The legendary racehorse Secretariat, often called “Big Red,” remains one of the most celebrated athletes in the history of sports. His record-breaking performance in the 1973 Triple Crown, particularly his astonishing 31-length victory in the Belmont Stakes, solidified his place in history. Long before modern genetic testing, a pervasive theory suggested the source of his unmatched speed and stamina lay in a unique physical trait. This enduring legend centers on the size of his heart, an organ rumored to be far larger than that of any other champion.
The Confirmed Measurement
The specific, documented weight of Secretariat’s heart was determined after his death in 1989, when the horse was euthanized due to laminitis. Though the organ was never actually placed on a scale, the head pathologist at the University of Kentucky, Dr. Thomas Swerczek, estimated its weight at approximately 22 pounds. This post-mortem finding was significant because the average heart weight for an adult Thoroughbred is considerably smaller, typically falling into the range of 8.5 to 9 pounds.
Dr. Swerczek reported that the heart itself appeared normal in structure, but its sheer size was unprecedented in his experience. The 22-pound weight meant Secretariat’s heart was roughly two and a half times the size of a typical racehorse’s heart. This physical anomaly provided a tangible explanation for the horse’s legendary power and enduring speed during the most demanding races.
Understanding the Biological Advantage
The physical size of Secretariat’s heart translated directly into a superior physiological capacity for athletic performance. A larger heart has a greater maximum stroke volume, which is the amount of blood pumped out to the body with each beat. This increased volume allows for a massive surge in cardiac output—the total volume of blood moved per minute.
During intense exercise, a horse’s heart rate can surge to over 220 beats per minute, and a large heart maximizes the efficiency of this rapid pumping. The large heart, particularly its left ventricle, the chamber responsible for pumping oxygenated blood to the rest of the body, can be dilated and conditioned to hold more blood. This capability ensures that the working muscles are flooded with oxygen at an accelerated rate.
The constant, efficient delivery of oxygen-rich blood helps to delay the onset of muscle fatigue and allows for faster recovery during a race. This superior oxygenation capability provided Secretariat with the sustained energy needed to run at maximum speed for extended periods, a trait on full display during his record-setting Belmont Stakes performance.
This concept is known as the “X-Factor” hypothesis, suggesting that an unusually large heart is a trait passed down through the female lineage via the X chromosome. While the existence of a single “Big Heart Gene” is still debated, research supports the idea that some bloodlines are predisposed to developing disproportionately large hearts. This genetic predisposition, combined with rigorous training, produces horses with the greatest potential for speed and endurance.
Secretariat’s Place Among Champions
Secretariat’s 22-pound heart places his physiology in a unique category when compared to other champion racehorses whose hearts were measured post-mortem. The Australian champion Phar Lap, known for his exceptional stamina in the 1930s, had a heart that weighed 14 pounds (6.35 kg). This measurement was considered remarkable, being nearly twice the average for a horse.
Sham, Secretariat’s contemporary and main rival who finished second in both the Kentucky Derby and Preakness Stakes, also possessed an abnormally large heart, reportedly weighing 18 pounds. While Sham’s heart was notably larger than the average Thoroughbred’s, it was still significantly smaller than Secretariat’s. The difference between 18 and 22 pounds likely represented the marginal physiological advantage that allowed Secretariat to pull away in the Belmont Stakes.
This finding solidified the idea that exceptional heart size is a significant predictor of elite performance in Thoroughbreds. The discovery contributed to the focus on maternal bloodlines in breeding programs, as owners and trainers sought to introduce the genetics associated with the “X-Factor” into new generations of champions. Secretariat’s physiology provided a scientific explanation for his dominance.