Effects of free mobilization and low- to high-intensity treadmill running on the immobilization-induced bone loss in rats.

After an immobilization period of 3 weeks, the effects of free remobilization (8 weeks) as well as low- and high-intensity treadmill running on the bone mineral content (BMC) and density (BMD) of the hindlimbs of Sprague-Dawley rats (n = 70) were studied using a dual-energy x-ray absorptiometric scanner. In the low-intensity running program, the rats were allowed to move freely in the cage for 1 week, after which they started to run on a treadmill twice a day for 7 weeks. The speed of the treadmill was 20 cm/s, with an uphill inclination of 10°. The running time was gradually increased from 20 minutes per session to 45 minutes per session. In the high-intensity group, the program was similar, with the exception that the speed of the treadmill was 30 cm/s, with an uphill inclination of 30°. Immobilization for 3 weeks produced a significant BMC and BMD loss in the immobilized left femur and tibia (mean loss 9.6%, p < 0.001) but did not affect the right free limbs. Both low- and high-intensity running restored mineral content in the immobilized limb; however, an average 5% difference (p < 0.05) in mineral content of the right and left limb bones persisted. In the running groups, the values for the immobilized left limbs were at the same level or exceeded (range 3.8–11.6%, p < 0.05–0.01) and those of the free right limbs exclusively exceeded (range 5.3–15.9%, p < 0.05–0.01) the corresponding values of the age-matched control rats. In the group with free remobilization, the BMCs and BMDs always remained below those in the controls (range –3.2 to –12.6%, p values ranging from NS to p < 0.01). Thus, this study showed the need for greater than normal activity to restore the BMC and BMD from disuse to normal.