Abstract
When a human grasps an object with a hand, the thumbnail plays an important role. The purpose of this research was to measure thumbnail strain when grasping an object (cylinder, sphere, and flat plate) and to determine whether asymmetric strain distribution occurs in the radioulnar direction. In addition, we examined the thumbnail surface (central, radial, and ulnar parts). Each axis of the biaxial strain gauges was aligned to the axial and transverse directions, respectively. When object size was small, the distribution of strain in the axial and transverse directions was asymmetric; however, when object size became large, the asymmetric 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.
