Photo: Anne M. Eberhardt

Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.

On the surface, a horse moving through his paces often looks smooth and effortless. Inside that horse, however, are several body systems working tirelessly. At a recent veterinary meeting, Kenneth W. Hinchcliff, BVSc, PhD, Dipl. ACVIM, professor and dean of the Faculty of Veterinary Science at the University of Melbourne, Australia, gave attendees an inside look at how horses' bodies work during exercise and what's needed to let the horse reach his athletic potential.

"A large number of physiologic and anatomic features act in concert to endow the horse with extraordinary athletic capacity," Hinchcliff said at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La. "Optimal athletic performance is dependent upon optimal integrated functioning of these physiologic and anatomic features."

Making several comparisons between horses and cattle (a similarly sized animal with lesser athletic potential), Hinchcliff discussed several anatomic and physiologic functions in detail and explained how they contribute to achieving optimal athletic performance:

Aerobic Capacity: "The maximal aerobic capacity (VO2max) of horses is approximately 2.6 times that of similarly sized cattle," Hinchcliff explained.

Although horses and cattle have a similar maximum heart rate, horses have a larger maximum cardiac output and stroke volume (the amount of blood pumped per heartbeat), a higher hemoglobin concentration (which indicates the blood has a higher oxygen-carrying capacity), and larger lungs (about twice the size of cattle lungs with a 1.6 times larger gas exchange surface). Combined, these features allow the horse to maximize blood oxygenation in the lungs, blood oxygen transport capacity, and oxygen delivery abilities, he said.

Substrate Usage: Horses require substrate to fuel metabolic processes and muscle contraction during exercise, specifically carbohydrates and fatty acids, Hinchcliff said.

"The body uses fatty acids to fuel metabolic processes related to exercise up to 40% to 60% of VO2max," he said.

After that, metabolism is solely powered by carbohydrate oxidation. Thus, ensuring a horse consumes adequate amounts of carbohydrates and fatty acids should help him reach his athletic potential.

Oxygen Delivery: The circulatory system is responsible for transporting oxygen to the tissues throughout the horse's body, Hinchcliff said. Oxygen carrying capacity is increased (sometimes by as much as 50%) when needed by an increase in the circulating hemoglobin concentration, he added, resulting from contractions of the spleen in anticipation of exercise. Not surprisingly, Hinchcliff noted that horses that have undergone a splenectomy generally have a reduced ability to carry out strenuous exercise.

Gait Efficiency: "Energetically efficient gaits are challenging for large animals because of the slow rate of contraction and low power output of their muscles," Hinchcliff said.

Horses, however, are able to utilize elastic stored energy in tendons and muscles to make their gaits more efficient than other animals of similar size, such as cattle.

Coordinating Body Functions: Hinchcliff relayed that in order to achieve maximal performance, all the physiologic and anatomic body functions must work in conjunction with each other. "In most cases, maximal performance requires that these body systems operate at or close to their maximum capacity," he added.

In horses with performance-limiting health issues, such as lameness or heaves, just one system functioning improperly can further decrease performance. Conversely, in most healthy horses, he noted, one system functioning below par generally won't cause a significant decrease in performance.

Certain issues can and will limit performance, however.

Performance Limiting Factors

"For healthy horses the actual performance limiting factor depends on the type of exercise and its duration," Hinchcliff said. For instance:

  • Oxygen transport capacity likely limits racehorses' performance when they're moving at top speed;
  • Thermoregulation could be implicated for performance limitation for three-day event horses; and
  • Maintaining fluid and electrolyte levels could limit endurance horses' performance.

"An understanding of what likely limits performance for each breed and use of horse is important to understanding not only the physiology of that form of exercise, but also in determining the likely causes of poor performance in animals with clinical disease," he said.

Take-Home Message

A number of physiologic and anatomic systems must coordinate to allow a performance horse to reach his maximum athletic potential. Several factors can limit performance, but an understanding of these issues can help owners get the most from their equine athletes.