The effects of jump training on bone morphological and mechanical properties were investigated in immature bones of female Fischer 344 rats, Five-week-old rats were divided into control or five jump-trained groups comprised of 5-, 10-, 20-, 40-, and 100-jump groups, representing the number of jumps per day, The rats were jump-trained 5 days/week for 8 weeks, and the height of jump was increased to 40 cm progressively, The femur and tibia in the 5;jump group had significantly greater fat-free dry weights per body weight and maximum loads at the fracture tests than those in the control group, The tibia in the 5-jump group also had significantly larger cortical area at the cross-sectional analysis, Although a slight tendency toward increase according to the number of jumps per day was observed, there were few differences in bone morphological and mechanical parameters among the 10-, 20-, and 40-jump groups, The present results indicate that a large number of strains per day is not necessary for bone hypertrophy to develop in rats.
