Figure 3:

Extensor moment arm of the extrinsic extensors at the PIP joint as a function of PIP joint angle. The black line indicates the moment arm of the nominal model; grey lines indicate the moment arms for the models where the diameter of the joint kinematic constraint was decreased to simulate volar translation of the lateral slips of the extensor mechanism. In all models, the moment arms of the EDCI and EIP are identical. The kinematic constraint is a part of the model of the geometry of the joint. Note that in full extension (0° PIP flexion), the muscle-tendon path is initially not in contact with the joint. All simulations have the same moment arms until the muscle-tendon path reaches the kinematic constraint, at which point the moment arm of that model diverges because the muscle-tendon path wraps over its surface. As the diameter of the constraint is decreased, the muscle-tendon path moves closer to joint center of rotation before making contact, decreasing the moment arms of the extensor muscles about the joint.