There are many challenges to overcome when conducting spaceflight experiments with mammals. For example, it is difficult to distinguish whether reproductive changes are due to gravity or other conditions aboard the spacecraft, such as increased radiation, noise, isolation, disrupted circadian rhythms, and stress. Responses to decreased gravity on orbit versus increased gravity during landing are difficult to separate because subjects are not immediately recovered after flight.
Thus, observed physiological changes may reflect recovery or landing (acute hypergravity) responses rather than in-flight (hypogravity) effects. Reproductive studies are also hampered by the short duration, high costs, and limited opportunities of spaceflight. To address some of these problems, scientists have designed simulation models of spaceflight.
One ground-based model for simulating hypogravity exposure in humans is bed rest with a 6° head-down tilt to initiate shifts in body fluids comparable to those experienced during orbital spaceflight. Bed rest is a convenient method that is reasonably effective at reproducing the major symptoms of hypogravity experienced during spaceflight. In rodent studies, the Morey-Holton hindlimb suspension (HLS) model has been used to simulate the major physiological effects of hypogravity.