Functional workspace for precision manipulation between thumb and fingers in normal hands.

Prehensile functions of hand are based on thumb-finger relationships which are regarded as an essential element in various manipulations of our daily living activities. Although the maximal workspace provides clinicians a way to comprehend the ranges of digital movements, little is known about the "functional workspace" based on thumb-finger relationships. This study defines the functional workspace of the precision thumb-finger grasp as the range of all possible positions in which thumb-tip and each fingertip can simultaneously contact each other. We present a quantitative method for measuring the functional workspace of the human hand. The maximal motion trajectories of thumb-tip and fingertips of twenty subjects were recorded using a video-capture system. The functional workspace of the precision manipulation was calculated via numerical methods based on the maximal workspaces obtained of the thumb-tip and fingertip motions. The ratios of the functional workspace with respect to the maximal workspace of the index, middle, ring and little fingers were calculated as 33.7%, 27.1%, 23.5% and 19.1%, respectively. Although the present approach is still a descriptive work which might require more validations or evidences to justify, the results obtained may become normal standards for practical use in objective handicapped authentications, insurance claims, and rehabilitation programs as well as criteria for ergonomic design considerations in the near future.