The concept of VO2 max was first introduced by Nobel Prize-winning physiologist A.V. Hill in 1923, who found that the amount of oxygen consumed by his muscles increased as he ran at a steadily increasing pace – up to a point. Eventually, his oxygen consumption would plateau, even if he continued to run faster and faster until he reached exhaustion.
That plateau, he argued, represented his body’s “maximal oxygen uptake,” or VO2max. Vo2Max (also maximal oxygen consumption, maximal oxygen uptake, peak oxygen uptake or aerobic capacity) is the maximum capacity of an individual's body to transport and use oxygen during incremental exercise, which reflects the physical fitness of the individual. The name is derived from V - volume, O2 - oxygen, max - maximum. Vo2Max is expressed either as an absolute rate in litres of oxygen per minute (l/min) or as a relative rate in milliliters of oxygen per kilogram of bodyweight per minute (ml/kg/min).
Under the Cardiovascular/Anaerobic model, as proposed by Hill, it is believed that Vo2Max is generally restricted by oxygen supply to the muscle/mitochondria rather than by the capacity of the muscle/mitochondrial to oxidate during maximal exercise (leading to an exercise-induced arterial hypoxemia). Essentially, cardiac output, the ability of the heart to pump oxygenated blood into the muscle, is the limiting function.
Among research in support of this contention is that pericardiectomy, which allows greater diastolic filling, elevates maximum oxygen uptake; however, more recent research also seems to indicate that all identifiable stages on the oxygen transport pathway, from pulmonary diffusion to oxidative phosphorylation in skeletal muscle mitochondria, materially influence maximum uptake. Thus, if a high cardiac output is to be of benefit (a large heart with outstanding contractibility), all the other steps must function better too. In the decades since, VO2max has become a standard tool to assess aerobic fitness. Researchers in all fields, including the equine field, measure it using an incremental test to exhaustion: On a treadmill subjects start at an easy pace and get faster every minute or so until they’re unable to continue. There are three conventional possibilities on what causes the plateau.
The heart cannot pump any more blood. It has reached its maximum performance.
The muscle is unable to extract any more oxygen from the blood
The lungs cannot extract any more oxygen from the inspired air.
Over the decades, debate has raged among exercise physiologists about the ultimate location of the bottleneck. Is it the cardiovascular capacity?, mitochondria?, lung capacity? The debate still rages however a more recent theory, proposed by exercise physiologist and author Dr. Tim Noakes, suggests that the limit may not be physical at all.
The Central Governor theory is a regulatory model of performance proposed by Dr. Noakes that applies to exercise physiology in all fields, including thoroughbreds. The traditional definition of fatigue used by exercise physiologists is an inability to either continue a pre-defined amount of work or equal a previous level of work, despite a strong desire and effort by the subject to do so. While the definition is good as far as it goes, it is not accurate. Even though a horse may not be able to sustain a set work load, they can continue at a lesser one, so fatigue is not absolute, rather it falls on a relative scale. In Dr. Noakes’s “central governor” theory, the decision to slow down or stop during exercise isn’t the result of an absolute physical limit. Instead, the brain applies the brakes proactively to prevent the horse from reaching these limits. After all, if if a horse really did run to the absolute edge of its physical limits, it would be dead.
Muscles contract because they receive a signal from the brain to do so. If they do not receive the signal they do not contract. In theory then, muscle activation would go on an upward slope in the face of increased workload, but in fact the opposite is true. 100% of available muscle fibers are not activated at the end of exhaustive endurance tests (the counter to this is that the limitations of oxygen delivery makes this goal impossible). Instead, the number of muscle fibers activated falls during exhaustive exercise. The argument that Noakes raises is that the central nervous system, in protecting homeostasis, reduces the force output by reducing the neural drive to the muscles. Subconsciously, the brain computes the metabolic cost to continue at the current pace and compares that to the existing physical state. Based on this information, the brain adjusts the optimal pace so that the event is completed in the most efficient manner, while maintaining homeostasis.
How does this all relate to elite thoroughbred performance? Owners and trainers talk about a 'will to win' in a horse. We see it every day where a Champion racehorse will turn back another horse of similar quality in a deep stretch drive, pushing themselves to their physiological limits. Using Dr. Noakes theory, the Champion horse uses their brain to allow themselves to produce an output that is better than the competition. Such performance can however have its drawbacks. Horses cannot tell us when they have effectively 'red-lined' in their performance, putting their maximum effort out and pushed themselves to the limit. Handicappers, especially those using Ragozin numbers can identify absolute peak performances for a horse and often these types of performance peaks can never be repeated. Without scientific evidence to suggest this, we would contend that after leading with splits of 22.85, 46.41, 1:10.54 and 1:35.48 when the 3yo filly Rachel Alexandra turned back the game Macho Again in a stretch drive in the Woodward Stakes G1 she was pushing herself to her absolute physiological limit. It may not have been visually her most impressive performance, but in terms of physiological output, it may have been her most taxing. While she did win races after this performance, it is pretty clear even in terms of speed figures that her performances subsequent to the Woodward were not nearly as good as her performances prior.
While the Central Governor Theory is yet to be tested in horses (Thoroughbreds or Standardbreds would be a great model), intuitively it makes sense. It has been known for some time that the best thoroughbreds do not have the highest Vo2Max values and that there are other factors including 'Will to Win' that define elite athleticism. After all, if failure of oxygen delivery was the ultimate limit of equine exercise performance, then it follows, that the horse with the highest Vo2Max values must be the most successful. This would mean that we could do away with all competition and merely award those with the highest tested Vo2Max values.