We conducted a cross-sectional study to clarify the effects of physical activity on physical fitness. Physical fitness tests were offered to workers of companies manufacturing automobile parts from December, 1992 to December, 1993. The subjects were 1, 217 male and 600 female workers who participated in the physical fitness test. From this group, we analyzed 1, 048 male and 522 female workers who answered a physical activity questionnaire. The questionnaire included age, sex, working posture, physical activity during working time and physical activity during leisure time. The physical fitness tests were composed of grip strength, standing trunk flexion, foot balance with closed eyes, jumping reaction time and step tests. We classified the subjects into low performance group and high performance group for each test. In addition, we classified the subjects into different groups by age (16-29 years old, 30-49 years old and 50-69 years old), by working posture (standing and sitting), as well as by activity levels during working time and leisure time physical activities (inactive and active). Thus, we examined potential risk factors for the physical fitness by a multiple logistic regression model.<br>The results were as follows:<br>1. Standing work was a significant risk factor for grip strength, foot balance with closed eyes and jumping reaction time in male workers, and standing trunk flexion, foot balance with closed eyes, jumping reaction time and the step test in female workers.<br>2. Physical activity during working time was not related to physical fitness in male and female workers.<br>3. Inactivity during leisure time was a significant risk factor for standing trunk flexion, foot balance with closed eyes, jumping reaction time and the step test in male workers, and grip strength and foot balance with closed eyes in female workers.<br>4. Young age (16-29 years old) was a significant risk factor for grip strength, standing trunk flexion, foot balance with closed eyes, jumping reaction time and the step test in male and female workers. Middle age (30-49 years old) was a significant risk factor for foot balance with closed eyes and jumping reaction time in male workers, and standing trunk flexion, foot balance with closed eyes, jumping reaction time and the step test in female workers.