Deltoid
Origin: Clavicle (collarbone), acromion, and spine of the scapula.
Insertion: Deltoid tuberosity of the humerus (upper arm bone).
Action: The deltoid muscle is responsible for shoulder abduction when the arm is moved away from the body, as well as flexion and extension.
Supraspinatus
Origin: Supraspinous fossa of the scapula.
Insertion: Greater tubercle of the humerus.
Action: Initiates shoulder abduction and helps stabilize the shoulder joint.
Infraspinatus
Origin: Infraspinous fossa of the scapula.
Insertion: Greater tubercle of the humerus.
Action: Performs external rotation of the shoulder joint.
Teres Minor
Origin: Lateral border of the scapula.
Insertion: Greater tubercle of the humerus.
Action: Assists in external rotation of the shoulder.
Subscapularis
Origin: Subscapular fossa of the scapula.
Insertion: Lesser tubercle of the humerus.
Action: Performs internal rotation of the shoulder joint.