Deltoid Muscle: Location and Actions

The deltoid muscle is a large, triangular, course, and thick muscle which gives the shoulder its shape and contour. Its name is often reported to have derived from the Greek letter Delta (Δ) but it actually derives from the Latin word deltoides 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. ¹Howell SM, Imobersteg AM, Seger DH, Marone PJ. Clarification of the role of the supraspinatus muscle in shoulder function. J Bone Joint Surg Am. 1986;68A:398–404.,²Doyle, James R., and Michael J. Botte. “Chp. 2: Muscle Anatomy.” Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 92-94. Print.

See Deltoid Trigger Points

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.”

Note that although muscles may have a stabilizing effect on joints there is not just one mechanism by which they do this. So although the deltoid is thought to be an important stabilizer it is difficult to be one hundred percent sure how it exerts this effect. The four mechanisms through which muscles provide dynamic stability to joints are:
1) passive tension from the bulk of the muscle itself over the joint
2) contraction which compresses the articular surfaces
3) movement of the joint through muscle action which pulls on the ligaments which then provide stability through their constraints
4) a barrier effect of the contracted muscle
Mechanism two is thought to be the main dynamic stabilizing action of the deltoid. The muscle does not appear to be important in inferior stability of the joint.

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 its 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 shoulder 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. ³ Di, Giacomo Giovanni. “Part 3: Glenohumeral Joint.” Atlas of Functional Shoulder Anatomy. Milan: Springer, 2008. 63-66. Print.,⁴Palastanga, Nigel, Derek Field, and Roger Soames. “Part 2: The Upper Limb.” Anatomy and Human Movement: Structure and Function. Edinburgh: Butterworth Heinemann/Elsevier, 2006. 74-76. Print.,⁵Hammer, Warren I. “Chp. 3: The Shoulder.” Functional Soft-tissue Examination and Treatment by Manual Methods. Sudbury, MA: Jones and Bartlett Pub., 2007. 33-45. Print.,⁶Floyd, R. T., and Clem W. Thompson. “Chp. 3: The Shoulder Joint.” Manual of Structural Kinesiology. Dubuque, IA: WCB/McGraw-Hill, 1998. 41. Print.,⁷Doyle, James R., and Michael J. Botte. “Chp. 2: Muscle Anatomy.” Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 92-94. Print.

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 its 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 supraspinatus and deltoid function synchronously although the supraspinatus, 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. One study that examined the functional role of the supraspinatus with the deltoid in functional planes of motion was a study by Howell, et al. in the Journal of Bone and Joint Surgery, American volume in 1986. They concluded, that the supraspinatus and deltoid muscles are equally responsible for producing torque about the shoulder joint in the functional planes of motion. ⁸Hammer, Warren I. “Chp. 3: The Shoulder.” Functional Soft-tissue Examination and Treatment by Manual Methods. Sudbury, MA: Jones and Bartlett Pub., 2007. 33-45. Print.

Deltoid Muscle: Origins, Insertions, and Actions

Origin:

• Anterior fibers: anterior and superior surfaces of outer third of clavicle and anterior acronium. Many people, especially those who engage in bodybuilding, call these the “front deltoid”. They are also sometimes referred to as the “clavicular fibers” by texts in reference to their origin on the clavicle. You may often see muscle fibers being identified by their origin in this way.
• Middle fibers: lateral margin of the acronium. The middle fibers of the deltoid are frequently referred to as the “medial deltoid” by lifters, bodybuilders, and experts alike but this is anatomically incorrect as the term medial refers to something that is toward the midline of the body. The middle fibers of the deltoid are in fact the furthest away from the midline of the body so the term “medial” should not be used to describe them as it has a specific biomechanical and anatomical meaning and does not just refer to something that is “in the middle of two other things”. It is more correct then, to refer to these fibers as the “middle fibers” and this is how they are usually termed. Some people, however, call them the “lateral deltoid” to refer to their position toward the side of the body. They are also sometimes referred to as the “acromial fibers” in reference to their origin on the acronium.
• Posterior fibers: inferior edge of almost 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. ⁹Floyd, R. T., and Clem W. Thompson. “Chp. 3: The Shoulder Joint.” Manual of Structural Kinesiology. Dubuque, IA: WCB/McGraw-Hill, 1998. 41. Print.,note]Palastanga, Nigel, Derek Field, and Roger Soames. “Part 2: The Upper Limb.” Anatomy and Human Movement: Structure and Function. Edinburgh: Butterworth Heinemann/Elsevier, 2006. 74-76. Print.[/note],¹⁰Doyle, James R., and Michael J. Botte. “Chp. 2: Muscle Anatomy.” Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 92-94. Print.

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. ¹¹Behnke, Robert S. Chp. 3: The Shoulder. Kinetic Anatomy. Champaign, IL: Human Kinetics, 2001. 52 Print.,¹²Doyle, James R., and Michael J. Botte. “Chp. 2: Muscle Anatomy.” Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 92-94. Print.

• The deltoid, together with the supraspinatus, contracts when carrying heavy objects at the side to resist the strong downward pull. The deltoid is also constantly active in positioning the hands for everyday manual tasks, by producing forward flexion of the humerus. ¹³Doyle, James R., and Michael J. Botte. “Chp. 2: Muscle Anatomy.” Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 92-94. Print.

Deltoid Actions and Synergists

These are the primary shoulder movements that the deltoid is active in together with its 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

See Deltoid Trigger Points and Referred Pain