There are at least fifteen types of upper extremity tendon transfer procedures for combined peripheral nerve injuries that involve re-routing a single donor muscle to multiple recipient tendons. As a result, force and movement across finger joints become coupled, resulting in limited grasping abilities. Quantification and modeling of the donor muscle’s mechanical functioning after tendon transfers can contribute to understanding surgical implications and guide novel innovative solutions. Therefore, the goal of this thesis was to analysis the biomechanics of tendon transfers for combined peripheral nerve injuries, specifically the extensor carpi radialis longus-to-flexor digitorum profundus (ECRL-to-EDP) tendon transfer for high medium-ulnar nerve palsy, while leveraging the development of an implantable passive differential mechanism (PDM) to improve such surgeries. This goal was achieved by studying the chicken extensor digitorum longus muscle-tendon unit, a model that was considered analogous to the human four coupled tendon muscle-tendon unit after ECRL-to-FDP tendon transfer and developing a computational model that simulated the isometric muscle force generation through a single tendon, two tendons coupled in parallel, and a PDM. The former two tendon network configurations provided insight into the baseline mechanics of a typical muscle-tendon unit and a muscle-tendon unit where a single muscle is coupled to multiple tendons. The latter tendon network configuration highlighted the advantage of incorporating an implantable PDM into a tendon network.