The posterior deltoid is the spinal head of the deltoid, performing the opposite movements of the anterior head — extension, external rotation, and horizontal abduction. It is activated during pulling movements such as rows and reverse flyes and is frequently underdeveloped relative to the anterior deltoid in people who perform more pushing than pulling exercises, contributing to shoulder postural imbalance and anterior glenohumeral instability risk.
| Origin | Inferior border of the scapular spine |
|---|---|
| Insertion | Deltoid tuberosity of the humerus alongside the middle and anterior fibres |
| Nerve Supply | Axillary nerve (C5, C6) |
| Blood Supply | Posterior circumflex humeral artery |
| Actions | Extension of the arm; External rotation of the arm; Horizontal abduction of the arm; Assists in arm abduction in the scapular plane |
|---|
During the deceleration phase of throwing, the posterior deltoid provides the braking force against the shoulder being pulled anteriorly by inertia, working with the posterior rotator cuff to prevent anterior subluxation at release.
Posterior deltoid weakness contributes to the rounded shoulder posture and anterior glenohumeral instability seen in throwers and swimmers with training imbalances. Posterior deltoid strengthening through horizontal abduction and extension exercises is a key component of shoulder rehabilitation for anterior instability and rotator cuff impingement. The quadrilateral space through which the axillary nerve and posterior circumflex humeral artery pass is formed superiorly by the teres minor and inferiorly by the teres major, with the deltoid at the lateral boundary.
The posterior deltoid is palpable over the posterior shoulder as the rounded muscle covering the scapular spine, becoming firm during resisted arm extension and horizontal abduction.
Muscle belly injury from sudden eccentric loading during throwing deceleration producing posterior shoulder pain reproduced by resisted extension.
