Secretariat's heart was extraordinarily large compared to the average Thoroughbred horse's heart. While the typical Thoroughbred heart weighs about 8.5 pounds, Secretariat's heart was estimated to weigh around 22 pounds-nearly three times the normal size
. This estimate comes from the necropsy performed by Dr. Thomas Swerczek, who did not weigh the heart directly but made the estimate based on comparisons with the second-largest heart he weighed, belonging to Sham (Secretariat's rival), which weighed 18 pounds
. This enormous heart is believed to have contributed significantly to Secretariat's legendary stamina and racing ability. The trait of having a large heart is thought to be linked to a genetic factor known as the "x-factor," traced back to the historic racehorse Eclipse, whose heart weighed 14 pounds-much larger than average for his time
. In summary, Secretariat's heart weighed approximately 22 pounds, making it about 2.5 to 3 times larger than the average Thoroughbred heart, and this physical attribute is considered a key part of his exceptional racing performance.
