[Changes in muscle power following tendon lengthening and tendon transfer]

Each muscle consists of a tendon, the aponeuroses and the muscle fibers. Because of the anatomical architecture a force functionally adapted to the joint motion is generated by the muscle. If the functional length changes, the muscle adapts by altering the number of sarcomeres in a series. If the tendon is elongated without changing the functional range of motion at which the muscle is active on the joint, the passive tension is decreased. As a result, the number of sarcomeres in a series is reduced. The muscle produces less force and the range of active force production is narrowed. If by elongating the tendon, however, the functional range of motion of the muscle at the joint is moved and the tension remains unchanged, the muscle force can be preserved. A tendon transfer affects the muscle in a similar way. If the length of the muscle (and the tension) remains unchanged, the muscle force will be preserved. This prerequisite is fulfilled even better the more the architecture of the replacing muscle equals what is being replaced.