Abstract
The motion analysis of two-finger tip pinching using the thumb and index finger provides crucial data for designing the motion mechanism of electric prosthetic hands. The purpose of this study is to determine the joints that have high mobility during two-finger tip pinching by measuring the flexion angle of each joint. Ten Japanese men with normal hand were selected. CT images were obtained while the hands adopted the following four postures: a basic posture not pinching a cylinder, and three postures pinching wooden cylinders with different diameters (2, 10, and 30 mm). Three-dimensional bone models of the thumb and index finger were created using the CT images and used to measure the flexion angles of the joints. The flexion angles of the proximal interphalangeal and metacarpophalangeal joints of the index finger significantly decreased as the diameter of the cylinder increased. However, even when the diameter of the cylinder changed, the flexion angle of the distal interphalangeal joint of the index finger, and the flexion and rotation angles of all of the thumb joints did not change. When pinching objects of different sizes with a two-finger tip pinch, the posture of the thumb is fixed, and only the posture of the index finger changes. When designing the two-finger tip pinch motion for an electric prosthetic hand, it is sufficient to drive the joints of the index finger only.
Highlights
When designing an electric prosthetic hand that accurately simulates human motion with a high degree of freedom, it is necessary to compromise on certain other factors including weight, mechanism simplicity, robustness, and cost [1]
The results showed that the proximal interphalangeal (PIP) and MP joints of the index finger were moved and adjusted to postures that made it easy to pinch when cylinders of different diameter were grasped
The results showed that the flexion angles in all of the index finger joints changed significantly depending on the width of the object; those of the thumb did not show significant change
Summary
When designing an electric prosthetic hand that accurately simulates human motion with a high degree of freedom, it is necessary to compromise on certain other factors including weight, mechanism simplicity, robustness, and cost [1]. Achieving the most appropriate product overall is a trade-off, and it follows that the factors mentioned can be improved by reducing the degree of freedom while ensuring accurate replication of human movement. One such approach is to use a coupling mechanism that allows two joints to be moved with one motor by finding a special relationship between the flexion angles of the two related joints and connecting these joints with a linkage mechanism. Mechanism simplicity, and robustness can be achieved by reducing the number of joints in the electric prosthesis that need to be driven
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