It's not uncommon for an owner of a particularly keen horse to affectionately say he has a “big heart.” But if that animal is a sport horse that completes intense workouts, he might, quite literally, have a huge heart.

Often called the “athlete’s heart,” an enlarged heart in a horse is often accompanied by murmurs and arrhythmias, said Rikke Buhl, PhD, exercise physiologist at the University of Copenhagen, in Denmark, at the 2014 International Society for Equitation Science (ISES), held Aug. 6-9 in Bredsten, Denmark. And while that might sound worrisome, Buhl said it’s still not clear whether athlete’s heart is actually related to the sudden equine deaths that sometimes occur during or just after exercise.

Scientists do know, however, that big hearts often mean big wins.

“Our studies actually proved what lay people have been saying for a hundred years: Horses with big hearts win more races than those with small hearts,” Buhl said.

Case in point: After 1973 Triple Crown winner Secretariat died in 1989, a post-mortem examination revealed that his heart weighed 10 kilograms (22 pounds; for comparison, the heart of an average 1,000-pound adult horse weighs about 10 pounds), approximately 2% of his entire body weight. Both breeding and training have something to do with that, Buhl said. Thoroughbreds are selected for speed, which is associated with genetically larger hearts, but “after a certain age the heart will only continue to grow if the horse is in intense training,” she said. The intense training leads to athlete’s heart.

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