Purpose: The purpose of this study is to determine the optimum period and kind of exercise which are more effective for increasing the calcaneus osteo sono assessment index (OSI) in adolescent women. Methods: The subjects consisted of 600 female students aged from 19 to 21 studying at the Department of Food and Nutrition at Nakamura Gakuen University. The calcaneus OSI was measured by ultrasonic bone absorptiometry. The factors associated with an increased OSI were determined using piecewise linear regression analyses. Results: Experience in performing impact-loading exercise either in junior high school or high school was found to be a significant predictive factor for increased calcaneus OSI after adjusting for age and weight. Regarding other exercise-related factors such as subjective intensity, frequency, hours at a time, and total duration of exercise, a piecewise liner regression analysis showed that a total duration of over 7 yr and a high subjective intensity of the impact-loading exercise during high school and junior high school periods caused a significant increase in the calcaneus OSI. However, the OSI decreased when the subjective intensity of the impact- loading exercise during the high school period was reported to be 'very strenuous.' Conclusions: Impact-loading exercise in junior high school and high school is important for increasing calcaneus OSI
however, impact- loading exercise in high school with a 'very strenuous' subjective intensity was also found to possibly decrease the calcaneus OSI.