Object stability during human precision fingertip manipulation

Fingertip-based manipulation is one of the hallmarks of human hand function. However, there are clear limitations to the stability of these grasps, whether it is due to digit lengths or the friction between the object and the finger pad. The present work focuses on the stability of point-contact grasps during rotational movements of objects 50 mm and 80 mm in diameter over the range of the workspace. The mean angle achieved during all the trials for each digit ranged from 2° to 76°, with the thumb generally having larger mean angles. During the trials the object would often lose contact with individual digits, and we suspect that it is due to the angle that the object made with each of the digits. Object size had little significance predicting this angle, while the number of digits used, which digit, and axis of rotation were significant. Angles 0.125 s and 0.025 s prior to loss of contact between digit and object were regarded as unstable. Unstable angles in the negative elevation direction (angles that are often achieved when the digit is extended) occurred in 64% of the trials and in the positive elevation direction in 23% of the trials. In 57% of the trials unstable angles lied in the positive azimuthal direction and in 29% of the trials lied in the negative azimuthal direction. The four fingers, disregarding the thumb, were found to have very similar instability regions when manipulating the object about the X-axis (ulnar-radial) and the Y-axis (distal-proximal), where most of the trials, 72%, have had the majority of their unstable angles occurring in the negative elevation direction and 62% occurring in the positive azimuthal direction.