VO2max refers to the maximum rate of oxygen consumption measured during intense exercise. The concept that there exists a finite rate of oxygen transport from the environment to the mitochondria of exercising muscles began with Archibald Hill and Hartley Lupton in their 1923 paper titled, “Muscular Exercise, Lactic Acid, And The Supply And Utilization of Oxygen.” Since then, VO2max has become one of the most ubiquitous measurements in all of exercise science. VO2 max is calculated by the maximum milliliters of oxygen consumed in a minute divided by body weight in kilograms. So, for example if we had a 77kg athlete and they consumed 5,500ml of oxygen in a minute, they would have a VO2 max of ~71 ml/kg/min, which would give them comparable values to an elite rower. In essence, VO2 is giving us some kind of meaningful proxy for our ability to deliver and utilize oxygen within the body. Furthermore, if we were to go a step deeper, we could say that VO2 tells us something about the capacity of the respiratory, cardiovascular, and muscular systems working in concert.
In order to deliver and utilize oxygen at a high rate within the body we need...
The respiratory system to allow for adequate oxygen to enter the body and diffuse into the blood, as well as for Co2 to leave the body;
The cardiovascular system to act as an efficient pump and deliver oxygenated blood to the working muscles; and
Mitochondria rich muscles that can utilize delivered oxygen efficiently and at a fast enough rate.
This raises the question - what is limiting an individual's VO2max?
When athletes tell me they get their VO2max tested they expect that we'll be able to use that data in a meaningful way. While it can be useful to tell us how fit someone is, it's not actionable because it doesn't tell us WHY they ended up with that value. Were they incapable of delivering oxygen to the muscle quick enough which ended up being a rate limiting step for oxygen consumption, were they delivering sufficient oxygen to the muscle but they muscle couldn't consume it at a fast enough rate, or was the issue lagging gas exchange at the level of the respiratory system?
You can have three athletes with the same VO2 max, but very different underlying limitations. In this case all three of them would need to train in very different ways to improve. In the same way, you can have three athletes with a 200kg Back Squat with different sticking points - despite them having the same performance metrics, they all grinded through different parts of the lift - intuitively we know they would need different to emphasize different things in their training, but seldom is that concept applied to energy system training. If you understand what an athletes rate limiting step is, then you understand what the low hanging fruit they need to improve their performance is. This isn’t to say VO2 max testing doesn’t have any place in an athletes testing arsenal, but instead that it has a different role than it typically used. It’s a descriptive assessment and not a prescriptive one in the same way that working to a 1RM snatch tells you how strong you are in that lift, but not what you need to emphasize to improve.
Descriptive tests aim to quantity performance and include metrics like maximal power output, VO2max, and critical power. Of course, the descriptive tests we use need to be relevant to the athlete's goals. There is no use in having an athlete establish a functional threshold power if they are a track sprinter. While descriptive tests help tell us how ‘fit’ an individual is, and potentially allow us to get data on different intensity thresholds, they aren’t incredibly useful from a programming perspective. For that reason, I also use a prescriptive test. These tests are a bit more subjective in that they don’t always quantify an individual's performance. Instead, the purpose of a prescriptive test is to tell us why someone performed the way they did and how we might change that.
For example, an individual may perform a VO2max test and get a value of 77 ml/mg/min at a velocity of 19.6 km/h while running]. This is valuable data, but it doesn’t tell us why this athlete's VO2max isn’t 79 ml/kg/min, for example. To understand that, we may employ a prescriptive test like a 515 assessment using NIRS, metabolic gas analysis, and oximetry. This can help us understand whether someone is limited by their ability to uptake, transport, or utilize oxygen, and in turn, that can point us in the direction of what to focus on in training so we can then get to work and improve their VO2max.
We may also rely on basic data science principles in this process as well. For example, during the descriptive testing phase, we may identify that an individual needs to improve their maximal power output on the rower. With some basic data analysis tools, we may identify that the rate of change of muscle oxygen saturation has an inverse-linear correlation with maximal power output. Knowing that we can select training methods that we know improve an individual's maximal oxygen consumption rate, like repeat desaturation intervals for example. Then we can monitor their improvements in oxygen extraction and power output over time and determine when these two factors begin to uncouple from each other since maximal oxygen extraction is constrained by total blood flow. At that point we can reprioritize that athletes training to focus on the new ‘lowest hanging fruit’.
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