VO2 Max Essentials Part I: What Limits VO2 Max?

Is our max oxygen consumption constrained by the heart or the lungs?

Greetings.

Welcome to part I of a 3-part series on VO2 max and endurance performance.

Today, we’ll dive into the basics of VO2 max, it’s history, and discuss the factors that contribute to and limit VO2 max.

In parts II and III, we’ll look at the determinants of successful endurance performance and then discuss the types of training that are best for improving VO2 max, as well as some ways to estimate VO2 max using field tests and equations.

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There’s been a lot of talk lately about the importance of cardiorespiratory fitness for health and longevity. Recent studies have provided compelling evidence that the higher your cardiorespiratory fitness — which is most commonly assessed as VO2 max — the longer your life.

VO2 max refers to your maximum rate of oxygen consumption, and it’s typically measured during a maximal exercise test to exhaustion (they’re not fun, trust me). It is used as an indicator of an individual's cardiorespiratory fitness and is often used to demonstrate the effects of training. Most exercise training studies use VO2 max as a primary, if not a secondary outcome, and most effective training programs will increase VO2 max to some degree.

VO2 max is also used in the development of exercise prescription — exercise at a certain % of your VO2 max can be used to indicate low, moderate, and high-intensity exercise, though heart rate is most often used due to practical considerations.

https://difference.guru/difference-between-pulmonary-and-systemic-circulation/ 

VO2 max: a brief history

The concept of VO2 max was first described and defined in the 1920s by scientists A.V. Hill and Hartley Lupton. They proposed the VO2 max paradigm, which included the following key points:

- There is an upper limit to oxygen uptake in humans.

- There are individual differences in VO2 max.

- A high VO2 max is essential for success in middle- and long-distance running.

- VO2 max is limited by the ability of the cardiorespiratory system to transport oxygen to the muscles.

Hill and Lupton conducted careful measurements of oxygen consumption, observing that VO2 (oxygen consumption) increased with running speed up to a certain point, beyond which no further increase in oxygen intake could occur. This demonstrated the existence of an upper limit to oxygen consumption — VO2 max.

Today, it is universally accepted that there is a physiological upper limit to the body's ability to consume oxygen. Research has illustrated this concept by indicating a point during exercise whereby an increase in work rate no longer produces an increase in oxygen consumption — VO2 plateaus. VO2 does not increase indefinitely with increases in work rate or running speed.

While a VO2 plateau is used to indicate that someone has reached their max, approximately 50% of people do not demonstrate a plateau when pushed to maximal effort. However, the absence of a plateau doesn’t necessarily indicate a failure to reach the true VO2 max, and other criteria such as respiratory exchange ratio above 1.15 (indicating that nearly 100% of energy is being derived from the “burning” of carbohydrate or “anaerobic” pathways) and blood lactic acid levels are recommended to verify maximal effort, as well as someone’s rating of perceived exertion (which should be maximal at the end of the test if done properly).

The VO2 plateau observed in some individuals represents a leveling off in cardiac output and arterial–venous oxygen (a–v O2) difference. The a–v O2 difference is a measure of how much oxygen is being delivered to the muscles (arterial oxygen) and how much oxygen is present in the blood returning to the heart (venous oxygen). The difference indicates how much oxygen the muscles took up and utilized (oxygen extraction).

This concept, known as the Fick principle, is how VO2 max is measured in the lab. If we have a measure of the oxygen content in the inspired air and a measure of the oxygen content in the expired air, we know the amount of oxygen that was extracted during transport throughout the body.

A high VO2 max is commonly associated with elite long-distance runners. Indeed, interindividual differences in VO2 max were recognized early on, and researchers emphasized the importance of a high VO2 max for elite performers. They also recognized the influence of other physiological factors, such as running economy, on race outcomes. While VO2 max is certainly necessary for elite endurance performance, it’s hardly sufficient. 

We’ll discuss the factors determining endurance performance in a future post.

Over the years, many exercise physiologists have studied the determinants of VO2 max and arrived at a consensus supporting the original VO2 max paradigm. The prevailing view is that VO2 max in humans is primarily limited by the rate of oxygen delivery by the cardiorespiratory system, rather than the muscles' ability to take up oxygen from the blood or the lungs’ ability to oxygenate blood.

What limits VO2 max?