Hideki YAMAUCHI, Hiroyuki KAWAI, Shoji MASHIKO, Masaki KIMURA, Satoshi MIYANO, Kyozo YONEMOTO

Division of Physical Fitness, Department of Rehabilitation Medicine, Jikei University School of Medicine

(Received 17 June 2002; accepted 5 December 2002)

Abstract: Non-weight-bearing associated with a microgravity environment or bed rest induces musculoskeletal atrophy. It takes a longer time to restore bone mass than skeletal muscle mass. The purpose of the present study was to investigate whether isometric resistance exercise (IRE) can attenuate loss of bone mass during unloading and accelerate its recovery during reloading. Twenty-six Fischer 344 female 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 30min/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 femurs from four experimental and two age-matched control rats were evaluated with dual-energy X-ray absorptiometry, mechanical testing, and fat-free dry mass (FFDM) measurement. Bone mineral density (BMD) was measured in the whole femur and in 7 regions divided equally along the long axis of the epiphysis from proximal (R1) to distal (R7). After unloading, FFDM, bone mineral content (BMC), and BMD of the whole tibia decreased by 6.3%, 8.7%, and 6.0%, 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 each of the 7 evenly divided regions. The rate of decrease in BMD was region-dependent and was pronounced in the epiphysis and the metaphysis compared to the diaphysis. The BMD in R4, R6, and R7 remained less than that in the age-matched controls after reloading. No significant changes were observed in maximum breaking load, energy, and deformation after unloading and reloading. IRE showed no effects on bone loss after unloading and reloading. We concluded that 1) regional variations in bone responses to skeletal unloading exist, and 2) a longer time than the unloading period is required for recovery from osteopenia. Further studies are needed to develop more effective countermeasures against osteopenia and to aid its recovery.

Key words: unloading, reloading, bone mineral density, mechanical property, isometric exercise