The relationship between physical fitness and physical activity in abolescents was longitudinally investigated for 19 months. The subjects were 266 healthy males (industry workers) aged 15-17 years. Three groups of subjects were classified according to the difference of their physical activity time: Group A (23-45 min/week), B (45-90 min/week), and C (180-270 min/week). The physical fitness program containing weight training, circuit training and long distance running was carried out on group C for l8 months. The physical performance tests, consisted of 12 items, were tested in every six months on all subjects. The results were as follows: 1. In groups A and B, test results in flexibility, muscle strength, pull-up, ball throw and vertical jump were improved according to the age advanced, but endurance was depressed remarkably. Comparing to the mean value of high school boys, the ratios of improvement in performance tests were lower in all items except for muscle strengh and agility. 2. In group C, all performance test recordings were improved according to the age advanced. Agility and endurance were improved rapidly in a short time (6 months) after the physical fitness program was started. On the other hand, it took long time (12-18 moths) to improve the ability of 50m dash and vertical jump. 3. In muscle strength, group C was stronger than the control group (high school boys) in 16 years old, but no significant differences were found in quantitative increase for one year in hand grip, arm flex strength and significant differences in back strength and leg extensor strength were recognized. 4. After physical fitness program was carried out for 19 months, the maximum oxygen intake of group C (n=74) was recorded to be 3.08 l/min or 52.21 ml/kg.min.
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