Shoulder Joint Muscles 1) Deltoid O: Ant fibers: clavicle Middle: Acromion Posterior: Spine of Scapula I: Deltoid tuberosity A: Ant: abduction, flexion, horizontal adduction, internal rotation Middle: Abduction Posterior abduction, extension, horizontal abduction, external rotation N: Axillary 2) Pectoralis Major O: Medial half of clavicle, costal cartilage of 1st 6 ribs and sternum I: intertubercular groove of humerus A: Upper: internal rotation, horizontal adduction, flexion, abduction, adduction Lower: internal rotation, horizontal adduction, adduction and extension N: Upper : lateral pectoral Lower: Medial pectoral 3) Latissimus dorsi O: Posterior crest of ilium, sacrum, and lumber and T6-12, lower 3 ribs