Fatty acids and amino acids require large amounts of oxygen to burn and produce energy, while glucose needs only a small amount. Aerobic exercise is exercise in which the muscles use large amounts of oxygen supplied by the lungs. For many people, the calculation for maximum target heart rate is 220 minus their age. The gold standard of fitness is called VO2-max. This is the body's ability to transport oxygen from the lungs through the bloodstream and to utilize oxygen to convert calories to energy.
Anaerobic exercise is exercise in which little oxygen is utilized in the muscles to burn calories.
The primary source of calories for anaerobic exercise is glucose. Anaerobic exercise typically occurs for one or two minutes at the beginning of a vigorous workout when calories come from glucose stored in the muscles. For example, weight training is usually anaerobic. When the lungs and heart cannot keep up the supply of oxygen needed to burn fat, the muscles switch over to burning glucose. When the body is at rest, the muscles use calories and oxygen from the bloodstream.