The purpose of this study was to examine the bone stiffness of healthy girls during their growth period and factors affecting on such stiffness. The subjects were 302 secondary school girls in Tokyo. Parameters examined included the body characteristics (standing height, body weight, bone stiffness measured by ultrasound, muscle thickness measured with the ultrasound B-mode system), extension power of the lower limb (containing the knee and hip joints), and a questionnaire about the daily intake of milk, kinds of meals and number of years from menophania. These examinations were carried out in June 1997.
Results were summarized as follows : 1) From the observation of bone stiffness in secondary school girls, it appears that bone stiffness increases during the junior high school period. In other words, the bone stiffness of the subjects had almost reached on adult level by high school. 2) A significant positive correlation was recognized between chronological age and bone stiffness (r=0.365, p<0.05) . A positive correlation also existed between the years from menophania and bone stiffness (r=0.477, p<0.05) . These coefficients showed that the years from menophania correlate with bone stiffness more closely in comparison with chronological age. 3) In the period when short comparatively years from menophania, body characteristics, which were the index of maturity, correlated to bone stiffness. However, muscle thickness/power, which was related to exercise habit, became the major parameter correlating with bone stiffness. These results suggest that factors affecting bone stiffness should differ according to the growth period. 4) Comparison of different athletic clubs showed that the bone stiffness of volleyball players was higher than that of control. These data suggest that physical education class, which was given 3 times a week, is not enough for total body development in both quality and quantity. In other words, physical education should be better matched with total physical development, including the growth of healthy bones.