Research shows physical activity can slash the risk of death from heart disease. And while many regular exercisers enjoy biking, jogging on the treadmill or mashing the stair-stepper, some take their workout to the streets, signing up for and competing in marathons. But recent news reports point to a disturbing paradox: Some runners have collapsed during the twenty-six-point-two-mile race, often in the final miles… and even at the finish line.

To look for factors that might contribute to this trend, researchers screened sixty non-professional marathoners before and after the Boston Marathon. The group included forty-one men and nineteen women. Runners were screened less than one week before the marathon, and about twenty minutes after finishing the race. Researchers used a heart imaging tool known as echocardiography and blood tests to compare strain on the heart pre- and post-marathon.

Although heart images taken before the race were normal, afterward some runners showed abnormalities in how their hearts filled with blood. And while blood levels of a substance that indicates damage to the heart muscle were immeasurable before the race, more than sixty percent of participants had increased levels of the marker immediately after the race.

Interestingly, those who trained more miles per week leading up to their marathon tended to have fewer signs of this short-term damage to the heart muscle. So marathoners may want to take an important step before race day… carefully conditioning their bodies during training for the long run ahead.