Abstract
The purpose of this research was to measure thumbnail strain when grasping an cylindrical object and to determine whether asymmetric strain distribution occurs in the radioulnar direction. In addition, we examined the effects of object size and thumb-pressing force. Biaxial strain gauges were attached at three locations on the thumbnail surface (central, radial, and ulnar parts). Each axis of the biaxial strain gauges was aligned to the axial and transverse directions, respectively. The diameter of the cylindrical object were 50mm, 60mm, 80mm, 100mm, and 120mm. The thumb-pressing force were 1N, 3N, and 5N. When the diameter of the cylindrical object was small, the distribution of strain in the axial and transverse directions was asymmetric; however, when the diameter of the cylindrical object became large, the asymmetric nature of the strain distribution in transverse direction disappeared. The thumb-pressing force affected the ulnar strain, but not the radial strain. When grasping a small object, the ulnar surface of the thumb pad is an important contact surface. This was the factor in asymmetric strain distribution occurring in the radioulnar direction. When grasping a large object, the contact surface moves to the center of the fingerpad; however, it was assumed that the asymmetric strain distribution occurred because of the asymmetric nature of the internal structure of the thumb.
