Physical activity is beneficial for those living with heart disease. But too much intense exercise may up your risk of death from a heart attack or stroke, finds a new study appearing in BMJ.

The research: Among more than 1,000 people diagnosed with stable coronary artery disease, those couch potatoes who failed to exercise were about twice as likely to suffer a stroke or heart attack—and four times more likely to die from all causes—than people who exercised regularly, indicates 10 years of study data from a German research team. But death rates from stroke or heart attacks also doubled among heart disease patients who completed strenuous exercise—activities like cycling or speed hiking—on a daily basis.

What it means: While it's clear that inactivity is a killer for heart disease sufferers, the links between strenuous exercise and heart disease mortality rates are murkier, says study coauthor Ute Mons, of the German Cancer Research Center. While some recent data, including her group's study, indicate that intense physical activity could have some adverse effects on your heart, the mechanisms at work aren't clear. That said, it's possible that working out too strenuously may increase oxidative stress and inflammation, which could explain the rise in mortality among the most hard-core exercise buffs, she says.

The bottom line: There's no question that people living with heart disease benefit from regular physical activity, Mons stresses. Her study found that exercising 2 to 4 days a week—even if that exercise is intense—provides the greatest protection against heart attacks, stroke, and death from heart disease. "But the ideal amount [of exercise] will depend on individual factors," she says. Her advice? Talk to your doctor about the right amount of physical activity for your unique heart.