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MOMENT ARMS OF THE MUSCLES CROSSING THE ANATOMICAL SHOULDER David C. Ackland and Marcus G. Pandy, [email protected] Department of Mechanical Engineering, University of Melbourne, Australia INTRODUCTION The moment arm (MA) of a muscle force represents the mechanical advantage of a muscle and largely determines its role, for example, as a stabilizer or a prime mover. Many biomechanical models rely on accurate muscle-path and MA data to represent the anatomical shoulder during movement. To our knowledge, no study has investigated the instantaneous MAs of functionally distinct muscle sub-regions within the rotator cuff, pectoralis major or latissimus dorsi through scapula plane abduction (scaption), coronalplane abduction and forward flexion. Furthermore, muscle MAs have not been reported over a continuous range of humeral elevation exceeding 100o, as most studies ignore scapulothoracic motion. METHODS AND PROCEDURES MAs of the musculature spanning the glenohumeral joint were measured in 8 freshfrozen, entire upper extremities using the tendon excursion method (An et al., 1984). Specimens were mounted on a custom designed dynamic shoulder cadaver testing apparatus (DSCTA) designed to produce and quantify 6 degree-of-freedom glenohumeral joint motion by means of simulated muscle force and scapula rotation. Tendons of the following muscles and muscle sub-regions were exposed by resection: deltoid (anterior, middle, posterior), subscapularis (inferior, middle, superior), supraspinatus (anterior, posterior), infraspinatus (superior, inferior), latissimus dorsi (superior, middle, inferior), and pectoralis major (superior, middle, inferior). Nylon-lines were sutured to all tendons and passed through a pulley system to hanging weights of 10 N; the pulleys were positioned to reproduce each muscle’s line-ofpull, as determined by visual inspection and using a computational model (Garner and Pandy, 2001). Retro-reflective markers were placed on each hanging weight, and marker triads placed on the humerus and scapula. The humerus was passively elevated in the scapula plane, coronal plane and sagittal plane to 120o. During elevation, the scapula was rotated on the DSCTA to simulate scapulohumeral rhythm as reported by Inman et al., (1944). Tendon excursion (vertical trajectory of hanging weight) and joint angle were measured from the retro-reflective marker trajectories using a 6-camera Vicon motion capture system, and instantaneous muscle MAs were then computed from the gradient of the plot of tendon excursion vs. joint angle. Scapula and humeral coordinate systems were defined by digitizing bony prominences, as described in Garner and Pandy (2001). RESULTS Significant differences in MAs were reported across sub-regions of all muscles (p<0.01) (Table 1). The most effective elevators in abduction were the anterior and middle deltoid, while the most effective depressors were the posterior deltoid and inferior and middle latissimus dorsi (Figure 1A). In flexion, the superior pectoralis major was the most effective elevator (Figure 1B), while teres major and superior latissimus dorsi had the largest depressor MAs. Division of multipennate shoulder muscles of broad-origins into sub-regions highlighted distinct functional differences across those subregions. Most significantly, we found that the clavicular fibres of the pectoralis major were able to exert substantial elevator torque in flexion, whereas the sternal and lower costal fibres behaved as stabilizers and flexion antagonists. Muscle/muscle subregion Scaption CP abduction Flexion Ackland et al (2008) Ackland et al (2008) Ackland et al (2008) Superior Subscapularis AG AN AG AN AG AN 9.8 2.2 7.2 -9.5 35.3 -5.4 Middle Subscapularis 1.8 -2.4 1.3 -12.7 24.2 -0.6 Inferior Subscapularis -1.5 -9.5 -2.2 -16.6 10.4 -3.4 Anterior Supraspinatus 32.4 9.2 23.2 5.6 41.8 0.6 Posterior Supraspinatus 31.9 13.8 26.8 10.4 13.4 2.7 Superior Infraspinatus 22.2 7.1 13.4 5.6 7.1 1.7 Inferior Infraspinatus 12.2 1.9 10.9 3.8 4.2 -6.8 Teres minor 2 -0.8 5.1 -3.3 2.2 -18.7 Teres major -18.6 -47.3 -12.1 -46.1 -19.7 -54.4 Anterior Deltoid 39.3 2.1 30.2 2 40 11.6 Middle Deltoid 33.1 6.7 29.1 8.3 12.2 0 3 -14.9 2 -15.9 -16.3 -33 -32.9 Posterior Deltoid Superior Pectoralis major 30.2 3.1 11.2 -1.8 53.7 Middle Pectoralis major -2.9 -12.7 -17.7 -32.9 15.9 4.4 Inferior Pectoralis major -12.4 -22.2 -16.2 -33.6 1.9 -9.3 Superior Latissimus dorsi -7.8 -31.5 -2.1 -29.9 -0.1 -22.1 Middle Latissimus dorsi -6.4 -21 -10.1 -38.6 -0.6 -15.3 Inferior Latissimus dorsi -9.9 -28.9 2.6 -38.1 -2.9 -10.8 Table 1. Peak muscle MAs. Positive values signify agonistic muscle action; negative values signify antagonistic muscle action. (AG), maximum elevation agonistic MA; (AN), maximum elevation antagonist MA. A 00 20 40 60 80 100 120 Moment arm (mm) -5 -10 -15 -20 -25 -30 -35 -40 -45 Joint angle (deg) Superior Moment arm (mm) B DISCUSSION Middle Inferior 60 50 40 The MA data presented in this study may facilitate the design and validation of biomechanical models of the shoulder complex. The data presented are valid for an intact shoulder free of pathology and joint dysfunction; any condition that is likely to effect joint congruency may change muscle moment arm quantities due to eccentric joint centre of rotation. Demonstrating the torqueproducing potential of the glenohumeral joint musculature helps to establish a better understanding of the normal function of the shoulder joint and its surrounding structures, and therefore may provide knowledge for the design and implantation of joint prostheses and ligament replacements in diseased or injured shoulders. SUMMARY Quantifying the MAs of sub-regions of multipennate shoulder muscles provided biomechanical evidence for the torqueproducing potential of such muscles across the glenohumeral joint, as well as the moment inducing potential of the muscle fibers across their extensive origins. Such evidence cannot readily be obtained by approximating broadorigin muscles as single lines-of-force. Knowledge of MA differences between muscle sub-regions may assist in identifying the functional effects of muscle sub-region tears, assist surgeons in planning tendon reconstructive surgery, and aid in the development and validation of computational models. 30 REFERENCES 20 10 0 -10 0 20 40 60 80 100 120 An et al. (1984) J Biomech Eng 106, 280-2 -20 -30 -40 Joint angle (deg) Superior Middle Inferior Figure 1. MAs arms of (A) latissimus dorsi, and (B) pectoralis major. Black lines show scaption data and grey lines flexion data. Garner and Pandy (2001) Comp Meth Biomech Biomed Engin 4, 93-126 Inman et al (1944) JBJS 58, 1-30