Men who exercise strenuously may have a lower libido than those whose workouts are lighter, according to one of the first studies to scientifically delve into the relationship between men’s workouts and their sex lives.

For years, scientists and active people have debated whether and how exercise affects sexual desire and human reproduction.

But most past studies have centered on women. Typically, this research has found that when some female athletes, such as marathon runners, train intensely for many hours a week, they can develop menstrual dysfunctions. These problems seem caused by hormonal imbalances related to physical stress and frequently affect a woman’s interest in sex and her ability to conceive.

But such dysfunctions are rare and usually resolve after the athlete lightens her training load.

Less is known about the effects of exercise, especially heavy exercise, on men’s libidos and fertility. There have been hints that, in moderate amounts, physical activity increases the male body’s production of the hormone testosterone, which theoretically should ramp up sex drive. Other small studies, on the other hand, have suggested that lengthy and grueling training may blunt the levels of testosterone in a man’s bloodstream both immediately and over the long term.