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Research Papers

Contributions of the Individual Muscles of the Shoulder to Glenohumeral Joint Stability During Abduction

[+] Author and Article Information
Takashi Yanagawa, J. Erik Giphart, Michael R. Torry

 Steadman-Hawkins Research Foundation, Vail, CO 81657

Cheryl J. Goodwin

Department of Biomedical Engineering, The University of Texas at Austin, Austin, TX 78712

Kevin B. Shelburne1

 Steadman-Hawkins Research Foundation, Vail, CO 81657kevin.shelburne@shsmf.org

Marcus G. Pandy

Department of Mechanical Engineering, The University of Melbourne, Victoria 3010 Australia; Department of Biomedical Engineering, The University of Texas at Austin, Austin, TX 78712

1

Corresponding author.

J Biomech Eng 130(2), 021024 (Apr 14, 2008) (9 pages) doi:10.1115/1.2903422 History: Received December 05, 2006; Revised February 04, 2008; Published April 14, 2008

The aim of this study was to determine the relative contributions of the deltoid and rotator cuff muscles to glenohumeral joint stability during arm abduction. A three-dimensional model of the upper limb was used to calculate the muscle and joint-contact forces at the shoulder for abduction in the scapular plane. The joints of the shoulder girdle—sternoclavicular joint, acromioclavicular joint, and glenohumeral joint—were each represented as an ideal three degree-of-freedom ball-and-socket joint. The articulation between the scapula and thorax was modeled using two kinematic constraints. Eighteen muscle bundles were used to represent the lines of action of 11 muscle groups spanning the glenohumeral joint. The three-dimensional positions of the clavicle, scapula, and humerus during abduction were measured using intracortical bone pins implanted into one subject. The measured bone positions were inputted into the model, and an optimization problem was solved to calculate the forces developed by the shoulder muscles for abduction in the scapular plane. The model calculations showed that the rotator cuff muscles (specifically, supraspinatus, subscapularis, and infraspinatus) by virtue of their lines of action are perfectly positioned to apply compressive load across the glenohumeral joint, and that these muscles contribute most significantly to shoulder joint stability during abduction. The middle deltoid provides most of the compressive force acting between the humeral head and the glenoid, but this muscle also creates most of the shear, and so its contribution to joint stability is less than that of any of the rotator cuff muscles.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Diagram illustrating how any muscle force vector can be resolved into a compressive component and a shear component. (a) Two muscle force vectors, F1 and F2, applied to a body segment that moves via a one degree-of-freedom hinge joint relative to the ground. Here, F1 is considered to be an agonist and F2 the antagonist. (b) Equivalent system to that shown in part (a), where the muscle force vectors have been replaced by their respective compressive components, Fcomp1 and Fcomp2, and shear components, Fshear1 and Fshear2, plus the corresponding moments, M1 and M2, acting at the joint. Fcomp1 and Fcomp2 act normal to the articular surface of the joint in the region of joint contact, while Fshear1 and Fshear2 act in a direction perpendicular to the compressive forces, parallel to the face of the joint. Notice that the moments and shear forces from each muscle act in opposite directions.

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Figure 2

Musculoskeletal model of the shoulder used in this study. Eighteen bundles were used to represent 11 muscles crossing the shoulder in the model: anterior, middle, and posterior portions of deltoid; clavicular, sternal, and ribs portions of pectoralis major; supraspinatus; infraspinatus; subscapularis; teres major; teres minor; long head and short head of biceps brachii; long head of triceps brachii; thoracic, lumbar, iliac portions of latissimus dorsi; and coracobrachialis.

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Figure 3

Comparison of bone positions measured for one subject (gray lines) with optimized bone positions calculated in the model (black lines) for abduction in the scapular plane. (a) Angular displacements of the clavicle. (b) Angular displacements of the scapula. (c) Angular displacements of the humerus. Flexion, abduction, and external rotation are positive for the humerus. Bone positions assumed for the model humerus were identical with the measured positions obtained from the bone-pin experiments.

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Figure 4

Comparison of muscle activations predicted by the model (black lines) and measured EMG (gray lines) for seven muscles crossing the shoulder. Pectoralis major represents the clavicular portion of the muscle. The EMG data were normalized by the level corresponding to a maximum voluntary contraction (MVC). The thick gray lines are the means of the EMG data averaged over six subjects; the thin gray lines represent ±1 standard deviation.

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Figure 5

Individual muscle forces calculated in the model for abduction in the scapular plane

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Figure 6

Individual muscle torques calculated in the model for abduction in the scapular plane. The thick black line is the net torque applied by all the muscles crossing the shoulder, which is equal and opposite to the torque applied by the weight of the arm. All torques are expressed in the scapular reference frame.

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Figure 7

Components of the resultant glenohumeral joint-contact force calculated in the model for abduction in the scapular plane (top panel). The thick black line is the resultant glenohumeral joint-contact force. All forces are expressed in the glenoid reference frame. The diagrams in the bottom panel illustrate the magnitude and direction of the resultant joint contact force acting at the glenohumeral joint (line with arrow) for four positions (15deg, 45deg, 90deg, and 135deg) of humeral abduction. Also shown are the coordinate axes of the glenoid reference frame assumed in the model.

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Figure 8

Muscle contributions to the glenohumeral joint-contact force per unit of muscle force. (a) Compressive force, (b) anterior-posterior shear force, and (c) superior-inferior shear force. All forces are expressed in the glenoid reference frame.

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Figure 9

Muscle contributions to the glenohumeral joint-contact force calculated in the model. (a) Compressive force, (b) anterior-posterior shear force, and (c) superior-inferior shear force. Muscles not appearing in the graphs made negligible contributions to the joint-contact force. The thick black lines represent the total forces applied to the glenoid in each of the coordinate directions; these quantities are the same as the joint reaction force components shown in Fig. 7. All forces are expressed in the glenoid reference frame (see bottom panel of Fig. 7).

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Figure 10

Relative contributions of muscles to glenohumeral joint stability during arm abduction. (a) Stability ratio of each muscle in the anterior-posterior direction, and (b) stability ratio of each muscle in the superior-inferior direction. These results were obtained from those given in Fig. 9. Specifically, the stability ratio of a muscle in the anterior-posterior (superior-inferior) direction was found by dividing the muscle’s anterior-posterior (superior-inferior) shear force by its compressive force.

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Figure 11

Average stability ratios for some muscles in the anterior-posterior and superior-inferior directions. These results were obtained by averaging the values of stability ratio for each muscle over the range of abduction simulated by the model. The error bars represent one standard deviation. PMajC, clavicular portion of pectoralis major; AntDelt, anterior deltoid; MidDelt, middle deltoid; PostDelt, posterior deltoid; Supr, supraspinatus; Infr, infraspinatus; Subs, subscapularis; BicL, biceps long head. The average and standard deviation reported for AntDelt were based on data obtained for abduction angles greater than 30deg, as an anomalous result was obtained for this muscle at 15deg of abduction, which was likely due to the incomplete convergence of the optimization solution.

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