When an athlete stresses the aerobic system, the body adapts in several ways. Oxygen can move more quickly to each muscle. Oxygen is necessary to produce the maximum amount of energy from pyruvate. Besides the availability of oxygen, there are other important factors which encourage the muscle to use pyruvate for fuel and speed up the use of lactate in adjacent muscles and the blood stream. Endurance training increases the density of mitochondria in certain cells.
Mitochondria are the part of cells that convert pyruvate into energy.
The denser they are, the higher the capacity of the cell to use pyruvate as fuel and produce more energy aerobically. Many sports physiologists have used the analogy of a factory to describe mitochondria. When the mitochondria finish processing pyruvate there is a lot of energy available as well as common waste products such as water, carbon dioxide and heat. This process produces a lot of energy but it is too slow to sustain fast activity such as sprinting. Energy produced in the mitochondria will provide most of the energy for a marathon or a triathlon.