The act of clapping your hands out in front of you with your elbows fully extended is achieved by contracting the pectoralis major muscles.
The pectoralis major muscle is a thick, large fan-shaped muscle that is located in the chest area of the human body. The pectoralis major is the most superficial muscle in the chest area and its functions is associated with the movement and rotation of the shoulder joint. Body movements such as extending the arm across the front part of the body, lowering the arm to the back part of the body, and rotating the upper arm in an inward manner are achieved by the pectoralis major muscles.