Bioeffects of Microgravity and Hypergravity on Animals

Gravity alterations in space cause significant adaptive effects on the human body, including changes to the muscular, skeletal, and vestibular systems. However, multiple factors besides gravity exist in space; therefore, it is difficult to distinguish gravity-related bioeffects from those of the other factors, including radiation. Although everything on the Earth surface is subject to gravity, gravity-induced effects are not explicitly clear. Here, different research methods that have been used in gravity alterations, including parabolic flight, diamagnetic levitation, and centrifuge, are reviewed and compared. The bioeffects that are reported to be associated with altered gravity in animals are summarized, and the potential risks of hypergravity and microgravity are discussed, with a focus on microgravity, which has been studied more extensively. It should be noted that although various microgravity and hypergravity research methods have their limitations, such as the inevitable magnetic field effects in diamagnetic levitation and short duration of parabolic flight, it is evident that ground-based clinical, animal, and cellular experiments that simulate gravity alterations have served as important and necessary complements to space research. These researches not only provide critical and fundamental biological information on the effects of gravity from biomechanics and the biophysical perspectives, but also help in developing future countermeasures for astronauts.