The purpose of this paper is to develop a method of measuring 3D-motion of human upper extremity; that is, measuring an angle of the forearm pronation/supination as well as positions of the shoulder, elbow and wrist. An optical sensing system (OPTOTRAK) is utilized, by which 3D-positions of individual infrared LEDs are detected. Putting the LEDs on the shoulder, elbow and wrist, we measured these positions with OPTOTRAK. In order to detect the forearm pronation/supination, a ringshape attachment was fixed around the wrist; eight LEDs were placed every thirty degrees on the circumference. Using the measured positions of these LEDs, we calculated an angle of the forearm pronation/spination. We showed experimentally both reliability and availability of the present method.

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