Intraoperative Sarcomere Length Measurements Reveal Musculoskeletal Design Principles

Summary

This chapter describes a series of experiments in which sarcomere length was measured in human upper extremity muscles in order to understand the design principles of these muscles. Such measurements have been combined with studies on cadaveric extremities to generate biomechanical models of human muscle function and to provide insights into the design of upper extremity muscles.

Intraoperative measurements of the human extensor carpi radialis brevis (ECRB) muscle during wrist joint rotation reveal that this muscle appears to be designed to operate on the descending limb of its length tension curve and generates maximum tension with the wrist fully extended. Interestingly, the synergistic extensor carpi radialis longus (ECRL) also operates on its descending limb but over a much narrower sarcomere length range. This is because of the longer fibers and smaller wrist extension moment arm of the ECRL compared to the ECRB. Sarcomere lengths measured from wrist flexors are shorter compared to the extensors. Using a combination of intraoperative measurements on the flexor carpi ulnaris (FCU) and mechanical measurements of wrist muscles, joints and tendons, the general design of the prime wrist movers emerges: both muscle groups generate maximum force with the wrist fully extended. As the wrist flexes, force decreases due to extensor lengthening along the descending limb of their length-tension curve and flexor 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 full extension.

Similar measurements have been made intraoperatively during surgical tendon transfer procedures. These procedures are used to restore lost muscle function after head injury, peripheral nerve injury, and stroke. In transfers of the FCU to either the ECRL or extensor digitorum communis (EDC) muscles, the relatively short fibers of the FCU make setting sarcomere length a critical choice that determines the ultimate functionality of these transfers.

Taken together, these experiments demonstrate the elegant match among muscle, tendon, and joints acting at the wrist. Overall, the wrist torque motors appear to be designed for balance and control rather than maximum torque generating capacity.