Joint Contribution to Fingertip Movement During a Number Entry Task: An Application of Jacobian Matrix

in Journal of Applied Biomechanics
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Upper extremity kinematics during keyboard use is associated with musculoskeletal health among computer users; however, specific kinematics patterns are unclear. This study aimed to determine the dynamic roles of the shoulder, elbow, wrist and metacarpophalangeal (MCP) joints during a number entry task. Six subjects typed in phone numbers using their right index finger on a stand-alone numeric keypad. The contribution of each joint of the upper extremity to the fingertip movement during the task was calculated from the joint angle trajectory and the Jacobian matrix of a nine-degree-of-freedom kinematic representation of the finger, hand, forearm and upper arm. The results indicated that in the vertical direction where the greatest fingertip movement occurred, the MCP, wrist, elbow (including forearm) and shoulder joint contributed 10.2%, 55.6%, 27.7% and 6.5%, respectively, to the downward motion of the index finger averaged across subjects. The results demonstrated that the wrist and elbow contribute the most to the fingertip vertical movement, indicating that they play a major role in the keying motion and have a dynamic load beyond maintaining posture.

Jin Qin is with the Department of Work Environment, University of Massachusetts, Lowell, MA, and with the Liberty Mutual Research Institute for Safety, Hopkinton, MA. Matthieu Trudeau is with the Department of Environmental Health, Harvard University, Boston, MA. Bryan Buchholz is with the Department of Work Environment, University of Massachusetts, Lowell, MA. Jeffrey N. Katz is with the Department of Environmental Health, Harvard University, Boston, MA. Xu Xu is with the Liberty Mutual Research Institute for Safety, Hopkinton, MA. Jack T. Dennerlein is with the Department of Environmental Health, Harvard University, Boston, and with the Department of Physical Therapy, Northeastern University, Boston, MA. Address author correspondence to Jin Qin at jinqinwork@gmail.com.