Biomechanics of tendon transfers

There are just two variables that define the mechanical qualities of each muscle. One is its potential for generating tension, and the other is the distance through which tension can be exerted. Tension capability can be increased in a muscle by exercise. There is no way to increase potential for excursion except by reattaching a muscle at surgery with extra length and tension, at the risk of involuntary contractions during healing that may result in avulsion. Single muscle may control several joints in series but cannot selectively vary the torque delivered at each. Therefore, when working against distal opposing forces, it is necessary to have separate muscles to add torque to proximal joints; or it may be advisable to reduce the number of joints to be controlled by fusing one or more of them, or sometimes by depending on tenodesis or other passive support.