Scapular Spinal Part of Deltoid Muscle

Origin

The scapular spinal part of the deltoid muscle originates from the inferior aspect of the spine of the scapula.

Insertion

The muscle bellies of the acromial, clavicular, and scapular spinal parts of the deltoid muscle travel inferolaterally and converge to a single tendon. This tendon is short, thick and inserts onto the deltoid tuberosity of the humerus.

Key Features & Anatomical Relations

The deltoid muscle is the thick triangular muscle that gives the shoulder its rounded contour. It is composed of three heads:

- a clavicular part, which is the anterior unipennate portion;

- an acromial part, which is the middle multipennate portion;

- a scapular spinal part, which is the posterior unipennate portion.

The deltoid muscle is located:

- superficial to the glenohumeral (shoulder) joint, the proximal tendons of the biceps brachii and triceps brachii muscles, and the axillary nerve;

- deep to the deltoid fascia and the superior lateral brachial cutaneous nerve;

- lateral to the pectoralis major, trapezius and infraspinatus muscles.

Actions & Testing

Regarding the scapular spinal part of deltoid specifically, it is involved in multiple actions:

- extends the arm at the glenohumeral (shoulder) joint;

- laterally rotates the arm at the glenohumeral joint;

- transversely abducts the arm at the glenohumeral joint (i.e., it abducts the flexed arm along the transverse plane);

- also helps stabilize the glenohumeral joint during abduction of the arm.

The deltoid muscle can be tested by abducting the arm beyond the 30° position, against resistance, during which it can be seen and palpated (Standring, 2016).

References

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41st edn.: Elsevier Limited.