The list below outlines the main factors that affect VO2 Max. Some of these factors, such as your fitness level, are under your control but others are not.
Your genetic make-up has a very strong influence over your VO2 Max and it is ultimately what defines your upper limit for VO2 Max 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.
The average person's VO2 Max 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 VO2 Max declines by roughly 1% per year. At the age of 55 the average person has a VO2 Max that is approximately 27% less than that of a 20 year old. Although there is a negative correlation between VO2 Max and age, the available evidence indicates that the influence of a person's fitness level on VO2 Max 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.
Your VO2 Max is heavily influenced by fitness level. Depending on the nature of the training program adopted, an unfit person can improve their VO2 Max from 5% to 30%. For those following the recommended ACSM training guidelines for cardio-respiratory fitness a 15% increase in VO2 Max is common. The majority of improvements to VO2 Max will occur during the first 2 months of training. After this point VO2 Max 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 VO2 Max, 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 VO2 Max 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 VO2 Max scores.
Body Mass and Body Composition
Differences in body mass account for almost 70% of the differences observed in VO2 Max 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 VO2 Max 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 VO2 Max 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 VO2 Max, all else being equal, even when it is measured on a per unit mass basis.
There is an inherent disparity in the VO2 Max capabilities of men and women. Men have roughly 10% to 25% higher VO2 Max 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.
Keller, B.A., Katch, F.I. It is not valid to adjust gender differences in aerobic capacity and strength for body mass or lean body mass. Med Sci Sports Exerc 1991; 23:S167.