The determination of moments for extension of the wrist generated by muscles of the forearm.

In deciding on suitable tendon transfers to replace denervated muscle-tendon units, important considerations are the strength and effectiveness of possible substitutes. A method is presented by which the strength of the wrist extensor muscles and their moment arms can be determined. The method can be applied to other muscles at other joints. It involves the use of a force transducer which measures the combined forces of the three wrist extensors in an isometric contraction. This moment for wrist extension, measured in the living intact arm, is the same as the sum of the moments of the three wrist extensor muscles. The contribution of each muscle to the total moment is calculated from ratios that have been developed from a quantitative study of moment arms and muscle masses in sixteen cadaver limbs. It is suggested that the ratio of one moment arm to another is fairly constant from subject to subject, and that muscle masses also have sufficiently similar ratios to each other to serve as the basis for practical estimations by the surgeon. Thus the surgeon needs only one or two direct measurements of moments externally and only one or two skeletal measurements on any living subject to be able to estimate the effectiveness of a number of muscles on the basis of cadaver studies such as this, and to project the behavior of a muscle after it has been transferred to a position where it will have new moment arms.