JOURNALOFNEUROPHYSIOLOGY Vol. 7 1, No. 3, March 1994. Printed in U.S.A. In Vivo Measurement of Human Wrist Extensor Muscle Sarcomere Length Changes RICHARD L. LIEBER, GREGORY J. LOREN, AND JAN FRIDkN Department of Orthopaedics and Biomedical Sciences Graduate Group, University of California and Veterans Administration Medical Centers, San Diego, California 92161; and Departments of Anatomy and Hand Surgery, University of Umed, S-90185 Umeb, Sweden SUMMARY AND CONCLUSIONS 1. Human extensor carpi radialis brevis (ECRB) sarcomere length wasmeasured intraoperatively in five subjects usinglaser diffraction. 2. In a separate cadaveric study, ECRB tendons wereloaded to the muscle’s predicted maximum tetanic tension, and tendon strain was measured to estimate active sarcomere shortening at the expense of tendon lengthening. 3. As the wristjoint was passively flexed from full extension to full flexion, ECRB sarcomere length increased from 2.6 to 3.4 pm at a rate of 7.6 nm/degjoint angle rotation. Correcting for tendon elongationduring muscle activation yielded an active sarcomere length range of 2.44 to 3.33 ,um. Maximal predicted sarcomere shortening accompanyingmuscle activation was dependent on initial sarcomere length and wasalways co.15 pm, suggesting a minimal effectof tendon compliance. 4. Thin filament lengths measured from electron micrographs of muscle biopsies obtained from the same region of the ECRB muscles were 1.30 * .027 (SE) ,ccrn whereas thick filamentswere 1.66 t .027 pm long, suggesting an optimal sarcomere length of 2.80 pm and a maximum sarcomere lengthfor active force genera- tion of 4.26 pm. 5. Theseexperiments demonstrate that human skeletalmus- cles can function on the descending limb of their sarcomere length-tension relationship under physiological conditions. Thus, muscle force changes during joint rotation arean important com- ponent of the motor control system. INTRODUCTION The sarcomere represents the fundamental unit of force generation in skeletal muscle. Elucidation of the relation- ship between sarcomere length and isometric tension repre- sents one of the great accomplishments in muscle biophys- ics (Gordon et al. 1966). Less-well understood, however, are sarcomere length changes that occur during normal movement. Attempts to define such changes have involved sarcomere length measurements from fixed tissues (Rack and Westbury 1969; Rome et al. 1988; Rome and Sosnicki 199 1; Weijs and van der Wielen-Drent 1982), theoretical modeling based on geometric relationships (Delp et al. 1990; Herzog and ter Keurs 1988; Hoy et al. 1990), laser diffraction of isolated muscle-joint specimens (Lieber and Boakes 1988; Lieber and Brown 1993; Mai and Lieber 1990), and laser diffraction of intact muscles (Fleeter et al. 1985; Lieber et al. 1992b). Sarcomere lengths so obtained fall on all portions of the length-tension relationship: the ascending limb (Rack and Westbury 1969), the plateau region (Lieber et al. 1992b; Rome et al. 1988; Rome and Sosnicki 199 1 ), and the descending limb (Herzog et al. 199 1; Lieber and Brown 1992; Lieber and Boakes 1988; Mai and Lieber 1990). There is, therefore, no general agreement regarding the physiological operating range of sarcomeres. Understand- ing this range is important for a complete understanding of the physiological basis of neuromotor control. Although muscle force can be altered by the number and frequency of activated motor units, force can also be altered by sarco- mere length changes that are effective and predictable. Understanding these relationships in the human upper ex- tremity may provide unique insights into the design of the musculoskeletal system because hand and wrist function are highly specialized to perform manipulative tasks. Thus, the purpose of this study was to measure the sarcomere length-wrist joint angle relationship in patients undergoing surgical release of the extensor carpi radialis brevis muscle (ECRB). METHODS Patient inclusion criteria The five patients included in the study wereundergoing surgical lengthening of the ECRB tendon for treatment of chronic lateral epicondylitis (tennis elbow). Patientsrangedfrom 35 to 50 years of ageand included three men and two women (Table 1). All procedures performed were approved by the Committee on the Useof Human Subjects at the University of Umea and University of California, San Diego. Intraoperative laser device The device used was a modification of that originally described by Lieber and Baskin ( 1985) and Fleeter et al. ( 1985). A 5-mW helium-neonlaser beam ( Melles-Griot, model LHR-007, Irvine, CA) was aligned with a specially designed prism such that the beam projected normal to one prism face and wasreflected90”, exiting the other prism face (Fig. 1). The prism reflective surface wasaluminum coated( Melles-Griot, model 001 PRA/OO 1) to di- rect all availablelaserpower through the muscle. The device was calibratedusing diffraction gratings of 2.50 and 3.33 pm grating spacings placedat the location of the muscle fiber bundle (see below) directly on the prism. Diffraction order spac- ings from the t 1 st order and the t2nd order weremeasured to the nearest 0.1 mm usingdial calipers that corresponded to a spatial resolution of ~0.02 pm. In practice, repeatedmeasurement of diffraction order spacing resulted in a sarcomere length variability ofO.lO+0.21(SE)~m(n= 12measurements from two separate orders on three muscle biopsy samples by a single observer). Re- peatability of sarcomere length measurements between observers 874 0022-3077/94 $3.00 Copyright 0 1994 The American Physiological Society