Equine Sports Medicine

All About VO2max

Horses are considered to be elite athletes. This means that, pound for pound, the horse is a better athlete than any of our domestic species, including man. One of the most useful ways of measuring an individual's aerobic capacity - which is in turn a measure of athletic capacity - is to determine the body's maximal capacity for oxygen consumption, or the VO2max.

Why is oxygen so important?

Oxygen is the body's fuel. Without oxygen, your horse's body would slow and quit within a matter of seconds. The ability to consume available oxygen is an indicator of how well your horse's body can use fuel when he is working aerobically. VO2max refers to the amount of oxygen that a horse takes up at any given time. The V with a dot overlying it indicates that we are looking at a flow rate, which is a volume per unit of time. In this case, we express the flow rate as millileters of oxygen per minute (mls/min). We usually further determine the flow per unit of body weight (mls of oxygen per kilogram per minute, or mls/kg/min), so that we can normalize the measurement - that is, we can now compare large individuals to small individuals.  Back to the top

Is VO2max the most important contributor to athleticism?

Well, first, there are many factors that go into the making of an elite athlete. Coordination, the will to win, and muscle strength are just a few of these factors. Some sports, such as racing medium to long distances, or endurance, rely heavily on having a superlative VO2max. Other sports, such as barrel racing, jumping or polo, are more heavily dependant on anaerobic metabolism. However, if we look just at , we must consider the portions of the horse's body that serve merely to transfer oxygen, before it is ever used as fuel. First, the airways are essential in transferring O2, to the lungs. Examples of airway problems that impede the delivery of oxygen to the lungs include laryngeal paralysis and small airway inflammatory disease. Once the O2 is in your horse's lungs, the blood vessels in the lungs must absorb the oxygen (and in return give up carbon dioxide, CO2), and bring this oxygenated blood to the heart. The heart must send the oxygenated blood to the rest of the body. A well-trained heart increases in size and weight, and, ultimately, in strength of contraction. When your horse is at maximal exercise, the muscles of motion receive the majority of available blood, whereas the blood supply to internal organs (such as the intestines and the kidney) is at a minimum. Finally, the ultimate job of the blood supply is to deliver oxygen to mitochondria that exist within the cells. The mitochondria use oxygen to supply energy to the rest of the body.

Figure 1, Determinants of VO2Max

To summarize, your horse needs high capacity airways and healthy lungs, a large, powerful heart, a plentiful red blood cell supply, and an abundance of mitochondria. In comparison to less athletic species, even when we adjust for size, the horse has a greater lung capacity, a larger, more powerful heart, and more mitochondria within the muscles. What does this do for a horse's VO2max ? Well, the best human athlete has a VO2max of approximately 80 mls/kg/min. Compare this to a Thoroughbred racehorse, with a VO2max of 180 mls/kg/min. This means that is twice the best recorded values for humans even after correcting for size.  Back to the top

Figure 3, VO2max in an elite Thoroughbred horse

How do we measure VO2max?

To determine VO2max, we measure: 1) the concentration of oxygen in ambient air, and 2) the concentration of oxygen in the air that the horse breathes out. The difference between the two will be the amount that the horse has had to consume in order to produce energy for the body. In order to do this, we use what is termed an open respirometry system, and a high speed treadmill. The horse wears a mask that is attached by a series of tubing to an oxygen sensor. After the horse has been trained to the treadmill (this takes anywhere from a half hour to an hour), he begins an incremental exercise test (IET), otherwise known as a stress test. The horse begins at a walk, and at the end of each minute, the speed of the treadmill is increased until the horse is going at his fastest pace. The test is ended when the horse can no longer keep his position at the front of the treadmill. The amount of oxygen consumed, or VO2max , is measured at the end of each step. The VO2 corresponding to the fastest speed is termed the VO2maxBack to the top

Figure 2, Conducting an incremental exercise Test on the high speed treadmill

How can I help my horse to improve his VO2max?

The major improvements are made in the capacity of the heart to pump blood, the density and size of the blood vessels in the muscles, red blood cell mass, the size of the muscles themselves, and the density of mitochondria within the muscles. Interestingly, the lungs are the one link in the chain that do not have the capacity to improve, unless they were diseased in the first place. Thoroughbred racehorses are born with more of these attributes than are, for instance, Shetland ponies, and they will always be faster, no matter how hard the pony trains.   Back to the top

Figure 4, VO2max in an elite athlete Before and After 2 months of aerobic training