A dynamic model of hand was generated and used to study potential hand functions on hand positioning and grasp force and the outcome of surgeries such as tendon transfers and joint fusions, in situations typical of those encountered in FNS. In the model, the hand is being treated as a multi-body system including rigid segments (phalanges and palm) connected by joints. Each joint is subjected to muscle moments, produced by a muscle model, passive joint moment and external force moments. The external forces are the result of interaction forces between the hand segments and a grasped fixed or movable object. In particular, the model was used to study the non-prehensile tip and palmar pinch movements, and to simulate prehensile lateral and palmar grasp functions of the hand.

A quasi-linear viscoelastic model was found to described the joint passive moment. Model simulations showed that, in the absence of external forces, tip pinch posture is not obtained using only the extrinsic muscles. The currently used tendon transfer methods were found useful in normal hands but not significantly effective in quadriplegic hands. It was also found that activation of multiple thumb muscles (ADP, APB and FPL) without surgical alterations can provide functional lateral pinch with about 70% more grip force than the currently used method with joint fusion. A grasp protocol was introduced and shown successful in palmar grasp and hold of movable cylindrical objects using only extrinsic muscles.