Inter-digit co-ordination and object-digit interaction when holding an object with five digits

The current study investigated inter-digit co-ordination and object-digit interaction during sustained object holding tasks by using five, six-component force/torque sensors. The sum of the individual finger normal forces and the thumb normal force showed a parallel variation with a mean median correlation coefficient of 0.941. The normal force traces demonstrated the lowest coefficient of variation (about 9% as averaged across digits) as compared with other force/torque traces. The sum for the variances of the normal forces of the index, middle, ring, and little fingers was about 50% of the variance of the summed normal force of the four fingers. Of the five digits, the thumb, index, middle, ring and little fingers accounted for 50.0, 15.4, 14.6, 11.7 and 7.3% of the total normal force; and 39.4, 9.9, 19.3, 14.0 and 17.5% of the total vertical shear force (i.e. the load), respectively. The ratios of the normal force to the resultant shear force were 2.6, 4.5, 1.8, 2.2 and 1.3 for the thumb, index, middle, ring and little finger, respectively. The centre of pressure migration area of a single digit at the object-digit surface during object holding ranged from 0.30 to 1.21 mm². The current study reveals a number of detailed object-digit mechanics and multiple digits co-ordination principle. The results of this study may help to improve ergonomic designs that involve the usage of multiple digits.