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The humerus is a long bone in the upper arm that serves as an attachment point for 13 muscles, including the supraspinatus, infraspinatus, and teres minor. These muscles contribute to the coordinated actions and movement of the upper limb, such as the hand and elbow, and play a crucial role in transmitting forces generated by the muscles. The humerus also provides structural support to the arm and is integral to the overall movement and stability of the upper limb. Physiotherapists can design exercises to enhance the range of motion in the upper limb and address functional impairments by understanding the humerus' articulations and muscle attachments.
| Characteristics | Values |
|---|---|
| Muscle Attachments | Supraspinatus, Infraspinatus, Teres Minor, Coracobrachialis, Deltoid, Brachialis, Brachioradialis, Triceps Brachii, Teres Major, Pronator Teres, Latissimus Dorsi, Pectoralis Major, Rotator Cuff Muscles, Flexor and Extensor Tendons |
| Function | The Humerus serves as an attachment point for muscles and plays a role in transmitting forces generated by the muscles, facilitating movement and stability of the upper limb. |
| Anatomy | The Humerus is a long bone with a shaft and two extremities, consisting of three surfaces: Anterolateral, Anteromedial, and Posterior. |
| Range of Motion | The Humerus articulates at the Glenohumeral Joint, allowing a wide range of movements. It also facilitates Forearm Movements at the Elbow Joint, enabling flexion and extension. |
| Rehabilitation | Physiotherapists can design exercises to enhance the range of motion and address functional impairments post-humerus surgery or trauma. |
| Complications | Fractures of the Humerus can damage nearby nerves and blood vessels, leading to potential paralysis and sensory loss. |
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Pectoralis Major
The pectoralis major is a fan-shaped, paired superficial muscle found in the anterior chest wall, underneath the breast tissue. It is the superior most and largest muscle of the anterior chest wall. The pectoralis major has three parts: the clavicular part, the sternocostal part, and the abdominal part. All three parts converge laterally and insert onto the greater tubercle of the humerus. The muscle is innervated by the lateral and medial pectoral nerves, which stem from the brachial plexus.
The pectoralis major acts as an adductor and internal rotator of the humerus at the shoulder joint. The muscle's clavicular part helps to flex the extended arm up to 90 degrees, while the sternocostal part facilitates the extension of the flexed arm by pulling it downwards. The muscle also assists in forced inspiration. When the insertion is fixed, it may assist in elevating the thorax.
The pectoralis major muscle can be tested by adducting the arm at the glenohumeral joint against resistance. The muscle can also be palpated.
A pectoralis major tendon rupture is a rare shoulder injury, most commonly seen in weight lifters. This injury is becoming more common due to an increased emphasis on healthy lifestyles.
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Teres Major
The teres major is a small muscle of the upper limb that runs along the lateral border of the scapula. It is one of the seven scapulohumeral muscles that act around the glenohumeral joint to facilitate shoulder movement. The teres major muscle originates on the dorsal surface of the inferior angle and the lower part of the lateral border of the scapula. At its origin, the muscle is easily palpable and feels like a soft tissue structure on the inferior scapular angle. The fibres of the teres major muscle converge into a single tendon that inserts onto the intertubercular groove (medial lip) of the humerus. The tendon, at its insertion, lies behind that of the latissimus dorsi, from which it is separated by a bursa. The fibres of these two muscles run parallel to each other, and both muscles insert at the crest of the lesser tubercle of the humerus.
The teres major is supplied by the lower subscapular nerve (C5-C7), a branch of the brachial plexus. It is also supplied by the thoracodorsal nerve (middle subscapular nerve). The teres major is vascularised by the thoracodorsal branch of the subscapular artery and the posterior circumflex humeral artery. The teres major is relatively prone to the development of trigger points, which are local, permanent hypertensive areas with rigidification (myofascial pain syndrome). Common causes include poor stretching before physical activities, trauma (e.g. falling on the shoulder), and microtraumas through chronic inappropriate straining.
The main function of the teres major is to produce the movements of the humerus at the glenohumeral joint. It pulls the anterior surface of the humerus medially towards the trunk (internal rotation) and assists in the extension and medial rotation of the humerus. It can also extend the arm from a flexed position and provides a supportive role in the adduction of the shoulder. The teres major is commonly confused as a rotator cuff muscle, but it is not, because it does not attach to the capsule of the shoulder joint, unlike the teres minor muscle.
Injuries to the teres major muscle can result in pain and difficulty with activities that require sideways or backward movements of the arm. Isolated tears of the teres major are uncommon but may occur in baseball or cricket players, especially pitchers and bowlers. Treatment for teres major injuries includes proper warm-up, strengthening exercises, and improving posture.
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Latissimus Dorsi
The latissimus dorsi is a large, flat muscle on the back that stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The latissimus dorsi is the widest muscle in the human body and is relatively thin, covering almost all back muscles at the posterior trunk, except the trapezius. The muscle is responsible for extension, adduction, transverse extension, also known as horizontal abduction or horizontal extension, flexion from an extended position, and (medial) internal rotation of the shoulder joint. The latissimus dorsi also has a synergistic role in extension and lateral flexion of the lumbar spine.
The latissimus dorsi is innervated by the sixth, seventh, and eighth cervical nerves through the thoracodorsal (long subscapular) nerve. The thoracodorsal nerve, a branch of the posterior cord of the brachial plexus (C6 to C8 with C7 predominant), provides innervation to the latissimus dorsi. The muscle is supplied predominantly by the thoracodorsal artery, a continuation of the subscapular artery, which is a branch of the third part of the axillary artery.
The latissimus dorsi crosses the inferior angle of the scapula. A study found that, of 100 cadavers dissected, 43% had "a substantial amount" of fibres in the latissimus dorsi originating from the scapula. 36% had few or no muscular fibres, but a "soft fibrous link" between the scapula and the latissimus dorsi. 21% had little or no connecting tissue between the two structures.
The latissimus dorsi, pectoralis major, and teres major insert at the intertubercular groove of the humerus. They work to adduct and medially, or internally, rotate the humerus. The latissimus dorsi, along with the teres major, is active in the extension of the humerus. Extension and adduction are strongest when the motion is started from a position of partial flexion or abduction or a combination of the two motions. The muscle is active in moving the trunk anterior and superior when the upper extremities are fixed overhead, as in climbing or performing an activity such as a chin-up.
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Supraspinatus
The supraspinatus is the smallest of the four muscles that make up the rotator cuff of the shoulder joint. It is also the most superiorly located of the rotator cuff muscles, residing in the supraspinous fossa of the scapula, superior to the scapular spine. The supraspinatus muscle performs abduction of the arm, pulling the head of the humerus medially towards the glenoid cavity. It also helps to stabilise the shoulder joint by keeping the head of the humerus firmly pressed medially against the glenoid fossa of the scapula.
The supraspinatus muscle is frequently torn, making it the subject of extensive research. It is one of the most frequently damaged components of the rotator cuff, whether from acute injury or gradual degeneration. Bad posture and age are leading risk factors for tears, which can be either partial or full. Calcification of the supraspinatus tendon is a major contributor to shoulder pain and is often worsened by a tear.
A 2011 study found that the supraspinatus was consistently recruited prior to movement of the limb at all loads. The authors concluded that posterior rotator cuff muscles appear to be counterbalancing anterior translational forces produced during flexion, and that the supraspinatus is one of the muscles that consistently "initiates" flexion. This is supported by a 2016 study that found that 95.8% of patients who underwent arthroscopic treatment for rotator cuff calcification returned to their original functionality after a mean of 5.3 post-operative months.
The supraspinatus muscle has a complex architecture, with multiple ways of describing its structure. In 1993, it was described as including anterior and posterior muscle bellies, with the tendon of the anterior belly thicker and more tubular, and the posterior tendon flatter and wider. The muscle fibres of the posterior belly inserted directly into the flatter, thicker external posterior tendon, comprising 60% of the thickness of the supraspinatus tendon. The tendon stress attributed to the anterior and posterior muscle bellies indicated that the anterior tendon is subjected to 2.88 times greater stress than the posterior tendon. Histologic intratendinous structural differences were also observed between the two portions of the tendon, with the anterior portion demonstrating a double-layered interwoven pattern of fibres, while the posterior division had thin, dispersed fibres.
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Infraspinatus
The infraspinatus is a thick triangular muscle that is one of the four muscles of the rotator cuff. It originates from the medial three-quarters of the infraspinous fossa and the dorsal part of the scapula, with tendinous fibres arising from the ridges of the fossa. The infraspinatus muscle fibres converge into a tendon at the superolateral edge of the scapula, which crosses the posterior side of the shoulder and inserts onto the greater tubercle of the humerus. The infraspinatus is the second-largest anatomic footprint on the humerus out of the rotator cuff muscles.
The infraspinatus is a powerful external rotator of the humerus and stabilises the shoulder joint. It works together with the teres minor to oppose the upward pull of the deltoid muscle and stabilise the humeral head against the glenoid fossa. This prevents the upward displacement of the humeral head and impingement on the coracoid process. The infraspinatus also reinforces the capsule of the shoulder joint.
The infraspinatus receives arterial blood supply from the suprascapular and circumflex scapular arteries. The suprascapular nerve innervates the infraspinatus muscle. The infraspinatus is frequently fused with the teres minor.
The infraspinatus is involved in anterior-posterior force balance, providing the posterior force to balance the anterior force provided by the subscapular muscle. This force balance stabilises the humeral head during shoulder abduction. When the rotator cuff muscles are deficient, such as in the case of a torn infraspinatus tendon, the humeral head can become partially dislocated from the glenoid fossa, reducing the efficiency of the deltoid muscles' abduction action.
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Frequently asked questions
The proximal humerus is connected to the shoulder through the glenoid fossa of the scapula, forming the glenohumeral joint. The muscles that insert at the proximal humerus include the supraspinatus, infraspinatus, teres minor, and teres major.
The distal humerus articulates at the elbow joint to the radius and ulna in the forearm. Muscles that insert at the distal humerus include the deltoids, brachioradialis, coracobrachialis, pronator teres, and brachialis.
The muscles that insert at the humerus provide motion for the arm and upper body. They also provide structural support and stability to the glenohumeral joint, which is freely movable but inherently unstable and prone to dislocation.
Knowledge of the humerus' articulations and muscle attachments is crucial for physiotherapists to design effective rehabilitation exercises following humerus surgery or trauma. Additionally, fractures in the region of the humerus can cause damage to nearby nerves and blood vessels, resulting in sensory loss and impaired movement.
