The muscles of the shoulder can be divided into two groups – extrinsic and intrinsic. Extrinsic muscles originate from the torso and attach to the bones of the shoulder (clavicle, scapula or humerus), whereas intrinsic muscles originate from the scapula and/or clavicle and attach to the humerus. In this article, we shall be looking at the anatomy, innervation and actions of the intrinsic muscles of the shoulder.
The intrinsic muscles of the shoulder, or the scapulohumeral group, consist of six muscles – the deltoid, teres major, and the four rotator cuff muscles (supraspinatus, infraspinatus, subscapularis and teres minor).
The deltoid muscle is an inverted triangle shaped muscle located on the outer part of the shoulder. It has an anterior, middle and posterior part, with each part having different attachments, actions and innervation.
The rotator cuff muscles are a group of four muscles – supraspinatus, infraspinatus, subscapularis and teres minor – that originate from the scapula and attach to the humeral head. The resting tone of these muscles collectively act to ‘pull’ the humeral head into the glenoid fossa, giving the glenohumeral joint additional stability. In addition, the individual rotator cuff muscles have their own individual actions.
Rotator cuff tendonitis refers to inflammation of the tendons of the rotator cuff muscles. This usually occurs secondary to repetitive use of the shoulder joint. The muscle most commonly affected by rotator cuff tendonitis is supraspinatus, which ‘rubs’ against the coraco-acromial arch during abduction. Over time, this causes inflammation and degenerative changes in the tendon itself.
Treatment of rotator cuff tendonitis typically involves conservative measures such as rest, analgesia, and physiotherapy. In more severe cases, steroid injections and surgical procedures can be considered.
The shoulder is a complex joint in the human body, with multiple muscles that can be grouped into three main categories: the deltoid, rotator cuff muscles (supraspinatus, infraspinatus, subscapularis, and teres minor), and teres major. Each of these muscles has their own unique anatomy, attachments, innervation, and action, contributing to both the movement and stability of the shoulder joint. This article will discuss the anatomy and action of each of the shoulder muscles.
The deltoid is a large triangular muscle located at the shoulder joint. Anatomically, the deltoid can be divided into three parts: anterior, lateral, and posterior. The anterior fibres are involved in flexion and medial rotation of the arm, the posterior fibres are responsible for extension and lateral rotation, and the middle fibres act as the primary abductor of the arm (taking over from the supraspinatus which abducts the first 15°). Innervation is provided by the axillary nerve.
The teres major forms the inferior border of the quadrangular space, the ‘gap’ which the axillary nerve and posterior circumflex humeral artery pass through to reach the posterior scapular region. This muscle originates from the posterior surface of the inferior angle of the scapula, and attaches to the medial lip of the intertubercular groove of the humerus. Its action is adduction and extension at the shoulder, and medial rotation of the arm, and is innervated by the lower subscapular nerve.
The rotator cuff muscles are a group of four muscles that originate from the scapula and attach to the humeral head. Together, they act to ‘pull’ the humeral head into the glenoid fossa and provide additional stability to the glenohumeral joint, but each muscle also has its own individual action:
Rotator cuff tendonitis, or inflammation of the tendons of the rotator cuff muscles, usually occurs secondary to repetitive use of the shoulder joint. The most commonly affected muscle is the supraspinatus, which is subjected to rubbing against the coraco-acromial arch during shoulder abduction. This rubbing action can cause degenerative changes to the tendon over time, leading to inflammation. Treatment of rotator cuff tendonitis usually involves conservative measures such as rest, analgesia, and physiotherapy. In more severe cases, steroid injections and surgery may be considered.
The muscles of the shoulder are vital for the movement and stability of the joint, making their anatomy and action important to understand. Knowing the action of each of these muscles, as well as the clinical significance of the rotator cuff muscles, is necessary for the proper diagnosis and treatment of shoulder-related injuries.
