The heart of the Triple Crown winner, Secretariat, was estimated to weigh 22 pounds, compared to the average Thoroughbred heart, which is about 9 pounds. In the words of Dr. Thomas Swerczek, who performed the necropsy:
“We were all shocked. I’ve seen and done thousands of autopsies on horses, and nothing I’d ever seen compared to it. The heart of the average horse weighs about nine pounds. This was almost twice the average size, and a third larger than any equine heart I’d ever seen. And it wasn’t pathologically enlarged. All the chambers and the valves were normal. It was just larger. I think it told us why he was able to do what he did.”
Secretariat’s world record of 2:24.0 for a mile and a half on a dirt track, set in the 1973 Belmont Stakes, still stands. The second-fastest time for that race (set by Easy Goer in 1989 ) is 2:26.0. Secretariat won the Belmont Stakes by an estimated 31 lengths; he would have beaten Easy Goer by about 10-12 lengths or so, assuming Easy Goer could have avoided exhausting himself trying to keep within striking distance of Secretariat (which is what happened to Secretariat’s 1973 competition).
Secretariat also owns the records for the Kentucky Derby and the Preakness, meaning he ran fast enough to have beaten every other runner who has ever run the three races, including every other Triple Crown winner.