Deltoid Muscle: Location, Actions and Trigger Points

Ground Up Strength
The deltoid muscle is a large, triangular, course, and thick muscle which gives the shoulder it's shape and contour. Its name is often reported to have derived from the Greek letter Delta (Δ) but it actually derives from the Latin worddeltoides which means "triangular in shape or form" and was taken from the shape of the letter delta and the word eidos (oid) meaning shape or form. The deltoid is the principal abductor of the arm at the glenohumeral (shoulder) joint and also flexes and extends the humerus. The deltoid is the largest and probably the most important muscle of the shoulder complex. 6, 5

The deltoid has three major parts: anterior, middle, and posterior, which, based on their origins can be considered the clavicular, acromial, and scapular divisions. These parts cover the upper (proximal) part of the humerus, converging to a thick tendon to insert on the lateral surface of the humerus bone. All three sections differ in structure and function but work in concert to produce important movements at the shoulder joint.

 

The deltoid is also an important dynamic stabilizer of the glenohumeral joint but this action has not been as extensively studied in its movements. It appears to provide anterior stability by compression of the humeral head against the glenoid fossa during 90° abduction and external rotation. Since the true plane of abduction is in line with the blade of the scapula, it is said that the deltoid provides dynamic stability during abduction in the "scapular plane."¹.

It decreases stability, however, in the coronal plane where is tends to produce an upward shearing or traction effect on the head of the humerus, producing impingement on the acronium. The rotator cuff, namely the subscapularis, infraspinatus, and teres minor produce a synergistic downward pull to offset this upward translation of the humeral head. In other words, the rotator cuff muscles are very important to stabilize the humeral head when counterbalancing this pull by resisting the upward shearing of the deltoid. There are still many questions about the deltoid's role in dynamic stability versus it's role as a de-stabilizer and much of the data is conflicting. However, it is clear that the deltoid, acting alone, would be unable to function properly as a mover of the shoudler joint. The rotator cuff is the main means of holding the humeral head centered in the glenoid fossa during most daily functional movements and tasks of the shoulder. 1,2,3,5

The Glenohumeral (Shoulder) Joint

The deltoid is active during any lifting movement and contracts statically during most everyday tasks. As important as the muscle is, however, the importance of the smaller rotator cuff muscles, as mentioned above, must also be considered in it's actions, with the supraspinatus being important during abduction. However, the frequent claim of the supraspinatus being the primary adductor of the shoulder during the first 30 degrees, after which the deltoid takes over, has been questioned by several studies and EMG data. It is more likely that the suprispinatus and deltoid function synchronously although the suprispinatus, if maximally contracted, may be able to elevate the arm to the initial 30°. Although EMG data shows the activity of the deltoid and the supraspinatus progressively increasing of abduction motion, the supraspinatus is still usually reported as the "initiator" of abduction during normal tasks.²,3

Deltoid Muscle, Side View

The deltoid is activated for long periods of time during keyboard work and driving. If a keyboard or work surface is set too low or too high, this activation is increased. Driving with the hands on the top of the steering wheel primarily activates the anterior deltoid. Trigger points can develop from over-use through these mechanisms or through the abuse the muscle frequently receives in resistance training, as many bodybuilders and strength trainees dedicate entire workouts to the shoulder alone. Since the deltoid is also active during most other upper body movements during other workouts, over-use can easily produce TP's. The trigger points cause pain in the deltoid muscle itself which is felt as a deep pain the the shoulder.
Trigger points may also develop after direct impact trauma to the muscle during sports. The functional characteristics of the deltoid and the supraspinatus becomes much more complicated during sporting movements such as throwing or with sports requiring shoulder abduction and external rotation. The remainder of this article will cover the primary movements only as sport-specific functions are beyond its scope.

Deltoid Muscle: Origins, Insertions, and Actions

Origin:

• Anterior fibers: anterior and superior surfaces of outer third of clavicle and anterior acronium³
• Middle fibers: lateral margin of the acronium⁴
• Posterior fibers: inferior edge of almost the the scapular spine

Insertion: The anterior and posterior fiber portions converge into a thick tendon which inserts on the lateral surface of the humerus near its midpoint at the deltoid tuberosity. The middle portion, however, is multipennate and inserts via four to five intramuscular septa or tendinous expansions. 4, 2,5

Actions

The three portions of the deltoid can contract independently or together, depending on the action. The actions of the deltoid and its three parts will be considered first in terms of the primary actions of each group of fibers and then again by movement, together with synergists.

Fibers	Action
Anterior fibers	glenohumeral (shoulder) joint flexion, internal rotation, horizontal adduction, and abduction (in the coronal plane)
Middle Fibers	abduction of the shoulder joint and small role in flexion
Posterior Fibers	external rotation, abduction, adduction and extension of the glenohumeral joint

Notes on Deltoid Actions

• Contraction of the entire deltoid, with all its fibers, results in shoulder joint abduction but the middle fibers are usually considered to only be abductors. Some of these fibers are active in flexion, however. When the entire muscle contracts it can produce shoulder abduction to just beyond 90 degrees without scapular rotation, which must occur for full abduction to be possible.

• The lateral deltoid fibers have a multipennate arrangement which gives it greater strength over a short range of motion than the anterior and posterior fibers, which are fusiform, and are better suited to great speed over long ranges of motion.

• Contraction of the anterior fibers alone results in adduction, flexion, and internal rotation. Forward flexion and internal rotation of the humerus is carried out in conjunction with the pectoralis major.

• Contraction of the posterior fibers alone results in adduction, extension, and external rotation. Extension and external rotation of the humerus is carried out in conjunction with the lattisimus dorsi and teres major.

• The anterior and posterior portions can assist, by contracting together, with stabilization of the humerus by preventing the humeral head from leaving the plane of motion. (bibcite behnke)),5

• The deltoid, together with the supraspinatus, contracts when carrying heavy objects at the side to resist the strong downward pull. The deltoid is also contstantly active in positioning the hands for everyday manual tasks, by producing forward flexion of the humerus. 5

Deltoid Actions and Synergists

These are the primary shoulder movements that the deltoid is active in together with it's synergists.8 Please note that a synergist can have more than one type of role. For instance, some synergists may be stabilizers during a given action while others may have a redundant role or act as neutralizers.

Action	Deltoid Fibers	Synergists
Forward Flexion	Anterior	Pectoralis major, coracobrachialis, biceps
Extension	Posterior	Teres major, teres minor, latissimus dorsi, sternocostal fibers of pectoralis major
Abduction	All	Supraspinatus, Infraspinatus, Subscapularis, Teres minor, biceps long head when are laterally rotated
Horizontal Abduction	Posterior	Teres major, Teres minor, Infraspinatus
Horizontal Adduction	Anterior	Pectoralis major

Deltoid Trigger Points Causes and Symptoms

As stated above, trigger points can be activated in the deltoid by an impact trauma such as a direct blow or fall on the shoulder; or by over-use of the muscle. Also, any accident that traumatizes the deltoid muscle, such as reaching out to catch oneself during a fall, can activate trigger points. Unlike most other trigger points, however, deltoid trigger points do not refer pain to remote area. The pain tends to be concentrated in the immediate area of the TrP and occurs primarily during shoulder movements while not occurring often during rest. Continuous pain in the shoulder is more likely to come from some other underlying pathology or trigger points in other muscles that refer pain to the shoulder area. For instance, a continuous dull ache in the shoulder is more likely to trigger points in the rotator cuff muscles.

 

When pain occurs in the shoulder joint during passive mobilization with scapula rotation and elevation, as opposed to active shoulder movements, this is more likely to indicate a sprain or subluxation of the acromioclavicular joint, the pain pattern of which mimics the pain pattern of anterior deltoid trigger points.
The pain from deltoid trigger points occurs deep in the deltoid and there may be difficulty in raising the arm or reaching back with the arm at shoulder level. Trigger points of the deltoid tend to occur in the anterior and posterior portions of the muscle although the posterior part rarely develops trigger points alone as the result of activity but rather develops them in association with TrP's in other muscles. Trigger points may sometimes develop in the middle portion of the muscle but how often this occurs, according to various texts, is unclear. Davies reports that the lateral deltoid is the portion where trigger points most often occur while Travell and Simons, and others reports that they are rare. However, when they do occur, the multipennate arrangement of these fibers means that the trigger points are likely to be sprinkled anywhere along lateral upper arm. Davies erroneously reports that the middle fibers are more likely to develop trigger points because this portion of the muscle is the "largest part of the muscle and works the hardest." 9,12,10,11

However, the front deltoid is much more likely to receive overload and trauma during everyday activities and during exercise and sports and there is no basis for claiming that the largest portion of a muscle is the most likely to be overworked. While the arrangement of the lateral deltoid's fibers give it the most strength over a short distance this is due to it's functional roles and not necessarily it's every-day workload. Reliable information on the frequency of middle fiber trigger points over anterior and posterior ones has been difficult to locate for the purposes of this article. Check for trigger points anywhere in the lateral upper arm if you experience deep pain in this area and suspect lateral deltoid trigger points.

Trigger Point Figure 1: Deltoid Muscle Trigger Points
Side View, Anterior and Posterior

Anterior Deltoid Trigger Points

When you have pain and difficulty while combing your hair, eating, or, in general, bringing your hand to your face this may be due trigger points in the anterior (front) deltoid. This part of the muscle is highly likely to receive a traumatic impact in many sports and can be suddenly overloaded by reaching out to catch a fall, such as when one stumbles on the stairs. Overuse occurs in the workplace when having to hold heavy tools or any job that requires frequent forward reaching. Exercise and sports activities that require a great deal of forward shoulder flexion such as swimming, skiing, and ball throwing may overload the muscle. And, as mentioned, shoulder abuse by improper emphasis of the shoulders in resistance training can strain and overload the muscle, setting up trigger points.
Anterior deltoid trigger points are usually located high in the front margin of the muscle, in front of the glenohumeral joint, or lower toward the midpoint. The referred pain pattern is in the area of the front shoulder surrounding the trigger point with some spillover pain further down the arm and posterior to the TP. See Trigger Point Figure 1 above for trigger point locations and Figure 2 below for pain patterns.

Trigger Point Figure 2: Anterior Deltoid Muscle Trigger Points
and Referred Pain Pattern

Posterior Deltoid Trigger Points

The posterior deltoid usually develops TrP's in conjuntion with other muscles such as the long head of the triceps, the latissimus dorsi, and the terses major. They may also be over-exerted, as by poling during skiing or other activities where the arm is frequently extended toward the back. TP's may also be activated by local intramuscular injection of irritable solutions such as B vitamins, penicillin or various vacines, after which the TP's are self-sustaining.
Trigger points in the posterior deltoid tend to be located on the lower posterior margin of the muscle and upward toward the midpoint. The pain is referred to the immediate surrounding area of the TP in the back of the shoulder with some spillover pain further down the arm and anterior to the TP.

Deltoid Trigger Point Self-Treatment

Don't try to use your hands to massage your own deltoids because it is too difficult a position to apply pressure and your hands will become exhausted and over-used. Instead, use a tennis or lacrosse ball against the wall. Place the ball against the trigger point area, lean into it, and roll the ball up and down over the area. It may help to place the ball in an old sock so that the end of the sock can be used as a handle. Also, smaller hard rubber bouncy balls can be used when you are ready to apply more precise pressure.
Alternatively a Knobble self massage tool or an Index Knobber can be used.

Trigger Point Figure 2: Anterior Deltoid Muscle Trigger Points
and Referred Pain Pattern