According to the New York Times the study included only men who had been on the British national or Olympic teams and runners who had completed at least 100 marathons in their lives:
"All of the men had trained and competed throughout their adult lives and continued to work out strenuously. Twelve were age 50 or older, with the oldest age 67; another 17 were relative striplings, ages 26 to 40. The scientists also gathered a group of 20 healthy men over 50, none of them endurance athletes, for comparison. The different groups underwent a new type of magnetic resonance imaging of their hearts that identifies very early signs of fibrosis, or scarring, within the heart muscle. Fibrosis, if it becomes severe, can lead to stiffening or thickening of portions of the heart, which can contribute to irregular heart function and, eventually, heart failure."
The study found that the more marathons and ultra runs completed by the test subjects the more likely they had heart damage.
“How many people are going to join the 100 Marathon club” or undertake a comparable amount of training? he asked. “Not many. Too much exercise has not been a big problem in America. Most people just run to stay in shape, and for them, the evidence is quite strong that endurance exercise is good” for the heart, Dr. Paul Thompson, the chief of cardiology at Hartford Hospital in Connecticut and an expert on sports cardiology.