The effect of low-frequency jump-training on bone was investigated in intact (Intact) and ovariectomized (OVX) rats. Rats were divided into the following three groups for each experiments: sedentary (Intact-Sedentary and OVX-Sedentary), training once per week (Intact-1T and OVX-1T), and training three times per week (Intact-3T and OVX-3T) . The jump-training started at the age of 11 weeks for the Intact rats, and 12 weeks for the OVX rats, The jump-training protocol was 10 times day, 1 or 3 days week for 8 weeks, with a jumping-board height of 45 cm for the Intact experiment and 40 cm for the OVX experiment. After training, the fat-free dry weight, maximum breaking force, and bone morphometry in the right tibia were determined. The fat-free dry weight and maximum breaking force for the training groups in both experiments were significantly higher than those for the respective sedentary groups, though there were no significant differences between training groups. In the Intact experiment, the increases of cortical area at the tibia midlength in both training groups were especially remarkable. llowever in the OVX experiment, there were no significant differences between the sedentary and training groups in cross-sectional analysis. These results indicate that low-frequency jump-training had beneficial effects in term of increased bone mass and strength in the Intact rats and the OVX rats as well.

Mechanical stress imposed by physical exercise is known to play an important role in increasing bone mass and preventing osteoporosis. As repetitional loadings may diminish mechano-sensitivity of bone cell, understanding shifts in mechano-sensitivity is important for making an effective training program for bones. The primary purpose of this study was to investigate bone responses when rats performed 400 repetitions of jump exercise in total using different training programs over 40 days. The secondary purpose was to clarify whether loading magnitude affects the results. This study comprised two experiments (EX1, EX2). In each experiment, 60 female Wistar rats (10-weeks-old) were divided into 1 sedentary group and 4 exercise groups. The 4 exercise groups were exercised with 10 jumps every day (10 jumps/day), 20 jumps every other day (20 jumps/2 days), 50 jumps every 5 days (50 jumps/5 days) or 100 jumps every 10 days (100 jumps/10 days). Jump height was set at 40 cm in EX1 and 30 cm in EX2. After 40 days, the 10 jumps/day, 20 jumps/2 days and 50 jumps/5 days groups in EX1 and EX2 showed significantly greater bone mass and strength compared to each sedentary group, although differences in training effects between these 3 groups were little. The 100 jumps/10 days group in EX1 also showed significantly greater bone mass and strength compared to the sedentary group, but the training effect was less. The 100 jumps/10 days group in EX2 did not show significantly greater bone mass and strength compared to the sedentary group. These results suggest that exercise everyday or every other day is not always necessary for bone development.