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.

The extensive use of food additives has increased the phosphorous content of the modern diet, while calcium intake has remained similar to past levels according to the national standards of nutrient intake. Although exercise increase bone mineral content, the intake of phosphorus may change the exercise effect. The purpose of this study was to examine the effects of jump exercise on bone and phosphate-calcium metabolism in rats consuming high levels of dietary phosphorous. Forty-two male Wistar rats aged 8 weeks were fed either a high-phosphorus diet with a 2.0 P/Ca ratio or a normal diet with a 1.0 P/Ca ratio. Rats from each dietary group were then further assigned to undergo 8 weeks of jump exercise or to be sedentary controls. Two-way analysis of variance (ANOVA) revealed that the bone mineral content (P<0.001), strength (P<0.001), transverse thickness (P<0.001), and longitudinal thickness (P<0.001) of the tibial diaphysis were increased by jump exercise in both dietary groups. The concentrations of serum inorganic phosphorus (P<0.001), FGF23 (P<0.001), and 1-25 (OH) vitamin D (P<0.001) were increased by a high phosphorus diet, and the concentrations of serum total calcium (P<0.05) and 1-25 (OH) vitamin D (P<0.05) were increased by jump exercise in both groups. In conclusion, exercise is important to increase bone mass and bone strength in a high phosphorus intake state.