Responses of Bone Mineral Density to Isometric Resistance Exercise During Hindlimb Unloading and Subsequent Recovery.

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The objective of this study was to investigate whether isometric resistance exercise (IRE) can attenuate musculoskeletal atrophy during unloading and accelerate its recovery during reloading. Twenty-six female Fischer 344 rats, aged 16 weeks, had their hindlimbs suspended for 3 weeks (unloading) ; 12 of these rats were allowed subsequent cage activity (reloading) for 3 weeks with or without IRE. IRE (stationary support on a cylindrical grid inclined 60 or 80 degrees) was done for 30 min/day, 6 days/week, with an additional load of 30% or 50% body mass attached to the tail during the unloading and reloading periods. The tibial bone and hindlimb skeletal muscles from four experimental and two age-matched control groups were evaluated with dual-energy X-ray absorptiometry, mechanical testing, and muscle mass measurement. Bone mineral density (BMD) was measured in the whole tibia and in 7 regions divided equally along the long axis of the epiphysis from proximal (R1) to distal (R7) . After unloading, fat-free dry mass (FFDM), bone mineral content (BMC), and BMD of the whole tibia decreased by 8%, 10%, and 6%, respectively. FFDM and BMC, but not BMD, returned to the levels of age-matched controls during reloading. Unloading-induced decreases in BMD were observed in the regions from the proximal epiphysis to the diaphysis (R1 to R4) and the distal epiphysis (R7) . The rate of decrease in BMD was regionally specific and was particularly pronounced (12%) in the most proximal region (R1) . These findings indicate regional variations in responses of BMD to skeletal unloading. The BMD in R2 to R4 remained less than that in age-matched control after reloading. No significant changes were observed in maximum breaking load, energy, and deformation after unloading and reloading. Hindlimb-unloading induced loss of mass in the soleus (38%), plantaris (14%), gastrocnemius (25%), tibialis anterior (8%), extensor digitorum longus ( 8%), and rectos lemons (17%) muscles, but the mass of muscles, except for the soleus muscle, recovered during reloading. IRE ameliorated the loss of mass in the soleus and gastrocnemius muscles during unloading but did not promote the recovery of mass in any muscles during reloading. Moreover, IRE showed no effect on bone responses after unloading and reloading. This lack of beneficial effects of IRE on osteopenia may be due, in part, to insufficient exerciseinduced load. We concluded that 1) regional analysis of BMD can be used to assess local bone metabolism, 2) the response of BMD to altered loading conditions does not necessarily depend on the response of muscle mass, 3) recovery from osteopenia progresses more slowly than that from sarcopenia, and a longer time than the unloading period is required to restore BMD. Further studies are needed to develop more effective countermeasures against osteopenia and sarcopenia.

Responses of Bone Mineral Density to Isometric Resistance Exercise During Hindlimb Unloading and Subsequent Recovery.

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