Jump exercise during remobilization restores integrity of the trabecular architecture after tail suspension in young rats

Three-dimensional trabecular architecture was investigated in the femora of tail-suspended young growing rats, and the effects of jump exercise during remobilization were examined. Five-week-old male Wistar rats (n = 35) were randomly assigned to five body weight-matched groups: tail-suspended group (SUS; n = 7); sedentary control group for SUS (S-CON; n = 7); spontaneous recovery group after tail suspension (S + R-CON, n = 7); jump exercise group after tail suspension (S + R-JUM; n = 7); and age-matched control group for S+R-CON and S+R-JUM without tail suspension and exercise (S-CON+R-CON; n = 7). Rats in SUS and SCON were killed immediately after tail suspension for 14 days. The jump exercise protocol consisted of 10 jumps/day, 5 days/wk, and jump height was 40 cm. Bone mineral density (BMD) of the femur and three-dimensional trabecular bone architecture at the distal femoral metaphysis were measured. Tail suspension induced a 13.6% decrease in total femoral BMD (P < 0.001) and marked deterioration of trabecular architecture. After 5 wk of free remobilization, femoral BMD, calf muscle weight, and body weight returned to age-matched control levels, but trabeculae remained thinner and less connected. On the other hand, S+R-JUM rats showed significant increases in trabecular thickness, number, and connectivity compared with S+R-CON rats (62.8, 31.6, and 24.7%, respectively; P < 0.05), and these parameters of trabecular architecture returned to the levels of S-CON+R-CON. These results indicate that suspension-induced trabecular deterioration persists after remobilization, but jump exercise during remobilization can restore the integrity of trabecular architecture and bone mass in the femur in young growing rats.