To allow continuous and simultaneous measurements of tendon excursion and joint rotation angles, we developed a system using an electric potentiometer and an electromagnetic tracking device. Instantaneous moment arms of each tendon could then be calculated on the basis of the slope of the curve between tendon excursion and joint angular displacement. Five prime wrist motor tendons were studied on freshly frozen cadavers. Calculated tendon moment arms were found to be consistent throughout a full range of flexion-extension wrist motion and radioulnar deviation and corresponded closely to the anatomic location and orientation of the tendons. The flexor carpi ulnaris tendon provided the largest moment contribution to the wrist joint. Forearm rotation did not affect the function of the wrist tendon except for the extensor carpi ulnaris. During flexion-extension motion, the excursion of the extensor carpi ulnaris decreased significantly from 10 to 4 mm, with the forearm in neutral and pronated positions. During radioulnar deviation, the excursion of the extensor carpi ulnaris increased from 14 to 17 mm with the forearm in supinated and pronated positions.
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