Atrial fibrillation (AF)—in which the upper chambers of the heart contract weakly and rapidly—is associated with an increased risk of stroke and heart failure. Although exercise has been suggested as a preventive strategy, there aren't enough data to indicate whether it prevents AF in women.
To shed more light on the topic, Swedish researchers in 1997 asked about 40,000 women over 50 how much time they had spent exercising throughout their lives. The researchers then tracked the women for 12 years and noted that 2,915 (about 7%) had developed AF. They found that the risk of AF fell steadily with increasing activity. The women who exercised the most—more than four hours a week—were 15% less likely to have developed AF than those who exercised the least (less than one hour a week). Those who walked or biked 40 minutes or more a day had a 20% lower risk than those who rarely did either. The results were reported online by the British journal Heart on May 27, 2015.
This was a large observational study, not a controlled trial, so there is no guarantee that walking or biking will keep you from developing AF, but it does suggest another potential health benefit of exercise.