inactive sedentary individuals. SED: sedentary activities; LPA: light-intensity physical activity; MVPA: moderate-to-vigorous physical activity. Adapted from Bergouignan et al 2010.

Figure 2:. Preventive effect of exercise (resistance exercise or resistance plus aerobic exercise) against the bedrest-induced physiological and metabolic alterations.

− : no effect; + : partially protected; ++ : fully protected; ?: no data available; ± : no consensus.