HUMAN WRIST MOTORS - BIOMECHANICAL DESIGN AND APPLICATION TO TENDON TRANSFERS

Moment arm, muscle architecture, and tendon compliance in cadaveric hu man forearms were determined and used to model the wrist torque-joint angle relation (i.e. wrist torque profile). Instantaneous moment arms were calculated by differentiating tendon excursion with respect to jo int rotation. Maximum isometric tension of each wrist muscle-tendon un it was predicted based on muscle physiological cross-sectional area. M uscle forces were subsequently adjusted for sarcomere length changes r esulting from joint rotation and tendon strain. Torque profiles were t hen calculated for each prime wrist motor (i.e. muscle-tendon unit ope rating through the corresponding moment arm). Influences of moment arm , muscle force, and tendon compliance on the torque profile of each mo tor were quantified. Wrist extensor motor torque varied considerably t hroughout the range of motion. The contours of the extensor torque pro files were determined primarily by the moment arm-joint angle relation s. In contrast, wrist flexor motors produced near-maximal torque over the entire range of motion. Flexor torque profiles were less influence d by moment arm and more dependent on muscle force variations with wri st rotation and with tendon strain. These data indicate that interacti ons between the joint, muscle, and tendon yield a unique torque profil e for each wrist motor. This information has significant implications for biomechanical modeling and surgical tendon transfer.