Snippet from the article:
Two new studies published this week lend credence to the idea that when it comes to exercise, there apparently can be too much of a good thing.
In the first study, led by Dr. Nikola Drca (Karolinska University Hospital, Stockholm, Sweden), and published online May 14, 2014 in Heart, investigators report that men 30 years old, who exercised for more than five hours per week, had significantly higher risks of developing atrial fibrillation later in life compared to men who exercised less.
The report also showed that older adults who walked or rode their bicycle for about an hour per day had a significantly lower risk of atrial fibrillation compared with older adults who almost never participated in such recreational physical activity.
In the second study, led by Dr. Ute Mons (German Cancer Research Center, Heidelberg, Germany), also published in Heart, researchers studied the association of self-reported physical activity in 1038 subjects with stable coronary heart disease and confirmed previous findings of an increased risk of adverse events among inactive patients. However, they also found those who participated in daily “strenuous” physical activity had an increased risk of dying from cardiovascular causes.
I guess it boils down to: “Bike riding for older folks is good. Exercise for 30 year olds is bad.”