Prospective study of physical activity and risk of developing a stress fracture among preadolescent and adolescent girls.

OBJECTIVE: To identify predictors of developing a stress fracture among adolescent girls during a 7-year period.
DESIGN: Prospective cohort study.
SETTING: Adolescent girls living throughout the United States. PARTICIPANTS: A total of 6831 girls aged 9 to 15 years at baseline in the Growing Up Today Study, an ongoing prospective cohort study. MAIN EXPOSURES: Exposures were assessed by self-report questionnaires completed by adolescent girls in 1996, 1997, 1998, 1999, 2000, 2001, and 2003. The adolescent girls' history of stress fracture, including age when fracture occurred and site, were reported by their mothers, who are registered nurses, in 2004. Cox proportional hazards models were used in the analysis. Main Outcome Measure Incident stress fracture that occurred between 1997 and 2004.
RESULTS: During 7 years of follow-up, 267 girls (3.9%) developed a stress fracture. Independent of age, age at menarche, family history of fracture, and hours per week of low- and moderate-impact activity, hours per week of running (hazard ratio = 1.13; 95% confidence interval, 1.04-1.23), basketball (hazard ratio = 1.12; 95% confidence interval, 1.03-1.23), and cheerleading/gymnastics (hazard ratio = 1.12; 95% confidence interval, 1.02-1.22) were significant predictors of developing a stress fracture. No other type of high-impact activity was associated with an increased risk.
CONCLUSION: Girls who engage in running, basketball, cheerleading, or gymnastics should be encouraged to include varied training in lower-impact activities to decrease the cumulative amount of impact in order to minimize their risk of stress fractures.