Maximus Gage
The humerus is the longest bone in the upper extremity, and it plays a crucial role in the movement of the upper limb. It serves as an attachment point for 13 muscles, including the deltoid, supraspinatus, infraspinatus, teres minor, and subscapularis. These muscles facilitate abduction, adduction, flexion, extension, internal rotation, and external rotation of the shoulder joint. The deltoid muscle, in particular, helps move the arm forward, backward, and to the side. The humerus also enables forearm movements through its articulation with the radius and ulna at the elbow joint, contributing to the overall dexterity and functionality of the upper limb. Understanding the anatomy of the humerus is essential for physiotherapists to design effective rehabilitation programs for patients with humerus-related injuries.
| Characteristics | Values |
|---|---|
| Humerus bone structure | Consists of a head, anatomical neck, surgical neck, tuberosities, shaft (diaphysis), body, and two extremities (epiphysis) |
| Humerus function | Serves as an attachment point for 13 muscles, facilitating upper limb movement and stability |
| Muscle attachments | Supraspinatus, infraspinatus, teres minor, deltoid, latissimus dorsi, teres major, subscapularis, pectoralis major, coracobrachialis, biceps brachii, brachialis, brachioradialis, and more |
| Glenohumeral joint | A ball-and-socket joint formed by the head of the humerus, clavicle, and scapula, allowing a wide range of motions |
| Shoulder motions | Flexion, extension, abduction, adduction, internal rotation, and external rotation |
| Shoulder flexion | Forward movement of the shoulder, achieved by deltoid, pectoralis major, coracobrachialis, and biceps brachii muscles |
| Shoulder extension | Backward movement of the shoulder, opposite of flexion, achieved by deltoid, latissimus dorsi, and teres major muscles |
| Shoulder abduction | Moving the arm to the side, achieved by deltoid muscle |
| Shoulder adduction | Opposite of abduction |
| Shoulder internal rotation | Achieved by pectoralis major, latissimus dorsi, deltoid, teres major, and subscapularis muscles |
| Shoulder external rotation | Opposite of internal rotation, achieved by deltoid, teres minor, and infraspinatus muscles |
| Rotator cuff muscles | Supraspinatus, infraspinatus, teres minor, and subscapularis, stabilize the shoulder joint and are prone to injuries |
| Shoulder injuries | Adhesive capsulitis (frozen shoulder), bursitis, rotator cuff tears, shoulder impingement syndrome, strains, and more |
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Deltoid muscle
The deltoid muscle is the muscle forming the rounded contour of the human shoulder. It is also known as the 'common shoulder muscle' and is made up of three distinct sets of muscle fibres: the anterior or clavicular part (commonly known as the front delt), the posterior or scapular part (commonly known as the rear delt), and the intermediate or acromial part (commonly known as the side delt). The deltoid muscle is responsible for elevating the arm in the scapular plane, and its contraction also elevates the humeral head.
The deltoid muscle has three origins: the body of the clavicle, the spine of the scapula, and the acromion. The insertion is an arch-like structure with strong anterior and posterior fascial connections. The deltoid is supplied by the thoracoacromial artery, the circumflex humeral arteries, and the profunda brachii artery. It is innervated by the axillary nerve, which originates from the anterior rami of the cervical nerves C5 and C6.
The deltoid muscle has a variety of functions, including abduction, adduction, flexion, extension, internal and external rotation. The specific function of the deltoid muscle depends on which muscle fibres are activated. For example, the anterior deltoid is responsible for flexion, internal rotation, and horizontal adduction, while the posterior deltoid performs extension, external rotation, and horizontal abduction. The deltoid muscle is also involved in stabilizing the shoulder joint and preventing dislocations.
The deltoid muscle is susceptible to various abnormalities and conditions, such as tears, fatty atrophy, enthesopathy, adhesive capsulitis (frozen shoulder), axillary nerve palsy, and deltoid fibrosis. Deltoid muscle pain can affect individuals who perform repetitive overhead arm movements, such as swimmers and pitchers.
The deltoid muscle is an important muscle in the shoulder that facilitates a wide range of movements and helps stabilize the shoulder joint. Its functions and conditions are well-studied, and it plays a crucial role in the overall movement and stability of the upper limb.
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Teres minor
The teres minor is a narrow, elongated muscle of the rotator cuff. It is one of four muscles that make up the rotator cuff, the others being the supraspinatus, infraspinatus, and subscapularis. The teres minor originates at the lateral border and adjacent posterior surface of the scapula. It inserts at the greater tubercle of the humerus. The tendon of the teres minor passes across and is united with the posterior part of the capsule of the shoulder joint. The muscle is innervated by the axillary nerve.
The primary function of the teres minor is to modulate the action of the deltoid, preventing the humeral head from sliding upward as the arm is abducted. It also functions to rotate the humerus laterally. The teres minor is deep to the deltoid muscle. As a rotator cuff muscle, the teres minor stabilizes the ball-and-socket glenohumeral joint by helping to hold the humeral head (ball) in the shallow glenoid cavity of the scapula (socket). The teres minor also laterally or externally rotates the arm at the shoulder joint. As a lateral rotator, the teres minor is an antagonist muscle to medial rotation. Therefore, the teres minor is especially critical in stabilizing the shoulder during medial rotation to prevent anterior dislocation of the humerus.
The teres minor is supplied by the subscapular artery and one of its branches, the circumflex scapular artery, as well as the posterior circumflex humeral artery. The subscapular artery is the largest branch of the axillary artery. It travels caudally before dividing into two arteries: the circumflex scapular and the thoracodorsal. The circumflex scapular artery travels around the lateral border of the scapula between the subscapularis and teres minor. The posterior circumflex humeral artery is a more distal branch of the third portion of the axillary artery. It travels posteriorly with the axillary nerve through the quadrangular space, bounded by the teres minor superiorly, teres major inferiorly, the surgical neck of the humerus laterally, and the long head of the triceps brachii medially.
There are two types of rotator cuff injuries: acute tears and chronic tears. Acute tears occur as a result of a sudden movement, such as throwing a powerful pitch or falling on an outstretched hand at speed. Chronic tears develop over time and usually occur at or near the tendon as a result of the tendon rubbing against the underlying bone. Selective atrophy of the teres minor muscle has been observed in cases of quadrangular space syndrome, which causes excessive and/or chronic compression of the structures passing through this anatomical tunnel.
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Supraspinatus
The supraspinatus is one of the four muscles that comprise the rotator cuff of the shoulder joint. It is the smallest and most superiorly located of the rotator cuff muscles. The supraspinatus resides in the supraspinous fossa of the scapula, superior to the scapular spine. The tendon of the muscle extends laterally, passing under the acromion process and over the head of the humerus, blending into the glenohumeral joint capsule. It inserts onto the superior facet of the greater tuberosity of the humerus.
The supraspinatus works in conjunction with the other three rotator cuff muscles: the infraspinatus, teres minor, and subscapularis. Together, they form part of the dynamic stabilization for the glenohumeral joint. These four muscles act in a coordinated fashion to stabilize the head of the humerus on the shallow glenoid fossa and promote the structural integrity of the joint. The supraspinatus helps to resist the gravitational forces that act on the shoulder joint, pulling down on the weight of the upper limb. 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 is involved in the abduction of the arm, pulling the head of the humerus medially towards the glenoid cavity. It can abduct the arm from 0 to 15 degrees independently and assists the deltoid muscle to produce abduction beyond this range up to 90 degrees. The supraspinatus is also thought to play a role in initiating shoulder abduction. Additionally, it may weakly contribute to the lateral rotation of the humerus.
The supraspinatus is the most frequently torn rotator cuff muscle and has been the subject of extensive research. Calcification of the supraspinatus tendon is a common cause of shoulder pain and can be worsened by a supraspinatus tear. Arthroscopic surgery for full-thickness supraspinatus tears has been found to be effective in improving shoulder functionality and reducing pain.
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Infraspinatus
The infraspinatus is a thick, triangular muscle that occupies the chief part of the infraspinatous fossa. It is one of the four muscles of the rotator cuff, along with the supraspinatus, teres minor, and subscapularis. The infraspinatus muscle's main function is to externally rotate the humerus and stabilize the shoulder joint. It originates from the medial three-quarters of the infraspinous fossa and the dorsal part of the scapula, and it inserts on the middle facet of the greater tubercle of the humerus.
The infraspinatus muscle plays a crucial role in shoulder movement and stability. When the arm is fixed, it abducts the inferior angle of the scapula, allowing for full shoulder abduction. It also assists in carrying the arm backward. The infraspinatus is frequently fused with the teres minor muscle, and together they rotate the head of the humerus outward. This outward rotation is also facilitated by the insertion of the infraspinatus tendon at the middle facet of the glenoid tubercle, which exerts a lateral or external rotational force.
Additionally, the infraspinatus reinforces the capsule of the shoulder joint. It provides the primary muscle force for external rotation of the shoulder. The infraspinatus is involved in the anterior-posterior force balance, providing the posterior force while the subscapular muscle provides the anterior force. This balance helps to stabilize the humeral head during shoulder abduction.
Isolated weakness or atrophy in the infraspinatus muscle can be an indication of suprascapular nerve compression in the spinoglenoid notch. Myofascial release techniques applied to the infraspinatus muscle may help to alleviate shoulder pain. The infraspinatus muscle is also associated with referred pain to the middle and anterior deltoid regions.
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Rotator cuff muscles
The rotator cuff (RC) is a group of four distinct muscles and their tendons that provide strength and
The primary biomechanical role of the rotator cuff is to stabilise the glenohumeral joint by compressing the humeral head against the glenoid. The tendons of the rotator cuff muscles blend with the joint capsule and form a musculotendinous collar that surrounds the posterior, superior, and anterior aspects of the joint. This arrangement is important because it prevents the humerus from sliding inferiorly through the unprotected part of the joint during arm movements. By contracting, the rotator cuff muscles allow for a full range of motion while maintaining stability.
The rotator cuff muscles are also involved in the mobility of the shoulder joint, facilitating abduction, medial rotation, and lateral rotation. They help to enlarge the range of motion in the glenohumeral joint and avoid mechanical obstruction. Dysfunction of the rotator cuff muscles can lead to shoulder pain, impaired functional capacities, and a reduced quality of life. RC injuries are common and can occur at any age due to trauma or overuse from overhead activities.
The vascular supply to the rotator cuff muscles comes from the suprascapular artery, the subscapular artery, and the posterior circumflex humeral artery. The suprascapular artery supplies the supraspinatus and infraspinatus muscles, while the subscapular artery gives vascular supply to the subscapularis muscle. The humerus, as the longest bone in the upper extremity, serves as an attachment point for the rotator cuff muscles and facilitates the coordinated actions of the upper limb.
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Frequently asked questions
The deltoid muscle, the pectoralis major muscle, the coracobrachialis muscle, and the biceps brachii muscles all play a role in moving the humerus.
The deltoid muscle has three origins: the body of the clavicle, the spine of the scapula, and the acromion. The function of the deltoid muscle varies depending on which muscle fibres are activated. The anterior deltoid flexes and medially rotates the humerus, the middle deltoid abducts the humerus, and the posterior deltoid performs extension and external rotation of the humerus.
The pectoralis major muscle is involved in shoulder flexion, which is the movement of the shoulder in a forward motion. An example of this is reaching forward to grasp an object.
