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The deltoid and the supraspinatus are the two main muscles that abduct the brachium, or shoulder. The deltoid muscle is divided into three parts: the anterior deltoid, the middle deltoid, and the posterior deltoid. The supraspinatus muscle, meanwhile, originates in the supraspinous fossa and inserts into the greater and lesser tubercles of the humerus. The coracobrachialis is another muscle that is involved in the abduction of the arm, although it primarily serves to adduct and flex the brachium.
| Characteristics | Values |
|---|---|
| Muscles that abduct the brachium | Deltoid and supraspinatus |
| Muscles that act on the brachium | Coracobrachialis, anterior deltoid, middle deltoid, pectoralis major, latissimus dorsi, teres major, teres minor, infraspinatus, subscapular, and more |
| Function of coracobrachialis | Flexion, abduction, and stabilization of the shoulder joint |
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What You'll Learn
Deltoid and supraspinatus muscles
The deltoid and supraspinatus muscles are both involved in arm elevation and abduction. The deltoid muscle is the largest contributor to arm abduction, with the supraspinatus assisting in the production of abduction up to 90 degrees. The supraspinatus is also active during arm abduction and contributes to glenohumeral elevation moments.
The deltoid muscle is the only shoulder elevator if the supraspinatus is torn and dysfunctional, so rehabilitation is often directed toward this muscle. The deltoid contributes to the glenohumeral elevation moment, and its activity increases with large elevation moment loading.
The supraspinatus is the smallest of the four muscles that make up the rotator cuff of the shoulder joint. It is considered the most superiorly located of the rotator cuff muscles and travels underneath the acromion. The supraspinatus has been reported to play a primary role in stabilizing the glenohumeral joint. It can press the humeral head against the concave glenoid, with its compressive muscle line of action and relatively small muscle moment arm.
A 2011 study found that the supraspinatus was consistently recruited prior to movement of the limb at all loads, suggesting that it "initiates" flexion. The Empty Can Test and the Full Can Test are commonly used orthopedic examinations for supraspinatus impingement or integrity of the supraspinatus muscle and tendon.
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Coracobrachialis
The coracobrachialis muscle is a slender muscle located in the upper medial part of the arm, within the anterior compartment of the arm. It originates from the coracoid process of the scapula and inserts onto the middle of the medial aspect of the body of the humerus. It is innervated by the musculocutaneous nerve, which arises from the anterior division of the upper trunk (C5).
The main function of the coracobrachialis muscle is to produce flexion and adduction of the arm at the shoulder joint. It also assists in resisting deviation of the arm from the frontal plane during abduction. The muscle is classified into distinct superficial and deep layers, with some individuals having a fully divided muscle, while others have incomplete or no separation of the layers.
The coracobrachialis is not one of the most prominent arm muscles, and it doesn't play a significant role in moving the arm. However, it can become tight from certain activities and can be a source of shoulder or arm pain. It is involved in flexion, bringing the upper arm forward, and adduction, bringing the arms back down to the sides.
The coracobrachialis also plays a role in internal rotation, bringing the arms inward towards the body, and shoulder stability, holding the upper arm bone within the shoulder socket during rotation. It can be strained by activities such as push-ups, swinging on gymnastic rings, or carrying heavy bags at the sides. Rupture of the coracobrachialis muscle is extremely rare and is typically caused by direct trauma to the contracted muscle.
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Anterior deltoid
The deltoid muscle, often referred to as the 'common shoulder muscle', forms the rounded contour of the human shoulder. The anterior deltoid, also known as the front deltoid or clavicular part, connects to the clavicle (collarbone) and helps move the arm forward.
The anterior deltoid is one of three distinct sets of muscle fibres that make up the deltoid. The other two are the posterior or scapular part (rear deltoid) and the intermediate or acromial part (side deltoid). The anterior deltoid's fibres arise from the anterior border and upper surface of the lateral third of the clavicle, lying adjacent to the lateral fibres of the pectoralis major muscle.
The anterior deltoid has several important functions. Firstly, it assists the pectoralis major in flexing the shoulder and helps to internally (medially) rotate the humerus, working in tandem with muscles like the subscapularis, pecs, and lats. Secondly, the anterior deltoid plays a crucial role in stabilising the shoulder joint, preventing subluxation or dislocation of the head of the humerus, especially when carrying heavy loads. This stabilisation helps protect the shoulder joint and prevents injuries.
To strengthen the anterior deltoid, specific exercises can be performed. One such exercise involves standing next to a wall with the back and elbow straight, then gently pushing the arm against the wall as hard as comfortably possible without causing pain. Holding this position for 5 seconds and repeating 10 times on each side can help improve strength, provided it is done without discomfort. Other exercises include using dumbbell weights, resistance bands, or performing shoulder diagonal PNF (Proprioceptive Neuromuscular Facilitation) exercises.
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Middle deltoid
The deltoid muscle is the muscle forming the rounded contour of the human shoulder. It is also known as the 'common shoulder muscle'. Anatomically, the deltoid muscle 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 works alongside other shoulder muscles, such as the rotator cuff muscles, to help perform a variety of movements.
The middle deltoid, also known as the side delt, connects to the acromion, a bony knob on the shoulder blade. This muscle helps move the arm out to the side, as well as up and down. For example, you use your side delts when doing jumping jacks.
The deltoid muscle as a whole is involved in arm abduction, which means raising the arm out to the side of the body. It also compensates for lost arm strength in the event of an injury, such as a rotator cuff tear. The deltoid muscle helps to stabilise the shoulder joint, preventing dislocations as you lift your arm or carry weight with your arms at your side.
Deltoid muscles cover the front, side, and back of the shoulder joint. They are superficial, meaning they are close to the surface of the skin. The deltoid muscle receives its blood supply from the posterior circumflex humeral artery.
Deltoid muscle pain can affect individuals who perform repetitive overhead activities, such as swimmers, pitchers, and painters. Conditions that may affect the deltoid muscles include adhesive capsulitis (frozen shoulder), axillary nerve palsy, bursitis, and deltoid fibrosis.
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Posterior deltoid
The deltoid muscle forms the rounded contour of the human shoulder and is also known as the common shoulder muscle. It is made up of three distinct sets of muscle fibres: the anterior or clavicular part (commonly known as the front deltoid), the posterior or scapular part (commonly known as the rear deltoid), and the intermediate or acromial part (commonly known as the side deltoid).
The posterior deltoid is a set of posterior fibres that arise from the lower lip of the posterior border of the scapula spine. From this origin, the fibres converge toward their insertion on the deltoid tuberosity on the middle of the lateral aspect of the humerus shaft. The posterior deltoid is involved in externally rotating the shoulders, as well as extending the humerus behind the body. This muscle is targeted during exercises such as rowing and pull-ups, and it helps to stabilise the shoulder joints.
To strengthen the posterior deltoid, exercises that involve pulling motions are recommended. One such exercise involves pulling ropes toward the chest while bending the elbows 45 to 90 degrees away from the sides. This movement can also be performed with a resistance band. Additionally, exercises that target the rear deltoid can be performed using gym equipment. For example, holding a pair of dumbbells with palms facing each other, one can raise their arms up and out, similar to spreading wings, until parallel to the floor.
The deltoid muscle as a whole plays a crucial role in stabilising the shoulder joint and preventing subluxation or dislocation of the humeral head, especially when carrying heavy loads. All heads of the deltoid work together to produce abduction of the shoulder joint, enabling the arm to be lifted to the front, side, and back.
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Frequently asked questions
The deltoid and the supraspinatus are the two main abductors of the shoulder.
The supraspinatus extends and stabilises the shoulder joint.
The deltoid muscle is responsible for shoulder flexion, extension, abduction, and internal and external rotation.
The coracobrachialis, anterior deltoid, and middle deltoid muscles are also involved in shoulder abduction.
An example of shoulder abduction is lifting your arms out to the side, as in the motion performed when doing jumping jacks.
