Lines of action and stabilizing potential of the shoulder musculature

Abstract

The objective of the present study was to measure the lines of action of 18 major muscles and muscle sub-regions crossing the glenohumeral joint of the human shoulder, and to compute the potential contribution of these muscles to joint shear and compression during scapular-plane abduction and sagittal-plane flexion. The stabilizing potential of a muscle was found by assessing its contribution to superior/inferior and anterior/posterior joint shear in the scapular and transverse planes, respectively. A muscle with stabilizing potential was oriented to apply more compression than shear at the glenohumeral joint, whereas a muscle with destabilizing potential was oriented to apply more shear. Significant differences in lines of action and stabilizing capacities were measured across sub-regions of the deltoid and rotator cuff in both planes of elevation (P < 0.05), and substantial differences were observed in the pectoralis major and latissimus dorsi. The results showed that, during abduction and flexion, the rotator cuff muscle sub-regions were more favourably aligned to stabilize the glenohumeral joint in the transverse plane than in the scapular plane and that, overall, the anterior supraspinatus was most favourably oriented to apply glenohumeral joint compression. The superior pectoralis major and inferior latissimus dorsi were the chief potential scapular-plane destabilizers, demonstrating the most significant capacity to impart superior and inferior shear to the glenohumeral joint, respectively. The middle and anterior deltoid were also significant potential contributors to superior shear, opposing the combined destabilizing inferior shear potential of the latissimus dorsi and inferior subscapularis. As potential stabilizers, the posterior deltoid and subscapularis had posteriorly-directed muscle lines of action, whereas the teres minor and infraspinatus had anteriorly-directed lines of action. Knowledge of the lines of action and stabilizing potential of individual sub-regions of the shoulder musculature may assist clinicians in identifying muscle-related joint instabilities, assist surgeons in planning tendon reconstructive surgery, aid in the development of rehabilitation procedures designed to improve joint stability, and facilitate development and validation of biomechanical computer models of the shoulder complex.

Fig. 1

Specimen mounted on the dynamic shoulder cadaver-testing apparatus (DSCTA). Triads of retro-reflective markers were rigidly attached to the humerus and scapula to enable three-dimensional motion capture measurement of shoulder joint kinematics during abduction and flexion. RF, rotary frame of the DSCTA; P, pulleys; M, 10 N free weight; PB, scapular potting block; SMT, scapular marker triad; HMT, humeral marker triad. The scapula potting block and pulleys were rigidly fixed to the rotary frame.

Fig. 2

(A) Bone-embedded scapular reference frame used in this study. The negative x axis corresponds to the medial (compressive) direction, the y axis to the anterior direction and the z axis to the superior direction (see Appendix). The projected angles of the muscle force vectors were calculated with respect to the scapular reference frame defined in (A) in both the scapular plane (B) and the transverse plane (C). Directions of all muscle force vectors were measured anti-clockwise from the mediolateral (x) axis.

Fig. 3

Muscle lines of action in the scapular and transverse planes as a function of humeral elevation angle during abduction. In the scapular plane, a muscle with a line of action greater than 180° had the potential to apply a superior shear force to the glenoid; conversely, a line of action less than 180° indicates that the muscle had the potential to apply an inferior shear force. In the transverse plane, a muscle with a line of action greater than 180° had the potential to apply a posterior shear force to the glenoid; a line of action less than 180° indicates that the muscle had the potential to apply an anterior shear force. The two diagrams between the graphs illustrate how the line of action of a muscle, described by the angle θ, is defined in the scapular and transverse planes. θ is measured anti-clockwise from the mediolateral (x) axis.

Fig. 4

Muscle lines of action in the scapular and transverse planes as a function of humeral elevation angle during flexion. In the scapular plane, a muscle with a line of action greater than 180° had the potential to apply a superior shear force to the glenoid. In the transverse plane, a muscle with a line of action greater than 180° had the potential to apply a posterior shear force to the glenoid. The two diagrams between the graphs illustrate how the line of action of a muscle, described by the angle θ, is defined in the scapular and transverse planes. θ is measured anti-clockwise from the mediolateral (x) axis.

Fig. 5

Averaged superior and anterior stability ratios of muscles crossing the glenohumeral joint during abduction and flexion. For superior stability ratios, positive bars indicate a muscle with an average superior stability ratio (a superior shear component). For anterior stability ratios, positive bars indicate a muscle with an average anterior stability ratio (an anterior shear component). Muscles with superior and anterior stability ratios of < 1 were defined as potential joint stabilizers (stabilizing region).