| 1 | Strength training increases the size of these muscle fibers. | |
| 2 | For example, when an athlete trains, her muscle fibers break down. | |
| 3 | Muscles are composed of cells called muscle fibers. | |
| 4 | This extra muscle fiber trauma is a result of higher intensity training. | |
| 5 | They have muscle fibers of various kinds that contract and expand at varying speeds and strengths. |