During an exercise, higher intensities require higher uptake of oxygen. During a race, the faster one pedals or runs the higher levels of oxygen one must consume. VO2max is the limit at which point one's body cannot process any more oxygen. VO2max is measured in ml/kg/min.
VO2max is the athlete's aerobic limit. Once the intensity increases beyond the VO2max the body uses anaerobic methods to produce energy. This causes lactic acid to be produced quickly. Energy production at this level is short and for a few minutes only. Usually, lactic acid is produced even before the athlete reaches his/her VO2max.
An average sedentary person has a VO2max of approximately 40 ml/kg/min. As a comparison, Lance Armstrong had a VO2Max of above 83. An increase of 5% to 20% in VO2max is achievable through regular endurance training.