The human skeletal system in weightlessness: A review of research data,hypotheses, and the possibility of predicting the state in long-term (Interplanetary) missions

The long-term research of human skeletal system during spaceflight on the orbital station Mir and International Space Station (ISS) was summarized. The amount of bone mass and body composition was measured using a noninvasive method, dual-energy X-ray absorbtiometry (DXA) or osteodensitometry. Theoretically expected loss of bone mass in tubular structures of the lower part of the body during space flight with a duration of five to seven months is described by the phenomenon of fast-developing but reversible osteopenia and is considered a manifestation of functional adaptation of bone tissue to the changing mechanical load on the skeleton. A high individual variability of changes and stability of individual nature of the ratio of bone mass changes in different segments of the skeleton independently of the type of orbital station has been demonstrated. A strict dependence of bone mass changes on the flight duration cannot be established, and there are no grounds for calculating the probability of reaching the critical level of demineralization for the duration of flight increased to 1.5–2 years. There is even less probability to predict changes in bone structure (quality), which, together with the loss of bone mass, determine the risk of fracture. The data indicating that the DXA method is insufficient for such prognosis are presented. The main areas of research that would optimize the development of the project of interplanetary mission in terms of preservation of the mechanical function of the skeleton are considered.