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The deltoid muscle is a large, thick, triangular-shaped muscle that forms the rounded contour of the human shoulder. It is a key shoulder stabilizer and is responsible for arm abduction, flexion, and extension. The deltoid is made up of three distinct sets of muscle fibers: 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). These muscle fibers work alongside other shoulder muscles, such as the rotator cuff muscles, to produce a variety of movements. In this article, we will explore the muscles that pair with the deltoids and discuss their functions and interactions.
| Characteristics | Values |
|---|---|
| Muscle type | Skeletal |
| Muscle shape | Triangular |
| Muscle size | Large |
| Muscle weight | 192 grams on average |
| Muscle parts | 3 |
| Muscle fibres | 7 |
| Muscle insertion areas | 2 or 3 |
| Muscle blood supply | Axillary artery |
| Muscle nerve supply | Axillary nerve |
| Muscle functions | Abduction, flexion, extension, stabilisation, compensation for lost arm strength |
| Muscle injuries | Tears, strains, paralysis |
| Muscle treatments | Ice, painkillers, physical therapy, surgery |
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What You'll Learn
Pectoralis major
The pectoralis major is the superior most and largest muscle of the anterior chest wall. It is a thick, fan-shaped muscle that lies underneath the breast tissue and forms the anterior wall of the axilla. The pectoralis major is the most superficial muscle in the pectoral region.
The pectoralis major consists of two heads: the clavicular and the sternocostal. The clavicular head originates from the anterior surface of the medial half of the clavicle, while the sternocostal head originates from the anterior surface of the manubrium and body of the sternum, the anterior surface of the superior six costal cartilages, and the superior part of the aponeurosis of the external oblique muscle. The upper and lower fibres of the pectoralis major insert into the crest of the greater tubercle of the humerus. The two heads of the pectoralis major have different nervous supplies. The clavicular head receives its nerve supply from the lateral pectoral nerve, while the sternocostal head receives innervation from the C7, C8, and T1 nerve roots via the lower trunk of the brachial plexus and the medial pectoral nerve.
The pectoralis major has four primary actions responsible for the movement of the shoulder joint. Firstly, it is responsible for flexion of the humerus, as in throwing a ball underhand or lifting a child. Secondly, it contributes to adduction of the arm at the glenohumeral joint. Thirdly, it assists in medial rotation of the humerus. Lastly, the pectoralis major is responsible for keeping the arm attached to the trunk of the body. The clavicular part of the pectoralis major is close to the deltoid muscle and contributes to flexion, horizontal adduction, and inward rotation of the humerus. On the other hand, the sternocostal part is antagonistic to the clavicular part, contributing to the downward and forward movement of the arm and inward rotation when accompanied by adduction.
The pectoralis major is highly vascularised and has been used in flap repairs in neck surgery. It is also utilised in soft tissue reconstruction of the neck and face following trauma or tumour-related operations. Pectoralis major tendon rupture is a rare shoulder injury, most commonly observed in weightlifters performing exercises such as the bench press.
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Supraspinatus
The supraspinatus muscle is the smallest of the four muscles that make up the rotator cuff in the shoulder joint. It is the most superiorly located of the rotator cuff muscles and resides in the supraspinous fossa of the scapula, above the scapular spine. The supraspinatus muscle is involved in abduction of the arm, pulling the head of the humerus medially towards the glenoid cavity and preventing it from slipping inferiorly. It also helps to stabilise the shoulder joint by keeping the head of the humerus firmly pressed against the glenoid fossa of the scapula.
The supraspinatus muscle works in cooperation with the deltoid muscle to perform abduction, particularly when the arm is in an adducted position. The supraspinatus muscle is active in the initial phase of abduction, from 0 to 15 degrees of arm abduction, and assists the deltoid muscle in abduction beyond this range. The deltoid muscle becomes increasingly more effective at abducting the arm as the angle of abduction increases, and it becomes the main agonist beyond 15 degrees.
The supraspinatus muscle is frequently torn, whether from acute injury or gradual degeneration, and is often associated with shoulder pain. Bad posture and age are leading risk factors for supraspinatus tears. Calcification of the supraspinatus tendon is a common cause of pain and can be effectively treated with arthroscopic surgery, leading to improved shoulder function and a significant decrease in pain.
The supraspinatus muscle is involved in the initiation of shoulder flexion, as it is consistently recruited prior to movement of the limb. It counterbalances anterior translational forces produced during flexion, working in conjunction with the posterior rotator cuff muscles. The integrity of the supraspinatus muscle and tendon can be assessed through orthopedic examinations such as the Empty Can Test and the Full Can Test.
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Teres major
The deltoid muscle is the muscle forming the rounded contour of the human shoulder. Anatomically, the deltoid 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 teres major is a small muscle 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 is positioned above the latissimus dorsi muscle and assists in the extension and medial rotation of the humerus. It is also a medial rotator and adductor of the humerus, assisting the latissimus dorsi in drawing the previously raised humerus downwards and backwards. The teres major is commonly confused as a rotator cuff muscle, but it does not attach to the capsule of the shoulder joint. However, it does play a role in stabilizing the glenohumeral joint.
The teres major is supplied by the thoracodorsal branch of the subscapular artery and the posterior circumflex humeral artery. It is innervated by the lower subscapular nerve, derived from C5, C6, and C7 nerve roots.
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Teres minor
The deltoid muscle is a thick, triangular shoulder muscle that gets its name from its shape, which resembles the Greek letter 'delta' (Δ). It is a skeletal muscle, and tendons attach it to bones. The deltoid muscle works alongside other shoulder muscles, such as the rotator cuff muscles, to help perform various movements.
The teres minor is a narrow, elongated muscle of the rotator cuff. It originates from the lateral border and adjacent posterior surface of the scapula and inserts at the greater tubercle of the humerus. The upper fibres of the muscle culminate in a tendon that inserts onto the greater tubercle of the humerus, and the tendon blends into the capsule of the shoulder joint. The lower fibres of the muscle insert directly into the humerus, just below the greater tubercle. The teres minor is deep to the deltoid muscle.
The primary function of the teres minor is to modulate the action of the deltoid muscle. It prevents the humeral head from sliding upward as the arm is abducted. It also functions to rotate the humerus laterally and is an antagonist muscle to medial rotation. The teres minor is, therefore, critical in stabilising 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. The circumflex scapular artery travels around the lateral border of the scapula between the subscapularis and teres minor, supplying the teres minor along its route. The posterior circumflex humeral artery is a more distal branch of the axillary artery. It travels posteriorly with the axillary nerve through the quadrangular space, supplying the teres minor.
The teres minor is innervated by the axillary nerve. Damage to the fibres innervating the teres minor is clinically significant. The axillary nerve is sometimes damaged during surgical procedures, and it may also be injured by anterior dislocation of the head of the humerus.
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Subscapularis
The subscapularis muscle is a large, triangular-shaped muscle that fills the subscapular fossa and inserts into the lesser tubercle of the humerus and the front of the capsule of the shoulder joint. The subscapularis is the largest and strongest of the four rotator cuff muscles, which include the supraspinatus, infraspinatus, and teres minor muscles. The subscapularis is covered by a dense fascia that attaches to the scapula at the margins of its attachment on the scapula. The muscle's fibres pass laterally from its origin before converging into a tendon of insertion, which intermingles with the glenohumeral (shoulder) joint capsule.
The primary function of the subscapularis is the internal rotation of the humerus, which is the long bone in the upper arm. It also assists in shoulder adduction and extension in certain positions. When the arm is raised, the subscapularis pulls the humerus forward and downward, and when the humerus is in a fixed position, the insertion of the subscapularis can act as an origin, producing abduction of the inferior border of the scapula. As part of the rotator cuff, the subscapularis plays a crucial role in stabilising the shoulder joint by contributing to the fixation of the proximal humerus during movements of the elbow, wrist, and hand.
The subscapularis is supplied by the upper and lower subscapular nerves (C5-C6), which are branches of the posterior cord of the brachial plexus. The primary blood supply to the subscapularis muscle is the subscapular artery, a branch of the axillary artery. The subscapularis may include up to three trigger points, with the two most common occurring near the outside edge of the muscle. Referred pain from trigger points in the subscapularis muscle concentrates in the posterior shoulder region, with spillover into the shoulder blade region and down the back of the upper arm.
The Gerber Lift-off test, also known as Gerber's Test, is the established clinical test for examining the subscapularis for potential tears. Positive bear-hug and belly press tests also indicate significant tearing of the subscapularis. Mack et al. developed an ultrasonographic procedure that can explore almost the complete rotator cuff in six steps, including a detailed examination of the subscapularis muscle.
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Frequently asked questions
The deltoid muscles work with the rotator cuff muscles to help perform a variety of movements. The four rotator cuff muscles are the supraspinatus, infraspinatus, teres minor, and subscapularis.
The deltoid muscles are large, thick, triangular-shaped muscles that form the rounded contour of the human shoulder. They are also known as the 'common shoulder muscle'.
The deltoid muscle is made up of three distinct sets of muscle fibres: the anterior or clavicular part (front delt), the posterior or scapular part (rear delt), and the intermediate or acromial part (side delt).
The deltoid muscle is one of the main abductors of the shoulder, helping to lift the arm when reaching for objects. It also helps to stabilise the glenohumeral joint and prevent dislocations, especially when carrying heavy weights.
There are various exercises to strengthen the deltoid muscles, including standing next to a wall and gently pushing the arm against it, as well as dumbbell weight arm raises and resistance band exercises.
