The detrimental effects of unloading on bone have been well-documented. Bone loss experienced by astronauts and bedrest patients can be as high as 3% per month. To design effective countermeasures without significant side-effects, the mechanisms by which unloading reduces tissue quantity and quality need to be understood. To this end, this study investigated whether the length of the reambulation period between successive exposures to unloading can mitigate, at least in part, the detrimental impact on the musculoskeletal system.

A genetically diverse adult female mouse population (F2 of BALB/cByJ x C3H/HeJ) was chosen to generalize findings across individuals with distinct genetic make-up. At 16wks of age, 30 animals were randomly divided into 2 experimental groups of n=15 each. Fifteen additional animals served as age-matched ambulatory controls (cf). Both experimental groups were subjected to 3 exposures to hindlimb unloading (HLU) of 3wks each. The first experimental group (3wf) was given 3wks of normal reambulation (RA) period between unloading cycles, while the second group (9wf) was subjected to 3 times as much reambulation time, 9wks, between successive exposures to unloading.

All mice were subjected to activity monitoring, in vivo computed tomography at the proximal and diaphyseal femur and abdomen, and in silico mechanical stress analysis, prior to and after each HLU. Control mice were subjected to the same procedures during the experimental protocol. Activity level, tissue morphology, and mechanical stress levels recorded indicated consistent deterioration during disuse. Generally, cortical sensitivity to disuse was an order of magnitude higher than that of trabecular bone to compression. Von-Mises mechanical stress experienced in the femoral distal metaphysis largely echoed the trends exhibited by morphology, except that the cortex was more mechanically sensitive to unloading than trabeculae. Activity patterns indicated a remarkable similarity in spite of large differences in genetic makeup between individuals. Activity patterns were insensitive to the specific pattern of HLU/RU and showed only low to moderate correlations with changes in tissue morphology/stress. The magnitude of recovery tended to be proportional to the length of the RA period, with the 9wk recovery time being able normalize the tissue morphology to that of age-matched controls. Abdominal fat volume was greatly reduced by unloading while reambulation caused tissue recovery normal control levels even in mice subjected to only 3wk of RA.

In summary, a recovery period thrice as long as the duration of exposure can revert most harmful effects of disuse to the skeleton. Unfortunately, the longer reambulation period also restores bone's susceptibility to unloading while a much shorter reambulation period may suppress it. If confirmed by human studies, this data indicates a limited potential of optimizing reambulation periods between successive space missions.