Transcript:

[0:00]

Hi there, I'm Dr. Perse McCrae and today I'll be talking a little bit about equine exercise physiology.

[0:08]

When we look at horses competing in different disciplines or events, we can begin to understand or appreciate just how varied the skill sets as well as the athletic efforts are of each of those events. So if we think about what's required of a barrel racer, that's very different than what's required of our Standardbred trotters, our Thoroughbred racehorses, or our endurance racehorses. All of those efforts are also very different from the efforts required of our show jumpers, our dressage horses, or our polo ponies.

[0:40]

Despite this wide variety of skill sets and athletic efforts, horses seem able to perform very well in each discipline. Horses are naturally great athletes — but how do they do it? What makes them able to perform better than humans and many other mammals? There are some key physiologic attributes of the horse that allow them to perform at such a high level. Overall, horses have a very high maximal aerobic capacity, or high VO₂ max. They also have a very high oxygen-carrying capacity of their blood, large intramuscular energy stores, and high muscle mitochondrial volumes.

[1:23]

Exercise — whether it's jumping, a collected trot, or reining — is simply muscles contracting. Every time a muscle contracts, it uses energy in the form of ATP. The horse's metabolism must then increase to replace the used ATP, creating new energy so they can continue to exercise or perform. The metabolic rate depends on the supply of substrates, or starting ingredients — for example, glycogen stored within the muscle — as well as oxygen.

[2:00]

The horse needs to be able to get oxygen from outside its body — from the atmosphere — all the way to the mitochondria to produce energy to continue performing. This shows how integrated the cardiovascular and respiratory systems are in enabling the horse to exercise. I want to start with a little bit about the unique anatomy of the horse in terms of their upper airways.

[2:26]

If I asked you to go for a light jog, you'd probably breathe through your nose. But if I asked you to jog faster or run longer, eventually you'd likely switch to breathing through your mouth. This is because your body has an increased demand for oxygen, and you'll do everything possible to reduce resistance to airflow. Breathing through your mouth uses a larger opening, which has less resistance and allows more airflow.

[2:55]

Horses, however, cannot do this. They are obligate nasal breathers, meaning they only breathe through their nose — they don't breathe through their mouth. Instead, they employ other strategies to increase airflow by reducing resistance. One strategy is that the mucosa lining their upper airways changes to enlarge the passage. Another is that as horses gallop, they extend their head and straighten their neck to enable greater airflow and less resistance because it's a more direct path for the air.

[3:35]

The lower airway is composed of the trachea, which branches into two bronchi, which branch further into smaller bronchioles and eventually the alveoli. The alveoli are the site of gas exchange — the location where blood comes in contact with the oxygen the horse has breathed in, to pick it up and enter circulation. Horses have a very large set of lungs capable of moving large amounts of air, but the air must travel about two meters from nose to lungs. This means there are many locations where things can go wrong, limiting how much oxygen the horse can bring in and impacting performance.

[4:27]

VO₂ max, or maximal aerobic capacity, refers to the maximum rate of oxygen consumption — the most oxygen the horse can bring in and deliver to tissues for energy (ATP) production. This depends on the respiratory system's ability to bring oxygen into the lungs, the blood's ability to pick it up, and the cardiovascular system's ability to pump it to tissues.

[5:11]

Horses have a very large VO₂ max. In fact, a horse's VO₂ max is about 2.6 times greater than a cow's and about two to two-and-a-half times greater than a human athlete's. A Thoroughbred racehorse will have a VO₂ max of about 180–200 mL oxygen/kg/min, compared to the highest recorded in a human — a cyclist at 97.5 mL/kg/min. This explains part of why horses have such an amazing athletic capacity.

[5:48]

VO₂ max depends on the respiratory system working closely with the cardiovascular system. At rest, a horse will take about 16 breaths per minute; during exercise, this can increase to 100–180 breaths per minute, moving over 1,500 liters of air each minute. Heart rate also increases, from about 35 beats per minute at rest to 220–240 during exercise, moving 250–450 liters of blood per minute. This is possible not because the heart beats faster than in people — maximum heart rates are similar — but because the horse’s large heart ejects more blood per beat.

[6:57]

From a health and fitness standpoint, we’re interested in monitoring cardiovascular responses to exercise. We can use ECGs or heart rate meters to look at things like maximum heart rate or the speed at which heart rate reaches 200 beats per minute (V200). As a horse becomes fitter, they can reach higher speeds at the same heart rate. We can also look at heart rate recovery — how long it takes for heart rate to return to resting levels after exercise.

[8:11]

In addition to moving oxygen into the body and pumping blood effectively, horses also need to hold oxygen in the blood. This comes down to oxygen-carrying capacity — the ability of the blood to bind oxygen. When a horse begins to exercise, the spleen contracts, releasing about 12 liters of blood that is ~80% red blood cells. This increases hematocrit from ~32% at rest to ~65% during exercise, much like blood doping in humans, and enhances performance.

[9:46]

Beyond carrying oxygen, it must be delivered to muscles. Muscles have dense capillary networks for oxygen delivery, active enzymes for energy use, and high mitochondrial volume — about double that of cattle — for energy production.

[9:58]

Interestingly, during exercise, horses cannot fully meet oxygen demands, resulting in low oxygen and high carbon dioxide levels in the blood at high intensities. They tolerate this to reduce the work of breathing. At a gallop, horses are locked into a one-breath-per-stride ratio, coupling breathing with locomotion. This allows them to use existing movement — forelimb impact and spine motion — to aid inhalation and exhalation without extra energy cost.

[11:41]

Monitoring respiratory and cardiovascular health is important, even in non-elite horses. Respiratory disorders are the most common diagnosis in sport horses with poor performance and are a major cause of training disruption. Conditions like asthma can affect both performance and overall health. Cardiovascular disorders such as arrhythmias may be harmless or life-threatening — fatal arrhythmias are the most likely cause of sudden death. If a horse shows exercise intolerance, fatigue, or unwillingness to work, it’s important to work with your veterinarian to ensure these systems are performing well.