Ground-based models used to simulate hypogravity have facilitated the interpretation of spaceflight studies. In ground-based simulation models, stress effects can be controlled, which has important implications for research on reproduction because stress responses are known to adversely affect reproductive function. For example, in animals and humans increased cortisol excretion associated with stress can result in reduced serum T levels. Thus, it has been difficult to ascertain whether the reproductive changes are due to stress or to hypogravity associated with spaceflight.
Chronic stress in spaceflight rats was indicated by higher adrenal gland weights compared with controls. In studies of astronauts aboard the MIR Space Station, increased cortisol was reported during spaceflight. In ground-based studies utilizing bed rest to simulate hypogravity, Blanc et al. found elevated cortisol excretion during the initial 4 wk of bed rest. Vernikos et al. compared urinary cortisol excretion in men and women during 1 wk of bed rest and reported elevated cortisol in the men only.