Everyone knows exercise is fundamental for good cardiovascular health, but a new and surprising body of research is finding that too much exercise may also increase the risk of death from heart attack or stroke in patients with existing heart problems.
The new study, which was just published in the journal Heart, tracked a decade's worth of exercise habits and survival of more than 1,000 people with diagnosed -- but stable -- coronary artery disease. The majority of the study's participants were in their 60s and had attended a cardiac rehabilitation program to avoid future heart attacks or strokes.
Around 40 percent of the study participants did an hour of moderate-intensity aerobic activity 2 to 4 times per week; of the remaining 60 percent, half exercised at that level more than four times a week and the other half exercised less. Overall, 1 in 10 said they rarely or never exercised.
The researchers found those who were most sedentary were around twice as likely to have a heart attack or stroke as those who were regularly physically active. They were around four times as likely to die of cardiovascular events and all other causes.
But more surprisingly, those who did the most strenuous daily exercise were also more likely to die of a heart attack or stroke than people who engaged in more moderate activity.
"Moderate physical activity is the most protective for the heart," Dr. Nieca Goldberg, a cardiologist and director of the Joan H. Tisch Center for Women's Health at NYU Langone Medical Center, told CBS News. She said the findings of the study support what she recommends to patients.
"If you're a couch potato, that's something that's particularly dangerous for your heart. You're going to have a higher risk for heart attack and a lower survival rate." But she also warned against going to the opposite extreme. "You don't need to super-size your exercise because we find those people have also a higher risk of having heart problems."
Goldberg said starting an exercise program is found to be most beneficial to a person who has a history of sedentary behavior. "We see a tremendous amount of heart benefit, and you just have to keep it moderate, because there's very little gain when you go from moderate to very vigorous exercise."
She recommends sticking with exercise guidelines from the American Heart Association. The organization recommends at least 30 minutes of aerobic activity a day "where you feel your heart rate going a little faster, you feel yourself breathing a little bit faster but you can still talk to the person next to you," explains Goldberg.
She said patients who are unsure about their exercise program should be evaluated by a cardiologist. "Sometimes we recommend stress testing because that gives us an objective assessment of how much you're capable of and then we base our recommendations on that," she said.
Recommendations for exercise heart rate depend on both age and existing health conditions, but typically a person can exercise up to 85 percent of the maximum heart rate found on the stress test.
Whether you're jogging in a park or spinning at a gym there are a number of ways to monitor a workout's intensity, she said. "The simplest is a one to 10 scale. When you're using the one to 10 scale you should feel like you're exercising at a six, seven or eight to be at a moderate level of exercise."
Goldberg also said personal heart rate monitors can be helpful, which have a band that wraps around the chest and includes a wrist monitor. Additionally, there are a number of smartphone apps that can help track exercise intensity and duration.
"What we want you to do is be healthy and do a moderate level of exercise," she said.