Almost everyone has done push-ups at one time or another when in school. They are often used as a test of fitness, training for shoulder girdle and arm strength. However, how you execute the push-up dictates the muscle development that you receive.
Variant 1: Elbows out
The anterior deltoid, pectoralis major, and coracobrachialis are involved in horizontal adduction in the shoulder joint. In this action the upper arms move from a position in line with the shoulders to a vertical position under the shoulders. In the shoulder girdle, there is scapula abduction performed by the pectoralis minor and the serratus anterior.
In the elbow joint there is extension performed by the triceps brachii. In scapula abduction the scapulae move out to the sides of the body from a position alongside the spine. In elbow extension the arm straightens as the upper arm moves away from the forearm.
Variant 2: Elbows in close to the sides
In the shoulder joint the anterior deltoid and the upper pectoralis major are involved in shoulder joint flexion and upward rotation of the scapula. In this action the right scapula rotates counterclockwise and the left clockwise when viewed from the rear. At the same time the scapulae move out away from the spine toward the sides of the body. In the elbow joint there is extension, in which the arms straighten as the upper arms move away from the forearms.