By Michelle Colvin, Spinning® Master Instructor | United KingdomVO2 sounds complicated but it isn’t. ‘V’ stands for volume and ‘O2’ stands for oxygen. VO2 is the volume or amount of oxygen your body consumes. The fitter you are, the more oxygen your body burns and the higher the score.Peak VO2 is the measure of millilitres of oxygen consumed at maximal effort divided by a person’s weight in kilos per minute. VO2 max is an effective measure of fitness and your body’s maximum potential to perform work. It is impacted by (amongst other things) hereditary factors, age and health status.Just like a car engine, your body needs oxygen to mix with fuel to produce energy. Your lungs (engine) and heart (fuel pump) deliver oxygen to the individual muscle cells and combine with fuels (fat, carbohydrates) for the production of energy. One of the by-products (exhaust) of this energy creation is carbon dioxide. You breathe in oxygen and you breathe out carbon dioxide.At lower exercise intensities, your aerobic system uses fats and some carbohydrates as fuel along with a moderate amount of oxygen. Of these fuels, only carbohydrates have the capacity to be used as fuel without oxygen, or anaerobically. As the intensity of your exercise increases and you reach the capacity of your aerobic system, to bring oxygen into your body and you shift progressively to your anaerobic system. Your anaerobic system primarily uses carbohydrates (in the form of blood sugar or stored glycogen) as a fuel source and produces an increased amount of carbon dioxide exhaled.For example, if you are walking up a few flights of stairs, as you get to perhaps the third flight, you begin to switch from your aerobic system to your anaerobic system and will notice an increased demand for oxygen and you will breathe harder and more rapidly. If you continue to climb the next flight of stairs, you will notice a burning sensation (accumulation of lactic acid) in your leg muscles and as this lactic acid accumulates in the muscle, your body attempts to rid itself of this condition by buffering it with bicarbonate in the blood. This buffering process produces additional carbon dioxide in the blood, which causes you to breathe even harder. This point is typically referred to as your lactate or ventilatory threshold.