Shoulder Joint

(Lateral View)

In the following images the direction of the arrows indicates the direction in which force is applied to the bone when the muscle contracts.

The flexors of the shoulder include (from anterior to posterior) the acromiodeltoid, the spinodeltoid , and the teres major. The deltoid muscles have a slightly different angles of action and insert along the deltoid ridge of the humerus. The teres major inserts on the medial surface of the humerus.

One of the extensors of the shoulder is the supraspinatus (4) which inserts on the greater tubercle of the humerus.

Shoulder Joint

(Anterolateral view)

The forelimbs are held under the body by the muscles of the pectoralis group. The origins of these muscles are along the sternum and they insert along the pectoral ridge. Their action depends on the angle of their attachment. The pectoralis major, pectoralis minor, and xiphimumeralis all adduct and retract the humerus.

The acromiodeltoid flexes and rotates the humerus.

The supraspinatus
extends the humerus.

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