The mitochondrion is an important cell organelle. The most important function of a mitochondrion is to generate ATP. The mitochondria are therefore often called the cells’ power plants. Mitochondria are present in almost almost all cells, but the number of mitochondria in a cell depends on the amount of energy the cell needs, and therfore large numbers of mitochondria are present in muscle fibers. The concentration of mitochondria is dependent on muscle fiber type and the demands placed upon the muscle. In addition the number of mitochondria in a muscle fiber adapts to endurance training or low intensity high repetition resistance exercise, which both target the aerobic system and stimulate the mitochondria to replicate.