There are two skeletal muscle fiber types. Red muscle is slow but durable. White muscle is fast, but gets tired very quickly. Both fiber types are present in all muscles.

Weight training increases muscle size but does not increase the number of cells. It helps small muscle cells to grow. As a result of heavy weight training, the oxygen uptake ability of a muscle can be reduced, causing a reduction in the cell's endurance properties.

In contrast, long-term training with a low resistance increases the oxygen uptake of the muscles, improving their endurance.