Journal of Electromyography and Kinesiology 8 (1998) 93–100 Musculoskeletal balance of the human wrist elucidated using intraoperative laser diffraction Richard L. Lieber a,* , Jan Fride ´ n b a Departments of Orthopaedics and Bioengineering, Biomedical Sciences Graduate Group, University of California and Veterans Administration Medical Centers, San Diego, CA 92161, U.S.A. b Department of Hand Surgery, Go ¨ teborg University, Go ¨ teborg, Sweden Abstract This review describes a series of experiments in which sarcomere length was measured in human wrist muscles to understand their design. Sarcomere length measurements were combined with studies on cadaveric extremities to generate biomechanical models of human wrist function and to provide insights into the mechanism by which wrist strength balance is achieved. 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 due to 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. These experiments demonstrate the elegant match between 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.  1998 Elsevier Science Ltd. All rights reserved. Keywords: Sarcomere length; Wrist muscles; Musculoskeletal design 1. Introduction Limb movement results from mechanical interaction between skeletal muscles, tendons and joints. The anat- omy of these structures has been studied extensively at both the gross and microscopic levels [5,6,8,11,29]. Skeletal muscles are responsible for force generation during movement and have been shown to have a wide range of designs that appear to be matched to their func- tional tasks [3,15,27,32]. An added factor in understand- ing this system is that these muscles of varying design insert onto bones with a wide range of mechanical advantages yielding “torque motors” of designs that * Corresponding author. Tel.: 001 619 552 8585, ext. 7016; fax: 001 619 552 4381; e-mail: rlieber@ucsd.edu 1050-6411/98/$19.00  1998 Elsevier Science Ltd. All rights reserved. PII:S1050-6411(97)00025-4 result from unique juxtaposition of muscle and joint properties. Since relative muscle force is highly dependent on sar- comere length within the muscle, we have focused on sarcomere length measurements to provide insights into the design and function of the neuromuscular system. Prior studies of this sort in a variety of animal systems have yielded intriguing results. For example, it was dem- onstrated that fish fast and slow skeletal muscles operate near the peak of their power–velocity relationships at near optimal sarcomere length [24–26]. During loco- motion in cats, medial gastrocnemius and soleus muscles are activated in such a manner as to exploit their meta- bolic and force generating properties [31,32]. Finally, frog skeletal muscles appear to be designed so as to max- imize either power production during hopping [20] or moment transfer in the biarticular musculature [12,21].