The shoulder is a ball-and-socket joint, which gives it a full range of motion and high flexibility. The tendons in the shoulder are used both for movement and stability, and they assist the labrum--a cap of connective tissue--in keeping the head of the humerus, the arm bone, in the socket of the shoulder joint.
Deltoid
The deltoid muscle is the strongest muscle in the shoulder. As Shoulder Solutions explains, it is the muscle responsible for lifting the arm up and moving it away from the body (also known as abduction). This shoulder muscle has two tendons: one that attaches to the back of the shoulder blade, on what is called the spine of the scapula, and one that attaches midway down the humerus.
Biceps
Another tendon located in the shoulder is the tendon of the long head of the bicep muscle. Inner Body notes that this tendon attaches to the scapula (the shoulder blade) at two different points. The biceps muscle is responsible for flexing the arm at the elbow and also for supination, turning the arm so the palm faces upward.
Supraspinatus
The supraspinatus is a part of the the muscle cluster known as the rotator cuff. The rotator cuff tendons play a crucial role in the stability of the shoulder. They run from the shoulder blade to the humerus and form a protective cuff of connective tissue around the shoulder joint. The supraspinatus muscle, according to Sports Injury Info, is part of the superior rotator cuff. This muscle is responsible for shoulder elevation and is one of the more commonly injured muscles as a result of a tear in the rotator cuff.
Infraspinatus
The infraspinatus muscle is another member of the rotator cuff. Its tendon is part of the posterior rotator cuff, as the tendon runs along the back of the shoulder and attaches to the back of the humerus. The infraspinatus muscle is used to externally rotate the shoulder so the thumb is pointing away from the body.
Teres Minor
The teres minor tendon also is part of the posterior rotator cuff. Like the infrapsinatus, this muscle assists with external rotation of the shoulder. Its tendon attaches slightly lower on the humerus than the infraspinatus tendon.
Subscapularis
The subscapularis tendon is part of the anterior rotator cuff, because this muscle runs along the front of the shoulder and is attached via its tendon to the front of the humerus. Because this muscle is positioned on the opposite side of the shoulder compared to the posterior rotator cuff, it also has the opposite action: It internally rotates the shoulder.
