The 1910 Boston marathon was DeMar's first; he finished 2nd. Later in 1910 he was advised by a doctor that he had a heart murmur and should stop running within a year or two. The next year at the Boston marathon the doctors on the starting line advised him of his heart murmur and told him that he should drop out if he was fatigued, and that he should not run any more races. Nevertheless, he won in 2:21:39, a course record. DeMar was one of the twelve members of the U.S. marathon team in the 1912 Stockholm Olympics, where he ran poorly, finishing 12th, complaining that the coaching staff's dictatorial control over the athletes' training had harmed the team's performance -wikipedia.com
Clarence DeMar ended up as a 7x winner of the Boston Marathon-
Genetics or training?
"his heart upon examination weighed 340 grams, (normal is 300 grams)"
"left ventricular wall was 18 mm thick (normal is 10-12 mm)"
"right wall was 8 mm thick (normal is 3-4 mm)
"coronary arteries were estimated to be 2-3 x normal size"
Inside Running: Basics of Sports Phyiology, Costill, DL, 1986.