When energy supplies must be maintained during long duration exercise, that is anything over about three minutes in length, the anaerobic systems are supplementary to the breakdown of carbohydrate and fat by the aerobic system. This is the system that we rely on to keep us going through a full mountain marathon. This process provides most of our energy requirements throughout the day. At low to moderate levels of intensity of exercise, the body can continue for hours to recycle ATP, largely from these aerobic sources. Aerobic respiration is only possible within organelles called mitochondria, found within the muscle cell.
Training increases the number and size of the mitochondria in the muscles being used.
The body stores large amounts of energy in fat. Fat is a very rich energy store. One gram of fat stores 9 Kcals whereas one gram of carbohydrate stores only 4 Kcals. However the breakdown of fat to release energy utilises large amounts of oxygen. The body s limited ability to supply large amounts of oxygen to the muscles limits the intensity of exercise that can be fuelled by fat, thus the anaerobic systems which supply energy at a very high rate but do not use oxygen cannot utilise fat as fuel.