Thumbnail strain during pressing and plucking of a string

Abstract

Human fingernails play an important role in daily life and while playing instruments. We previously showed that the strain generated at the radial and ulnar parts of the nail was asymmetric when the thumb pad was pressed down on a flat surface. The purpose of the present study was to investigate whether nail strain is asymmetrical while pressing or plucking a string with the thumb. Biaxial strain gauges were attached to the surface of the thumbnail of 15 subjects; the gauges were attached to the proximal-radial, distal-central, and proximal-ulnar parts of the thumbnail. We measured the maximum strain generated while pressing or plucking a string with the thumb pad. The strain values obtained were then compared among the measurement locations. We also measured the effects of contact angles and contact points on the thumbnail strain. Upon investigating the effect of the measurement locations, we found that during string-pressing motion, the maximum strain was significantly lower in the proximal-radial-axial direction than in the distal-central-axial and proximal-ulnar-axial directions. This indicated that an asymmetric strain generated at the proximal-radial and ulnar sides. We also investigated the effect of the contact angle and found that the maximum strain varied with changes in contact angle. The investigation of the effect of the contact point revealed that the maximum strain during string-pressing motion varied with changes in the contact point, although no change was observed during string plucking. Asymmetrical thumbnail strain may have been caused by the asymmetrical nature of the flexor tendon insertion and the distal phalangeal tuberosity of the thumb. Uniform pressure applied to the thumb pad became asymmetric in the radioulnar direction because of the interaction between the thumb pulp, distal phalanx, and nail. This asymmetry may then have been transmitted to the nail.