Like the fourth letter of the Greek alphabet, delta, the deltoid (-oid = “like”) muscle is triangular in shape, with its apex pointing distally along the humerus. It is the most superficial muscle spanning the glenohumeral joint, and gives the deltoid region of the shoulder its rounded appearance.
Understanding the deltoid muscle, especially its functions and innervation, is very important in terms of differential diagnosis of weakness in musculature associated with the shoulder (glenohumeral) joint.
|Origins||Lateral 1/3 of clavicle (clavicular part), acromion (acromial part), spine of scapula (spinal part)|
|Insertion||Deltoid tuberosity of humerus|
|Innervation||Axillary nerve (C5, C6)|
Anterior (clavicular) part: flexion and internal rotation of the arm,
Middle (acromial part): abduction of the arm beyond the initial 15°
Posterior (spinal part): extension and external rotation of the arm.
|Testing||Resisted abduction with the arm abducted beyond 15 degrees|
This article will discuss the deltoid muscle, its bony attachments (origin and insertion), actions, and innervation, highlighting the components that can be helpful in testing for weakness and dysfunction in the muscle.
Origins and Insertion
The very broad origin of the deltoid actually means that functionally, the muscle can be broken down into three separate parts:
- The clavicular (anterior) part
- The acromial (middle) part
- The scapular (posterior) part
Unlike other muscles, for example the triceps brachii (tri = three, ceps = heads), the name of the deltoid muscle does not indicate this possible functional division.
Clavicular (anterior) part: Lateral 1/3 of the clavicle.
Acromial (middle) part: around the acromion.
Scapular (posterior) part: along the lateral 1/3 of the spine of the scapula
|Insertion||The deltoid tuberosity, located approximately halfway down the lateral aspect of the shaft of the humerus.|
One of the key points to understanding the deltoid muscle functionally is its action when the arm is fully adducted, or when the arm is resting at the side of the body when standing erect. In this position, as the muscle contracts, it will produce a line of force that will pull along the long axis of the bone, producing no movement. Instead, in this position, the deltoid will function to maintain joint stability by resisting forces pulling down on the humerus (i.e., resisting dislocating forces). For example, along with the rotator cuff muscles, the deltoid would resist the inferior displacement of the humerus from the glenoid cavity when carrying heavy objects such as suitcases or shopping bags.
As with the rotator cuff muscles, the deltoid muscle will also help to stabilize the glenohumeral joint by holding the head of the humerus in the glenoid cavity during movements of the upper limb.
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Abduction at the glenohumeral joint
The deltoid is often thought of, and talked about, as one of the main abductors of the arm. And this is true, but only once the supraspinatus muscle initiates the movement. The deltoid muscle does not become fully active in abduction until the arm has moved through approximately 15 degrees of abduction which is driven by the activation of the supraspinatus.
When all parts of the deltoid contract simultaneously, abduction of the arm occurs. However, the middle fibers of the deltoid muscle are responsible for the movement while the more anterior and posterior fibers will function to guide the arm through the abduction motion.
Flexion and internal rotation at the glenohumeral joint
The anterior, or clavicular, fibers of the deltoid muscle are active during flexion at the glenohumeral joint, for example to swing the upper arm forward during walking or running motions. These fibers are also active during internal/medial rotation of the glenohumeral joint.
Extension and external rotation at the glenohumeral joint
The posterior, or scapular, fibers of the deltoid muscle are active during extension at the glenohumeral joint. Using the same example as above, they would assist the latissimus dorsi muscle in swinging the arm posteriorly during walking and running motions.
In addition, these fibers will assist in external(or lateral) rotation of the arm. This is important from a functional standpoint as strengthening the posterior fibers of the deltoid muscle can help to offset the tendency of the shoulder to become internally rotated due to poor posture. Overdevelopment or tightening of muscles that internally rotate the arm can result in dysfunctional motion at the glenohumeral joint especially during overhead activities.
The deltoid muscle is innervated by one of the main branches of the brachial plexus, the axillary nerve (C5, 6). The axillary nerve branches from the posterior cord of the brachial plexus in the axilla and runs posterior to the humerus, around the surgical neck of the humerus.
The cutaneous branch of the axillary nerve (superior lateral cutaneous nerve of the arm) supplies an area of skin overlying the deltoid muscle, and can be a clinical manifestation of damage to the axillary nerve.
It is incredibly important to properly test the function of the deltoid muscle to accurately determine muscular or nervous injury! An inability to abduct the arm from a position in which the arm is resting at the side of the body does not indicate an injury to the deltoid muscle or the axillary nerve. An inability to initiate abduction of the arm (up to approximately 15 degrees of abduction) would indicate involvement of the supraspinatus muscle or the nerve innervating it, the suprascapular nerve.
To properly test the function of the deltoid and the axillary nerve, the arm must be beyond 15 degrees of abduction. Once the arm is in this position, the patient then pushes against resistance. If the muscle is functioning properly, contraction of the muscle should be felt near the acromion of the shoulder.
Axillary Nerve Injury
As the axillary nerve passes posteriorly in the axilla, it runs around the surgical neck of the humerus. Fractures in this region of the humerus can therefore, affect the functioning of the nerve, and consequently the deltoid muscle. In addition, the axillary nerve can also be damaged during dislocation of the glenohumeral joint or it can be compressed during incorrect use of crutches. Symptoms may include atrophy of the deltoid muscle, resulting in weakness and a loss of muscle tone, making the shoulder look flattened rather than rounded. In addition, there may be a loss of sensation to the skin laying over the deltoid muscle.
As the axillary nerve wraps around the surgical neck of the humerus, it passes from posterior to anterior, running transversely under the the deltoid muscle. An awareness of its location anteriorly is important during intramuscular injections in the deltoid muscle, and during surgical approaches to the shoulder to avoid injuring the nerve.
It may be that deltoid pain may be indicative of injuries involving other muscles that stabilize the glenohumeral joint, and structures other structures associated with improper functioning of the glenohumeral joint. The subacromial/subdeltoid bursa, lies deep to the deltoid muscle, between the acromion and supraspinatus tendon.
In overhead motions of the arm, the greater tubercle of the humerus approaches the acromion, especially when the arm is internally rotated. The subacromial/subdeltoid bursa can become irritated and swollen as it can be repeatedly pinched between the acromion and greater tubercle of the humerus. The bursa may then become distended and cause pain underneath the deltoid muscle.