The body undergoes a wide variety of physiological adaptations when exposed to an environment of microgravity. One of the major alterations in the musculoskeletal system during weightlessness is the loss of bone mass. The ability of the body to readapt to the Earth’s gravitational field after prolonged exposure to microgravity is remarkable, especially when considering the extended periods of time currently spent in space. However, there is still much to learn regarding the adaptive processes in space and their responsible mechanisms. Although many different countermeasures have been used to combat space flight-induced osteoporosis, none have been successful. It has been contended that in order to maintain the integrity of the skeleton, a component of loading must be present. In space, however, skeletal loading on the body is virtually eliminated and reflected in the fact that the body, and especially the lower extremities, lose bone mass. Current exercises protocols used in space to combat bone loss are not effective in providing the necessary axial loading. Therefore, activities which contain a component of high impact force must be investigated as a possible solution.

This study proposes that Jumping exercises, which are known to impart high impact loads to the body, will help to combat space flight-induced osteoporosis. The underlying hypothesis is that as the impact load and loading rate increases, so too does the internal strain in the bone. Previous investigations have determined the osteogenic threshold of strain but none have been successful in relating this level to the external forces during activity. Therefore, this study attempted to determine the relationship between the external forces and the internal bone strain during jumping activities.

A zero gravity simulator was constructed and jumping exercises were performed at four different gravity levels in the simulator and in IG. Twelve subjects, who represented the anthropometric characteristics of the current U.S. astronauts, were recruited for the study. Four of the subjects were instrumented with a calcaneal strain transducer in order to assess the level of strain during jumping. Ground reaction forces, tibial and calcaneal accelerations, and ankle and knee joint angles were recorded during jumping. Three different landings were performed at four zero gravity simulator tension levels and in IG in order to obtain a wide response in the measured parameters. Results obtained in these experiments were compared to drop test data on cadaveric feet. A theological model of the hind foot, including the fat pad and the calcaneal bone, was also developed in order to predict the strains in the bone and the effectiveness of the heel pad in absorbing the shock transient following high impact forces.

Results showed that strains in the calcaneus were significantly higher in jumping than the proposed osteogenic threshold. However, no clear relationship between the strain and the external ground reaction forces were seen. Differences existed in the level of the force, loading rate and accelerations between the zero gravity simulator conditions and IG suggesting that the mechanics of jumping is altered in microgravity. Differences were also found for different landing type with the flat-footed landing producing the highest magnitudes in the variables. Comparison of experimental variables to the cadaveric data showed higher magnitudes for the in vivo strains with no apparent differences in acceleration data. The theological model was successful in predicting bone strains akin to those found in vivo for similar peak forces. The model also showed a highly correlated relationship between force and strain.

The results of this investigation are important in light of the fact that they were able to show that the strain magnitude in the calcaneus exceeded the osteogenic level regardless of either tension level or jump type. This suggests that jumping may be a viable exercise for preventing bone loss in space. It may also prove to be a more cost and time effective alternative to exercise countermeasures in space.