VO2 and VO2max

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What are VO2 and VO2max?
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.
Note
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.

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The VO2 Abbreviation Explained
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.
The Basic Physiology of VO2

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.
VO2 During Exercise
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.
VO2max
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).
How is VO2max Measured?
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).
The method for measurement of VO2 can be summarized according to the following equation...
•    VO2 = [VI x %O2VI] - [VE x %O2VE]
where

VI = Volume of Inspired Air
%O2VI = Percent Oxygen in Inspired Air
VE = Volume of Expired Air
%O2VE = Percent Oxygen in Expired Air
The method described above is the most ideal for determining VO2max, but it requires access to sophisticated laboratory equipment that most people don't have. If you don't have access to a laboratory equipped for VO2max measurement, you can still reasonably estimate your VO2max by using any one of the four calculators on our VO2max Calculator page.
Plowman SA, Smith DL, Exercise Physiology: for Health, Fitness, and Performance, 2nd Edition, 2003, Glenview, IL.
What are Normal VO2max Values?
The table below displays standard VO2max classification ranges for both men and women.
VO2max (mL•kg-1•min-1) Classifications for Women
Age (years)    Poor    Fair    Good    Excellent    Superior
20 - 29    ≤ 35    36 - 39    40 - 43    44 - 49    50+
30 - 39    ≤ 33    34 - 36    37 - 40    41 - 45    46+
40 - 49    ≤ 31    32 - 34    35 - 38    39 - 44    45+
50 - 59    ≤ 24    25 - 28    29 - 30    31 - 34    35+
60 - 69    ≤ 25    26 - 28    29 - 31    32 - 35    36+
70 - 79    ≤ 23    24 - 26    27 - 29    30 - 35    36+
VO2max (mL•kg-1•min-1) Classifications for Men
Age (years)    Poor    Fair    Good    Excellent    Superior
20 - 29    ≤ 41    42 - 45    46 - 50    51 - 55    56+
30 - 39    ≤ 40    41 - 43    44 - 47    48 - 53    54+
40 - 49    ≤ 37    38 - 41    42 - 45    46 - 52    53+
50 - 59    ≤ 34    35 - 37    38 - 42    43 - 49    50+
60 - 69    ≤ 30    31 - 34    35 - 38    39 - 45    46+
70 - 79    ≤ 27    28 - 30    31 - 35    36 - 41    42+
Source:
V. H. Heyward, Advanced Fitness Assessment and Exercise Prescription, Fifth Edition, 2006, Champaign, IL: Human Kinetics.
Original Source:
The Cooper Institute for Aerobics Research, The Physical Fitness Specialist Manual. Dallas, TX. 2005.
How Can VO2max be Improved?
Basic aerobic endurance training that follows the ACSM's recommended guidelines for cardiorespiratory fitness training is known to improve VO2max. It is common for an average unfit person following the ACSM's guidelines to experience a 15% improvement in VO2max after 2 to 3 months of regular training. Although basic aerobic endurance training is usually sufficient for the general population, the most effective approach to improving VO2max, the one applied by many elite endurance athletes, is called high intensity interval training (HIIT).
How to Improve VO2max with HIIT
HIIT is a method of training that involves performing intervals of high intensity exercise interspersed with rest intervals of lower intensity exercise. The specific intensity and duration of each high and low intensity interval can be modified to suit the training goals of the individual, however, in the case of someone whose goal is to improve their VO2max, the most effective approach is to maximize the total amount of training time that is spent at, or close to, VO2max. The more time you spend at or near VO2max, the greater your VO2max improvements will be.
Creation of an HIIT Program
Step 1: Determining the Length and Intensity of the High Intensity Interval
To improve VO2max your high intensity intervals should be performed at an intensity of roughly 90% of VO2max (this correlates to approximately 95% of maximum heart rate). This is not absolute maximal exertion, but almost maximal exertion. The length of each high intensity period should be roughly 75% of the maximum amount of time that you could last at this intensity before fatiguing. To determine this, you'll need to time yourself while you perform a single trial interval at high intensity until you fatigue (do this a few days before beginning your HIIT sessions so that you have time to adequately recover, and be sure to properly stretch and warm-up before you do it). If, for example, you can last for 200 seconds (this is an example only, it might be more or less) before fatiguing during this trial period, then your actual high intensity intervals should be set to last for 150 seconds each.
Step 2: Determining the Length and Intensity of the Rest Interval
Now that you know how long your high intensity intervals will last, you need to figure out how long your rest intervals will last. The relative duration ratio of your rest to high intensity intervals should be based on your comfort and fitness level. If you are a beginner, set your rest to high intensity interval time ratio at 2:1 (i.e. your rest interval duration should be twice that of your high intensity interval). If you are well trained then you will ideally have interval time ratios of 1:1 or even 1:2 (i.e. rest interval the same duration as your high intensity interval or rest interval half the duration of your high intensity interval, respectively). Therefore, using the example from Step 1, if the length of your high intensity interval is 150 seconds, your rest interval would last for 300 seconds if you are a complete beginner, or 150 seconds if you are well trained, or perhaps 75 seconds if you are very well trained. During your rest intervals, you should carry on your exercise activity while reducing your intensity level to approximately 70% of maximum heart rate.
Step 3: Determining the Total Number of Intervals
Set the total number of intervals performed over the entire workout so that the total time spent at high intensity equals between 20 to 30 minutes. This variable should also be determined based on your comfort and fitness level. Therefore, using the example from Step 1, if your high intensity intervals last for 150 seconds (2.5 minutes) then you should perform at least 8 high intensity intervals to ensure a total of 20 minutes of high intensity activity. Start and end your training sessions with a rest interval (so you'll actually end up doing one more rest interval than the total number of high intensity intervals).
Step 4: Training Frequency, Activity Type, and Additional Notes
Since these types of training sessions are fairly demanding, you should perform them no more than 2 or 3 times per week. The type of exercise activity performed during your HIIT sessions doesn't really matter, as long as it is a full body type of activity (i.e. running, swimming, rowing, cross-country skiing, speed skating, etc...) and you can achieve the necessary steady intensity levels discussed above for the appropriate periods of time. If you are training to improve your performance in a particular type of exercise activity, you should be performing that particular type of exercise activity during your HIIT sessions to benefit from the effects of training specificity. As with all training programs, you should modify the parameters of your HIIT sessions within the guidelines discussed above to suit your fitness level as it progresses.
Helgerud J, Høydal K, Wang E, et al. (2007). "Aerobic high-intensity intervals improve VO2max more than moderate training". Med Sci Sports Exerc 39 (4): 665–71.
Douglas S, Pfitzinger P, Advanced Marathoning, 2nd Edition, 2009, Champaign, IL: Human Kinetics.
Factors that Affect VO2max
The list below outlines the main factors that affect VO2max. Some of these factors, such as your fitness level, are under your control but others are not.
Genetics
Your genetic make-up has a very strong influence over your VO2max and it is ultimately what defines your upper limit for VO2max improvements. The capacity of your circulatory system to deliver oxygenated blood to your muscles and also the specific physiology of your muscles are both genetically predetermined to a certain extent. For example, in regards to your circulatory system, hemoglobin (the molecule in your blood that binds and carries oxygen) concentrations are genetically influenced. As for your muscle physiology, the relative proportion of fast twitch and slow twitch fibers in your muscles is also genetically predetermined, and slow twitch muscle fibers are able to consume more oxygen than fast twitch muscle fibers.
Age
The average person's VO2max peaks at around the age of 18 and remains fairly level (only a slight decline occurs) until the age of 25. Beyond 25 years of age VO2max declines by roughly 1% per year. At the age of 55 the average person has a VO2max that is approximately 27% less than that of a 20 year old. Although there is a negative correlation between VO2max and age, the available evidence indicates that the influence of a person's fitness level on VO2max is stronger than the influence of their age.
Katch VL, Katch FI, McArdle WD, Exercise Physiology: Energy, Nutrition, & Human Performance, 6th Edition, 2007, Baltimore, MD.
Fitness Level
Your VO2max is heavily influenced by fitness level. Depending on the nature of the training program adopted, an unfit person can improve their VO2max from 5% to 30%. For those following the recommended ACSM training guidelines for cardiorespiratory fitness (see How To Improve Cardiorespiratory Fitness) a 15% increase in VO2max is common. The majority of improvements to VO2max will occur during the first 2 months of training. After this point VO2max will continue to improve, but at a slower pace.
Plowman SA, Smith DL, Exercise Physiology: for Health, Fitness, and Performance, 2nd Edition, 2003, Glenview, IL.
Form of Exercise
Since oxygen is ultimately consumed in the muscles during exercise, it follows that your VO2max, when measured, will vary in accordance with the specific form of exercise you are performing. For example, there is usually more total muscle mass active during running than during swimming, and so VO2max will generally be greater when measured during a running test than it would be if measured during a swimming test. Treadmill running type tests typically return the highest VO2max scores.
Body Mass and Body Composition
Differences in body mass account for almost 70% of the differences observed in VO2max test subjects. Since almost all body tissues consume oxygen, although some tissues more than others (i.e. muscle consumes more oxygen than fat), a person with a larger total body mass will be much more likely to consume more total oxygen than a person with a lower total body mass. This is the reason for which VO2max is generally measured on a per unit mass basis, it reduces the obvious disparities that will be observed in people of differing total body mass. However, while expressing VO2max on a per unit weight basis will control for differences in total body mass, it does not eliminate differences in body composition (i.e. one person may be more muscular than another). Since muscle consumes more oxygen than fat, a more muscular person would be expected to have a larger VO2max, all else being equal, even when it is measured on a per unit mass basis.
Gender
There is an inherent disparity in the VO2max capabilities of men and women. Men have roughly 10% to 25% higher VO2max capabilities than women, even when experimental adjustments are made to eliminate and/or minimize differences in total body mass, fat free mass, training history, or even differences in hemoglobin concentrations. The available data suggests that the differences are biologically predetermined and largely due to size differences in contracting muscles.