VO2max is the maximum amount of oxygen that can be consumed each minute during exercise. It is an indicator of cardiopulmonary fitness and a predictor of endurance potential

VO2 = HR x SV x (a-vo2 diff)

HR is heart rate in beats per minute.

SV is stroke volume or the amount of blood in ml. that is pumped from the heart each time it beats.

(a-vo2 diff) is the amount of oxygen in the blood as it travels in the arteries to the muscles minus the amount returning to the heart in the veins. Endurance athletes have the ability to extract and utilize higher amounts of oxygen in their muscles as compared to other athletes.

The Anaerobic Threshold or AT represents the transition from predominantly aerobic to predominantly anaerobic metabolism in the working muscles. When exercise intensity increases, energy (ATP) cannot be supplied to the muscles fast enough. Increased recruitment of fast twitch muscle fibers along with a lack of sufficient oxygen to completely break down carbohydrates begins to rapidly deplete glycogen stores. In addition, lactic acid is being produced in higher amounts. Lactic acid from the muscles is converted to blood lactate, a process that increases CO2 in excess of normal muscle metabolism. Blood lactate is then shuttled throughout the circulatory system and used by other tissues for fuel.

The anaerobic threshold is also referred to as the lactate or ventilatory threshold and signifies the point when there is a sudden increase in CO2 production, ventilation, and blood lactate levels. An endurance athlete wants his/her anaerobic threshold to be at a high percentage of their VO2max. The average untrained person has an AT of 55% of their VO2max while that of a trained endurance athlete is between 80-90%. With a high AT, you can exercise at a higher intensity while delaying the fatigue, soreness and decreased power that occur with increasing levels of acidity in the muscles and lactate in the blood.

The Aerobic Threshold or AeT signifies the point when the active muscles begin to use more glycogen than fatty acids to provide energy during exercise. This occurs at about 60-65% of VO2max in a well trained athlete. Energy is in the form of adenosine triphosphate or ATP. All of the body processes from digestion to muscular contractions use ATP. At this time, blood lactate levels rise slightly and breathing deepens a bit.

Since exercise intensity is low while training near the aerobic threshold, duration can be long. This increases muscular endurance, tendon and ligament strength, and the efficiency of the aerobic system. Exercise efficiency or economy means that less oxygen is required for the same activity or that the same muscle fibers provide more energy at any given rate of oxygen consumption. The amount of oxygen you needed to run easily at a 9 minute mile is now enough for you to run easily at an 8 1/2 minute mile. Training close to the aerobic threshold teaches the body to rely on and after adaptations that occur following training, store more triglycerides within the muscle and increase the amount of oxygen getting into the muscle. Endurance athletes are very efficient at using fat to provide ATP, sparing the use of muscle glycogen even at faster running or cycling speeds.