Longitudinal growth of muscle is difficult to measure because muscles are capable of contraction and also can be strectched.
- The second problem is that many muscle fibers are much shorter than the length of the muscle, and their ends must be found if the fiber length is to be measured.
- Longitudinal growth originates from the formation of new sarcomeres at the ends of the muscle fibers and the addition of new sarcomeres is regulated so that sarcomere length and the degree of overlap of thick and thin filaments is at an optimum for efficient muscle contraction, a regulation that may be mediated via dystrophin as a mechanochemical transducer. However, the system must be a very complex one, because sarcomere length changes during contraction differ between muscles and are related to the angular changes of nearby skeletal joints.
- Myofibrils usually insert obliquely at their ends so that new filaments can be added without interfering with the mechanical continuity of the fibrils at their attachement to the cell membrane.
- When investigating the source of new muscle fiber nuclei by injecting tritiated thymidine, the incorporation of new DNA is most active at the ends of the muscle, but there is evidence of minor growth zones associated with intrafascicularly terminating fibers in the belly of the muscle.
- The ratio of muscle belly length to free tendon length remains approximately constant during growth.