Whose heart is healthier: an extreme endurance athlete, a la Lance Armstrong, or a casual exerciser, a la your neighbor who likes to take her dog for 20-minute walks?

Dr. James O'Keefe, a cardiologist at the Mid America Heart Institute of St. Luke's Hospital in Kansas City, Mo., says it's probably the neighbor.

"A lot of people misunderstand that if moderate exercise is good, then more is better," O'Keefe says in an online video that explains his study published in the June issue of the journal Mayo Clinic Proceedings.

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The findings come on the heels of last week's report that exercise could actually be bad for some people.

Most benefits of exercise accrue at a moderate level, such as a 20-30 minute walk, O'Keefe says. To O'Keefe, an ideal-for-the-heart exercise pattern might look like this: a 2-4 mile run two or three times a week, plus cross training that includes strength training and yoga on the other days.

"It can be counterproductive to be doing extreme endurance athletic events," he says. If you're set on doing a marathon or an Ironman, he advises to make it a once-in-a-lifetime event.

The researchers found transient structural cardiovascular changes and elevations of cardiac biomarkers in endurance athletes: 12 percent of the marathon runners studied developed abnormal thickening or scarring of the cardiac tissue. That can indicate an increased chance of atrial and ventricular arrhythmias.

The news isn't all dire for endurance-crazed athletes: high-intensity interval training can provide fitness benefits that don't take as much of a toll on the heart as two hours of strenuous exercise, O'Keefe said.

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It's also not an excuse to couch-surf.

"We want people to understand that this in no way detracts from the importance of exercise," O'Keefe says. "Physically active people are much healthier than their sedentary counterparts…so much so that on average, they live seven years longer."