MUSCULOSKELETAL BALANCE OF THE HUMAN WRIST ELUCIDATED USING INTRAOPERATIVE LASER DIFFRACTION

This review describes a series of experiments in which sarcomere lengt h was measured in human wrist muscles to understand their design. Sarc omere length measurements were combined with studies on cadaveric extr emities to generate biomechanical models of human wrist function and t o provide insights into the mechanism by which wrist strength balance is achieved. Intraoperative measurements of the human extensor car-pi radialis brevis (ECRB) muscle during wrist joint rotation reveal that this muscle appears to be designed to operate on the descending limb o f its length-tension curve and generates maximum tension with the wris t fully extended. Interestingly, the synergistic extensor carpi radial is longus (ECRL)also operates on its descending limb but over a much n arrower sarcomere length range. This is due to the longer fibers and s maller wrist extension moment arm of the ECRL compared to the ECRB. Sa rcomere lengths measured from wrist flexors are shorter compared to th e extensors. Using a combination of intraoperative measurements on the flexor carpi ulnaris (FCU) and mechanical measurements of wrist muscl es, joints and tendons, the general design of the prime wrist movers e merges: both muscle groups generate maximum force with the wrist fully extended. As the wrist flexes, force decreases due to extensor length ening along the descending limb of their length-tension curve and flex or shortening along the ascending limb of their length-tension curve. The net result is a nearly constant ratio of flexor to extensor torque over the wrist range of motion and a wrist that is most stable in ful l extension. These experiments demonstrate the elegant match between m uscle, tendon and joints acting at the wrist. Overall, the wrist torqu e motors appear to be designed for balance and control rather than max imum torque generating capacity. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.