Secretariat is remembered as one of the most dominant Thoroughbreds in racing history. His physical prowess and seemingly limitless stamina became a subject of intense scientific fascination. This curiosity focused on his biological makeup as a potential explanation for his unparalleled ability on the track.
Secretariat’s Unmatched Racing Career
The 1973 Triple Crown campaign showed Secretariat’s physical superiority, establishing a benchmark that remains unbroken. His victory in the Kentucky Derby set a track record of 1:59 2/5 for the 1 1/4-mile distance. He then won the Preakness Stakes, with his time of 1:53 later officially recognized as a stakes record after a review of the race footage.
His dominance culminated in the Belmont Stakes, the final and longest leg of the Triple Crown at 1 1/2 miles. Secretariat won by a record 31 lengths, the largest margin of victory in history. He completed the race in a record time of 2 minutes, 24 seconds, which remains the fastest in American history for that distance.
The Post-Mortem Discovery: Heart Size
Secretariat’s heart size was determined after his death in 1989 at age 19. The necropsy was performed by veterinarian Dr. Thomas Swerczek as a requirement for the horse’s insurance policy, following his euthanasia due to severe laminitis. During the examination, Dr. Swerczek noted that Secretariat’s heart was large, though it was not formally weighed.
Based on his experience, the veterinarian estimated the heart’s weight to be approximately 21 to 22 pounds. This contrasts sharply with the average Thoroughbred heart, which typically weighs between 8.5 and 10 pounds. Secretariat’s heart was roughly two to two-and-a-half times the size of a normal racehorse’s, an anatomical anomaly that appeared to confirm the source of his stamina. The second largest heart Dr. Swerczek had examined belonged to Secretariat’s rival, Sham, estimated at 19 pounds.
The Biology of the “Super Heart”
The physiological advantage of a large cardiac muscle lies in its ability to enhance cardiovascular performance. A larger heart volume directly correlates with an increased stroke volume, which is the amount of blood pumped with each beat. This greater stroke volume boosts the horse’s cardiac output—the total volume of blood pumped per minute.
During peak exertion, an elite racehorse’s heart rate can exceed 240 beats per minute, increasing cardiac output from about 10 gallons per minute at rest to 65 to 75 gallons per minute. Secretariat’s large heart muscle would have allowed him to maintain this maximum output more efficiently and for longer durations. This superior pumping capacity translates to a higher maximal oxygen uptake ($\text{VO}_2\text{max}$), delivering more oxygenated blood to the working muscles. Enhanced oxygen delivery and the rapid removal of metabolic byproducts, like lactic acid, are key to sustaining speed and avoiding fatigue late in a race.
The Large Heart Gene Connection
The large heart size led to the development of the “X-Factor” or Large Heart Syndrome theory, which explains the genetic basis for this trait. This theory posits that the characteristic is linked to a specific gene carried on the X chromosome. Since male horses (XY) inherit their single X chromosome exclusively from their dam, the trait is believed to be passed down through the female line.
The X-Factor theory suggests Secretariat inherited this trait from his dam, Somethingroyal. Proponents trace this genetic line back through history to horses like the 18th-century stallion Eclipse. While widely discussed in Thoroughbred breeding circles, the exact mechanism of inheritance remains debated, and the theory is not universally accepted as the sole determinant of success.