Implications of muscle design on surgical reconstruction of upper extremities.

The basic anatomic and physiologic properties of human upper extremity muscles have been elucidated using a novel intraoperative sarcomere length measuring device in conjunction with quantitative anatomic and biomechanic models of these same muscles. We reviewed the genesis of these studies which began with development of the optical tools, validation using animal models, and application to human upper extremity surgery. Human muscles have a remarkable degree of specialization, even among synergists. Mechanical properties of human tendons provide another source of specialization such that the muscle-tendon unit does not simply retain the properties of a muscle plus a tendon. The operating range of different muscles is also a method whereby the balance of the joint can be maintained in the face of altered moment arms and muscle forces. The derivation of these principles is explained along with their importance in surgical tendon transfers where one muscle substitutes function for another.