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.

The relationships between physical activity and childhood body size, low physical fitness epidemic are still unclear. The purpose of this study was to examine the relationship between energy expenditure estimated by doubly labeled water (DLW) method and body fatness, physical fitness in children, and the relationship between physical activity levels and percent time spent in activities. 30 healthy Japanese children (20 boys and 10 girls) participated in this study. The total energy expenditure (TEE) and % body fat were measured by the DLW method over a 6-day period. The physical activity-related energy expenditure (PAEE) was calculated as (TEE × 0.90) – basal metabolic rate (BMR). The physical activity level (PAL) was also calculated as TEE/BMR. The physical fitness tests (8 items) were applied to evaluate fitness, and scores of each test were calculated as overall physical fitness score. The TEE was 2009.8 ± 272.6 kcal/day, the PAEE was 558.4 ± 206.1 kcal/day and the PAL was 1.61 ± 0.18. TEE per weight and PAEE per weight (PAEE/wt) was significantly negatively correlated with % body fat (r = - 0.626; r = - 0.400, respectively). These results suggest that increasing energy expenditure is important for achieving adequate body size. The PAEE/wt was most strongly correlated with physical fitness score (r = 0.680). The PAL was associated with percent time spent of inactivity ( r = -0.506), light-moderate activity ( r = 0.450) and vigorous activity ( r = 0.545). It was suggested that physically active lifestyle would be necessary for childhood health.

The purpose of this study was to examine the relationships between patterns of daily physical activities measured by accelerometer and physical activity level (PAL) in children. Firstly, activity intensities during incremental exercise were measured using a tri-axial accelerometer (HJA-350IT) in twenty one children aged 10.6 ± 0.9 years. As a result of receiver-operating characteristic curve analysis, the cut-off value for discrimination between walking and running activity was set at 7.2 METs of HJA-350IT. Secondly, total energy expenditure (TEE) in daily life was measured by doubly labeled water method, and durations of lifestyle, walking, and running activities were measured by the accelerometer in 6^th grade elementary school children (11 boys and 10 girls). TEE and physical activity level (PAL) were 2,021 ± 343 kcal/day and 1.56 ± 0.17, respectively. The average durations of lifestyle, walking and running activities were 188 ± 30 min/day (50.6 ± 6.0 %), 171±28 min/day (45.9 ± 5.0 %) and 13.3 ± 7.6 min/day (3.5 ± 1.8 %), respectively. The proportion of the duration of running activity was positively correlated with PAL (r = 0.615, p < 0.01), and the proportion of the duration of lifestyle activity was negatively correlated with PAL (r = -0.439, p < 0.05). There was no relationship between the proportion of the duration of walking activity and PAL (r = 0.300, n.s.). These results suggest that running activity is important to increase PAL more than ever in primary school children.