VO2 max refers to the maximum amount of oxygen that can be processed by the body in a certain time period - it is expressed as litres of oxygen per minute. In other words it is the capacity of the cardiovascular system and peripheral systems (muscle groups) to take in oxygen, deliver to muscle systems, process oxygen, and deal with the by-products. VO2 max can be tested by increasing pace every minute until the runner is going all out. Expired air is collected in a bag during the test - heart rate is taken at the end and lactate levels are measured two minutes after the end of the test when lactate is highest.
Along with lactate threshold and running economy it is an important determinant to distance running. It is most important for distances up to 10k.
So, how do you improve VO2 max? Interval training is the most efficient way to improve VO2 max. Usually this is done by running at 3k-5k pace (usually95-100% VO2 max) for 3-5 minute intervals with recovery times that are equal to, or a little less, than the time of the interval. The amount of total weekly interval-paced training should be up to 8 percent of total mileage according to Jack Daniels.