Feedback In Voluntary Closing Arm Prostheses

High rejection rates indicate that prosthetic users are not satisfied with the performance of their arm prostheses. In theory, one of the advantages of shoulder controlled prosthesis, compared to myoelectric prostheses, is that the user receives direct proprioceptive feedback about the opening width and pinch force of his prosthetic hand. However, the operating forces that commercially available voluntary closing prostheses require are too high, which leads to discomfort and disturbs the direct proprioceptive feedback. The purpose of this study was to find an optimal operation force, at which the prosthetic user receives optimum force feedback during comfortable prosthesis operation. During experimental research, subjects were asked to reproduce a certain reference force, with and without a visual representation of the force produced. The subject’s performances of blind generated forces regarding the reproducibility, stability and repeatability were evaluated to find an optimal cable force. The performances of male and female subjects, as well as the performances of subjects with and without arm defects were compared. As a result the optimal operation force level is found between 20 and 30 N for male and female subjects without arm defects. No differences in stability and repeatability performance are found between subjects with and without an arm defects. However, subjects with arm defects are found to have difficulties reproducing high force levels. In line with this, the reproducibility optimum is found between 10 and 20 N for subjects with arm defects. It is concluded that of today’s commercially available arm prostheses only one is capable of creating pinch forces at the optimal cable activation force level of 20-30 N. The created pinch forces of this prosthesis are not sufficient to handle objects in daily life. Future prosthesis design should not exceed cable forces of 30 N when realizing the desired pinch forces for daily activities. Therefore transmission ratios or servo mechanisms might be needed to optimize prosthesis design.