Muscle Biomechanics: Deltoid Muscle

This is another post in a long series that will reveal the lesser-known functions of our muscles.

In fact, almost every muscle in our musculoskeletal system performs more than one movement related to one or two joints.

Specifically, this post will analyze the functions of the Deltoid Muscle which is a thick, triangular shoulder muscle. It gets its name because of its similar shape to the Greek letter ‘delta’. 

Anatomy of the Deltoid Muscle

The Deltoid muscle has a wide origin spanning the clavicle, acromion, and scapula.

It passes inferiorly (below) surrounding the glenohumeral joint on all sides and inserts onto the humerus.


Lateral 1/3 of Clavicle (clavicular part), Acromion (acromial part), Spine of Scapula (spinal part).


Deltoid tuberosity of humerus

Innervation ( the motor neuron responsible for contraction )

Axillary nerve (C5, C6).


The deltoid has three distinct parts. The acromial part, sometimes also known as the middle or central, is the largest and the strongest. It is a multipennate muscle (having the fibers arranged at multiple angles in relation to the axis of force generation).

It arises as four intramuscular septa, which interdigitate with the three tendons at the insertion site (one each for the anterior, posterior, and middle parts). The four septa are connected by short, strong muscle fibers.

The clavicular (anterior) and scapular spinal (posterior) parts are both unipennate (those where the muscle fibers are oriented at one fiber angle to the force-generating axis and are all on the same side of a tendon) and converge directly onto the inserting tendon.

The deltoid muscle has a very broad origin and a narrow base, thus creating its triangular shape. The three parts of the deltoid each have a different origin:

· The clavicular (anterior) part originates from the superior surface and the anterior border of the lateral third of the clavicle.

· The acromial (middle) part arises from the lateral margin and superior surface of the acromion of the scapula.

· The scapular spinal (posterior) part originates from the lateral 1/3 of the spine of the scapula, on the crest.

The muscle fibers then run inferiorly towards the humeral shaft and converge to a narrow strong tendon. It inserts into the deltoid tuberosity located approximately halfway down the lateral aspect of the shaft of the humerus.

Actions of the Deltoid Muscle

The Deltoid muscle has many functions related to the shoulder girdle:

  1. Abduction of the arm – Vertically and horizontally
  2. Adduction of the arm – horizontally
  3. Flexion and Extension of the arm
  4. Internal and External Rotation of the arm

The acromial part “middle fibers” abducts the arm, while the clavicle and scapular parts aid with stabilization and a steady plane for abduction.

The Scapula part assists in horizontal abduction

The Clavicle part of the arm assists with Horizontal Adduction

The Clavicle and Scapula parts Flex and Extend the arm respectively

The Clavicle and Scapula parts act as Internal and External Rotators, respectively.

The deltoid overlies several other muscular structures: the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis), the pectoralis major, and the tendon of pectoralis minor, as well as tendons of coracobrachialis, both heads of biceps brachii and long and lateral heads of the triceps brachii muscle.

Deltoid’s Injuries

Deltoid muscle strain

  • Symptoms include sudden pain in the deltoid muscle at the front of the shoulder is common.
  • Pain is reproduced when lifting the arm from your side to the front keeping it straight against resistance.
  • Or when you lift your arm from your side up sideways against resistance if the strain is in the mid portion or top of the muscle.
  • Tenderness and swelling where the muscle is torn may also be visible and for very severe injuries bruising may develop.

A rotator cuff strain may have similar symptoms to a deltoid strain and is probably far more common so should always be considered.

What is a deltoid strain?

Muscle strains are categorized under grades 1, 2, or 3 depending on how bad they are.

Grade 1 deltoid strain:

Symptoms – You might have tightness in the muscles. You may be able to use your arms properly or do press-ups easily. You probably won’t have much swelling. Trying to lift your arm up sideways or to the front or back of the body probably won’t produce a lot of pain.

Grade 2 deltoid strain:

Symptoms – You probably cannot use your arm properly or do press-ups. You may get occasional sudden twinges of deltoid pain during activity. You may notice swelling. Pressing it causes pain. Lifting your arm up to the front, side, or back against resistance causes pain.

Grade 3 deltoid strain

Symptoms – You will be unable to move your arm and are likely to be in severe pain. Bad swelling will appear immediately. Contracting the deltoid muscle will be painful and there may be a bulge or gap in the muscle. Expect to be out of competition for 3 to twelve weeks or more.

Deltoid Muscle and Tendon Tear
Deltoid Muscle and Tendon Tear

Deltoid Injury Treatment

Grade 1 Deltoid Strain Treatment:

  • Apply ice or cold therapy and compression wrap for the first 24-72 hours (15 mins at a time).
  • After 72 Hours do Shock Termal Therapy alternating heat and cold.
  • Light exercises – 4 sets of 10 repetitions 3 times a week (for example). Gradually build up the weight to strengthen the muscles. The athlete should ease down on training for a week or two but it is unlikely they need to stop unless they are getting pain.
  • A doctor or sports therapist can use sports massage to speed up recovery as well as ultrasound or electrical stimulation.

Grade 2 Deltoid Strain Treatment:

  • Apply ice or cold therapy and compression wrap for the first 24-72 hours (15 mins at a time).
  • After 72 Hours do Shock Termal Therapy alternating heat and cold.
  • A doctor or sports therapist can use sports massage  techniques to speed up recovery and advise on a rehabilitation program.

Grade 3 Deltoid Strain Treatment:

  • follow the P.R.I.C.E. procedure: Protection;Rest;Ice;Compression;Elevation.
  • You should seek medical attention immediately to get the best plan of therapies with a specific muscle tear treatment and physio exercises.

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