Objective Previous studies have found that regular participation in intense physical activity increases the risk of developing atrial fibrillation (AF) in men, but it remains unclear how physical activity influences the risk of AF in women. We aimed to examine whether physical activity of different types and at different ages influences the development of AF in women.

Methods In the population-based Swedish Mammography Cohort, information about physical activity was obtained from 36 513 AF-free women (49–83 years old, median age 60 years) who had completed a questionnaire at study entry (1997). Participants reported their time spent on leisure-time exercise and on walking or bicycling throughout their lifetime (at study entry, and at 30 and 50 years of age). We used the Swedish National Inpatient Register (IPR) to determine whether the participants were diagnosed with AF. Cox proportional hazards regression models were used to estimate relative risks (RR) with 95% CI, adjusted for potential confounders.

Results During a median follow-up of 12 years (10th percentile 7.5 years, 90th percentile 12.0 years), 2915 cases of AF were diagnosed. The risk of AF decreased with increasing levels of leisure-time exercise at study entry (RR 0.85, 95% CI 0.75 to 0.95 for ≥4 h/week vs <1 h/week) and walking/bicycling (RR 0.81, 95% CI 0.72 to 0.92, for ≥40 min/day vs almost never).

Conclusions Physical activity is associated with a reduced risk of AF in women. Moderate amount of physical activity was sufficient to significantly reduce AF risk.