Sample and setting
We conducted a cross-sectional study of women with early-stage breast cancer between March 2008 and July 2009 at the Rowan Breast Cancer Center of the Abramson Cancer Center of the University of Pennsylvania (Philadelphia, PA, USA). The institutional review board of the University of Pennsylvania and the regulatory committee of the Abramson Cancer Center approved the study and all participants provided informed consent. Research assistants screened medical records and approached potential patients for enrollment at their regular follow-up appointments. After informed consent was obtained, each participant completed a self-administered survey and a blood sample was collected for telomere analysis. Patients were eligible for study inclusion if they were: 18 years or older; had a history of stage I, II, or III breast cancer; postmenopausal; currently or previously on aromatase inhibitors, and able to understand written English. The sample size in the original cross-sectional survey was 476, reflecting a 78% response rate among those eligible. Complete physical activity and telomere data were available for 392 of those patients. There were no differences between our sample and the original sample in terms of age, race, or education.