To perform Isometric shoulder extension exercise
In arm extension the humerus is rotated out of the plane of the torso so that it points backwards (posteriorly).
The muscles involved in this action are: latissimus dorsi and teres major, long head of triceps, posterior fibers of the deltoid.
Action: Adducts and medially rotates humerus; extends the shoulder joint as in pulling on oars of a rowboat; produces back swing of arm in such actions as walking or bowling; with hands grasping overhead objects, pulls body forward and upward, as in climbing; produces strong down strokes of arm as in hammering and swimming (nicknamed the “swimmer’s muscle”); aids in deep inspiration, sudden expiration as in sneezing and coughing, and prolonged forceful expiration as in singing or playing a wind instrument.
Teres Major: Extends and medially rotates humerus; contributes to arm swinging.
Origin :Inferior angle of scapula
Insertion : Medial lip of intertubercular sulcus of humerus
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.