Muscles run on ATP (Adenosine Triphosphate). This is produced by three energy systems during exercise.
Aerobic (Low intensity, endurance). Used from durations from about a minute to hours. This system uses oxygen and synthesizes ATP from stored fat and carbohydrates. It can produce energy until reserves are exhausted but produces ATP at a slower rate compared to other systems. The first hour or so of any exercise consumes muscle glycogen, afterwards fats become the principal source of fuel. In a race you will probably want to eat some sugars before all the glucose is consumed or risk “hitting the wall”.
Anaerobic Lactic (Glycolytic) Energy System. Used for durations from 10 seconds to max two minutes for medium to high levels of activity such as a sprint or short uphill. Oxygen is not used for fuel but lactic acid will build up quickly given a burning sensation in the muscles.
Anaerobic Alactic (ATP-CP) Energy System (High Intensity – Short Duration/Bursts). The system does not use oxygen, but rather your body’s CP (creatine phosphate) stores to create energy for a short duration such as a 100 meter sprint.