Deltoid muscle

The deltoid is a triangular muscle covering the shoulder joint. Its three parts help lift the arm and contribute to shoulder rotation and stability.

Anterior deltoid: The front portion of the deltoid, arising from the clavicle and attaching to the humerus. It helps flex and internally rotate the arm at the shoulder.
Humerus: The long bone of the upper arm, extending from the shoulder to the elbow. The deltoid inserts on the humerus and moves it at the shoulder.
Axillary nerve injury: Damage to the axillary nerve that can weaken the deltoid and impair shoulder sensation. Deltoid weakness and numbness over the outer shoulder can indicate this injury.
Supraspinatus: A rotator cuff muscle that initiates shoulder abduction and helps stabilize the humeral head. It starts arm elevation, while the deltoid supplies much of the force through a wider range.
Lateral deltoid: The middle portion of the deltoid, arising from the acromion and attaching to the humerus. It is the deltoid portion most associated with lifting the arm sideways.
Clavicle: The collarbone, a slender bone connecting the sternum to the scapula. The deltoid’s anterior fibers originate from its lateral end.
Shoulder dislocation: Displacement of the humeral head from the shoulder joint's normal position. Dislocation can stretch or injure the axillary nerve that supplies the deltoid.
Pectoralis major: A large chest muscle that adducts and internally rotates the arm at the shoulder. Its pull brings the arm toward the body, contrasting with the deltoid’s lifting action.
Posterior deltoid: The rear portion of the deltoid, arising from the scapular spine and attaching to the humerus. It helps extend and externally rotate the arm.
Scapula: The flat shoulder blade that links the upper arm with the clavicle and trunk muscles. Its acromion and spine provide origins for the deltoid’s middle and rear fibers.

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