| Functional neuromuscular stimulation (FNS) can restore hand function to individuals with a spinal cord injury at the C5/C6 level. However, integrated control of hand grasp and wrist movement in a FNS hand grasp neuroprosthesis is compromised due to: (1) electrical stimulation of the hand extrinsic muscles (i.e. grasp activation), (2) arm orientation in the gravitational field, (3) time dependent muscle properties, and (4) load in hand. Adequate voluntary (via residual control or muscle tendon transfer of brachioradialis to extensor carpi radialis brevis) and/or stimulated wrist extension moments will be able to counterbalance these disturbances, thus making integrated hand and wrist control feasible. To evaluate the different wrist extension moments, the net wrist moment was measured in seven neuroprosthesis users during grasp activation combined with voluntary and/or stimulated wrist extension. Transferring the tendon of the paralyzed extensor carpi ulnaris to the extensor carpi radialis brevis increased isolated stimulated wrist extension, while combined voluntary and stimulated wrist extension was the most effective in reducing wrist flexion generated by grasp activation.;With the knowledge that adequate wrist extension can be incorporated in the hand grasp neuroprosthesis, a feedforward controller was designed to provide individuals with independent control of hand grasp and wrist movement. The feedforward controller was designed using artificial neural networks that modeled the relationship between muscle stimulation and the corresponding changes in grasp and wrist posture. In simulation studies, the controller was successful in generating the desired grasp and wrist parameters with minimum error. In clinical trials on one neuroprosthesis user, offset errors between the desired and actual grasp force and wrist angle were present (due to arm orientation and time varying muscle properties), however, the desired hand and wrist coordination pattern was produced. Gravitational disturbances at the hand and wrist were corrected by adding an arm orientation input to the control scheme (demonstrated in simulations), or by voluntary wrist extension (demonstrated in clinical experiments). Based on these results, integrated hand grasp and wrist control will likely be possible in the neuroprosthesis, resulting in a more natural and functional grasp for the neuroprosthesis user. |
