Feedback in voluntary closing arm prostheses

Abstract

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 myo-electric prostheses, is that the user gets direct proprioceptive feedback about opening width and pinch force of his prosthetic hand. However, commercially available voluntary closing prostheses require too high operating forces, which leads to discomfort and disturbs the direct proprioceptive feedback. The purpose of this study is to find an optimal operation force, at which the prosthetic user gets the best force feedback during comfortable prosthesis operation. During experimental research, subjects where 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 defect were compared. As a result the optimal operation force level was found between 20 and 30 N for male and female subjects without arm defect. No differences in stability and repeatability performance were found between subjects with and without an arm defect. However, subjects with arm defect were found to have difficulties to reproduce high force levels. In line with this, the reproducibility optimum was found between 10 and 20 N for subjects with arm defect. 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 with 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.