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The Deltoid Muscle is a thick, triangular shoulder muscle whose main actions are Abduction (Vertically and horizontally), Adduction (horizontally), Flexion, and Extension of the arm.
Another important action is the Internal and External Rotation of the arm.
It gets its name because of its similar shape to the Greek letter ‘delta’.
The Deltoid muscle has a wide origin spanning the clavicle, acromion, and scapula.
It passes inferiorly (below) surrounding the glenohumeral joint and inserts into the humerus.
Origins
Lateral 1/3 of Clavicle (clavicular part), Acromion (acromial part), Spine of Scapula (spinal part).
Insertion
Deltoid tuberosity of the humerus.
Innervation ( the motor neuron responsible for contraction )
Axillary nerve (C5, C6).
Structure
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 three intramuscular septa, which interdigitate with the three tendons at the insertion site (one each for the anterior, posterior, and middle parts). The three septa are connected by short, strong muscle fibers.
The clavicular (anterior) and scapular spinal (posterior) parts are both unipennate (those where the muscle fibres are oriented at one fibre 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.
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.
The Deltoid muscle has many functions related to the shoulder girdle:
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.
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.
Grade 1 Deltoid Strain Treatment:
Grade 2 Deltoid Strain Treatment:
Grade 3 Deltoid Strain Treatment:
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