Muscles in Shoulder

muscles in shoulder

The muscles in the shoulder are the main stabilizing and flexing joints of the arm. The rotator cuff consists of three muscles: the Abductor moment arm of the anterior deltoid, the Pectoralis major, and the Teres minor. All of these muscles are connected to each other. The Abductor moment arm of the anterior deltoid is responsible for the shoulder’s adduction and abduction motion. The teres minor is the narrow muscle at the underside of the upper arm, overlapping the teres major and infraspinatus. The Subscapularis is the largest of the rotator cuff muscles. Other muscles in the shoulder area include the pectoralis minor, which connects to the third and fourth ribs, and the pectoralis major

Abductor moment arm of the anterior deltoid

The musculature of the shoulder has twelve degrees of freedom (DOF). The muscles of the posterior deltoid, middle deltoid, subscapularis, and infraspinatus generate forces that produce both movements and rotations. In most cases, they function as a force couple, generating opposing forces and neutralizing each other’s antagonistic effects.

In addition, the posterior deltoid’s peak extensor moment arm decreased as flexion angle increased, suggesting that its function in the posterior deltoid is less well-defined. The posterior deltoid, on the other hand, was a weaker agonist. Both muscles contributed to the flexion moment arm of the posterior deltoid.

Pectoralis major

Pectoralis major and minor muscles of the shoulder are closely related. They originate from the third to fifth ribs and attach to the scapula, a structure in the shoulder that stabilizes and moves the arm. Together, the two muscles form the pectoralis major and pectoralis minor, which work to lift the shoulder. The pectoralis minor and pectoralis major form a triangle.

During a seated position, the scapula cannot touch the table and the anterior chest wall may be elevated. Abducted scapula and medially rotated humerus are common in patients with this condition. Similarly, a weak pectoralis major muscle limits shoulder horizontal abduction. Therefore, when measuring pectoralis major muscles in shoulder mobility, the arm should be held in a 155 degree adduction position.

Teres minor

Although the teres minor muscle is not commonly included in routine electromyographic studies, its integrity influences postoperative shoulder function. Until now, evaluation of teres minor integrity has been largely based on physical examination. Although various radiologic imaging tests can evaluate the status of teres minor, no studies have specifically evaluated its anatomy at the humeral insertion. In addition, patients with shoulder instability and pain may be at increased risk of teres minor atrophy.

There are a few different causes of teres minor atrophy, including large and massive rotator cuff tears. However, in this study, we looked at shoulder MRIs from a middle-aged population and a population with rotator cuff tears. Despite these differences, we did not find any significant differences in the incidence of teres minor atrophy between the groups. In contrast, in our study, no shoulder with teres minor atrophy had any large or massive rotator cuff tear.

Subscapularis

When you have pain in the shoulder, you should try to determine if your subscapularis muscle is causing the problem. Using a mirror, assess your posture and try to see if it is at an angle against a wall. Ideally, your feet should be slightly forward and your palm should be in contact with the wall. If it is not, you may end up experiencing pain for several days.

The SSC tendon passes under the coracoid process, a small finger-like structure on the upper side of the scapula that points forward. It works together with the acromion to stabilize the shoulder joint. The tendon of the subscapularis passes under this structure, which can be described as a “roller-wringer” effect. The SSC muscle is a multipennate muscle, and its tendon is the most common site of injury.

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