To perform the isometric shoulder external rotation :
The muscles involved in this action are infraspinatus and teres minor, posterior fibers of deltoid.
Deltoid Muscle :
This thick muscle covers the shoulder protecting the joint. It assists in many movements of the arm, moving it forward, backward, and outward abduction.
It arises from two bones and three places: from the lateral third of the clavicle, from the acromion, and from the spine of the scapula. The fibers cover the shoulder joint to merge as a small tendon which inserts onto the deltoid tuberosity, a small projection on the lateral aspect of the shaft of the humerus, about midway down.
Teres Minor: Modulates action of deltoid, preventing humeral head from sliding upward as arm is abducted; adducts and laterally rotates humerus.
The origin is from Lateral border and adjacent posterior surface of scapula
The insertion is into Greater tubercle of humerus; posterior surface of joint capsule
Infraspinatus: Modulates action of deltoid, preventing humeral head from sliding upward; laterally rotates humerus
The origin is from Infraspinous fossa of scapula
The insertion is into Greater tubercle of humerus