VO2 (or oxygen consumption) is a measure of the volume of oxygen that is used by your body to convert the energy from the food you eat into the energy molecules, called adenosine triphosphate (ATP), that your body uses at the cellular level. VO2max (or maximal oxygen consumption) is simply the maximum possible VO2 that a given person can achieve. VO2 and VO2max are important in the context of exercise, because they are a measure of your body’s ability to generate ATP, and ATP is the energy source that allows your muscles to continue working while you are exercising. Therefore, by definition, a VO2max measurement is ultimately a measure of your cardiorespiratory fitness level.

During exercise, ATP can be generated without oxygen, but only for short periods of time. For aerobic endurance (i.e. cardiorespiratory) exercise the primary method of ATP generation requires oxygen. You can read more about ATP in the Exercise Energy Systems article, but for the purposes of this article all you really need to know is that ATP fuels your muscles during exercise and your muscle cells need oxygen during aerobic endurance exercise to generate ATP.

The V in VO2 stands for volume, and in fact it is usually displayed in technical texts with a dot above it to indicate that it represents not just a volume, but a volume rate (i.e. a volume per unit of time). The O2 in VO2 is the chemical formula for oxygen in its most stable form, the form that is found in the air we breathe.

So how does your body “consume” oxygen? Breathing in is only the first step in oxygen consumption and, in fact, not all of the oxygen you breathe in gets consumed. The amount of oxygen in the air that you breathe in is approximately 20.95% by volume (the other major components of air are nitrogen, argon, and carbon dioxide at 78.08%, 0.93%, and 0.03% by volume respectively). If you consumed all of the oxygen that you breathed in you would expect to find that the amount of oxygen in the air that you breathed out would be 0%, but it is typically in the 15% to 18% range, depending on what you are doing and a number of other factors. To actually consume the oxygen you’ve inhaled your body needs to make use of it in the cellular respiration process that generates ATP. Before your cells can use oxygen to generate ATP, the oxygen must first be delivered to your cells. Delivery of oxygen to your cells requires pulmonary (or lung) ventilation to bring the oxygen into your body, diffusion of the oxygen from the air in your lungs into the blood in the capillaries that surround them, cardiac (or heart) contraction to pump the blood through your body to your muscles, and diffusion of the oxygen from your blood into your muscle cells. Once oxygen has been delivered to your muscle cells it is used in the cellular respiration chemical reaction to produce ATP energy. The byproducts of this cellular respiration chemical reaction are water and carbon dioxide, that’s why we are said to breathe in oxygen and breathe out carbon dioxide.

When you exercise your muscles are working harder than normal and, as a result, they require more energy than normal. Since the ATP energy used by your muscles is generated with the aid of oxygen, it follows that an increase in exercise intensity will result in an increase in muscular oxygen demands. Therefore, increased exercise intensity ultimately corresponds to an increased VO2. This is the reason that your breathing gets progressively faster and deeper as your exercise intensity increases, your body is trying to provide more oxygen to your working muscles so that they can generate enough ATP energy to keep you moving.

During aerobic/endurance exercise, as your exercise intensity increases, so does your VO2. If you continue to increase your exercise intensity you will eventually reach a point of maximal exertion (i.e. you cannot work any harder than you already are). Your VO2 at this stage of maximal exertion is called your maximal oxygen consumption (VO2max). It is the maximum volume of oxygen that your body is capable of consuming and converting to energy for your working muscles. VO2max is considered to be the gold standard by which one can measure his or her cardiorespiratory fitness level. A person who is fit, in the cardiorespiratory sense of the word “fit,” would have a higher VO2max than someone who is less fit. However, it is important to understand that if you have a larger VO2max than someone else, it does not necessarily mean that you could beat them in, for example, a marathon race. What it means is that your body is more able to absorb and use oxygen to generate energy for your muscles, and this will certainly give you an edge, but what you are actually capable of doing with that energy depends on many other factors (the mechanical efficiency with which you run at a given speed, for example).

VO2 is essentially the difference between the amount of oxygen that a person inspires and the amount of oxygen that they expire. Therefore, to accurately determine someone’s VO2 you need to know the volume of air that they inspire and expire and the percentage of oxygen in the inspired and expired air (remember, the percentage of oxygen in the inspired air is an already known constant, so you really only need to measure the percentage in the expired air). Sophisticated laboratory equipment is required to make these measurements while a subject performs a gradual progression to maximal physical exertion (generally this is done on a treadmill). Once maximal exertion is reached (i.e. the subject cannot work any harder) VO2 will, after a slight delay, be observed to plateau. The highest measured VO2 at this plateau stage represents the subject’s VO2max.

Results for VO2 measurements are generally displayed in L•min-1 (i.e. litres per minute, representing the volume of oxygen consumed by your entire body each minute) or, to account for differences in total body mass, in mL•kg-1•min-1 (i.e. millilitres per kilogram per minute, representing the volume of oxygen consumed each minute per kilogram of your body mass).