Fitness can be measured by the volume of oxygen you can consume while exercising at your maximum capacity. VO² max represents the maximum amount of oxygen in millilitres per kilogram of weight that a person can use in one minute converting fuel to energy.  Those who are fit have higher VO2 max values and can exercise more intensely than those who are not as well conditioned.

As there is a direct correspondence between oxygen usage and energy consumed it represents the practical upper limit of energy available to an exercising muscle.  This upper limit is largely genetically determined but training can increase it by up to 20% of non-trained capacity.  Numerous studies show that you can increase your VO2 max by working out at an intensity that raises your heart rate to between 65 and 85% of its maximum for at least 20 minutes three to five times a week.

This means running quite fast. Interval sessions with ‘on’ times up to a maximum of 5-8 minutes are the best mechanism for this. If you run near your upper limit for some time then your body will make adaptations and try and increase it. Running longer than about 8 minutes, you won’t be running hard enough (close to your VO2max performance) so you won’t stress this system.

A mean value of VO2 max for male athletes is about 3.5 litres/minute and for female athletes it is about 2.7 litres/minute.

See more definitions in the Definitive Guide to Running Terms.