The aim of this cross-sectional study was to investigate the age at which exercise exerted a favorable effect on peak bone mass in young women. The subjects were 24 college athletes (CA), 19 sedentary college students (CN), and 29 high school athletes (HA). Athletes participated in at least 9 h of exercise per week. Lumbar bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry (DXA). The history of exercise in puberty and adolescence was estimated by a questionnaire as to weekly participation in exercise, for example, field hockey, basketball, tennis, or soccer. The CA group showed significantly higher lumbar BMD than the CN or HA group (1.230 ± 0.112 g/cm2 vs 1.164 ± 0.120 g/ cm2, or vs 1.164 ± 0.088 g/cm2; P ± 0.05). In both the CA and CN groups, those who had a history of exercise during junior high school showed a significantly higher BMD regardless of the history of high school exercise. In the HA group, those who had exercised during junior high school also showed a significantly higher lumbar BMD than those who had not. These data suggested that exercise during junior high school has a favorable effect on peak bone mass, regardless of the history of high school exercise.
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