The term 'UPPER BODY MUSCLE' can be broken down into three parts: PEC, which is a 3-letter acronym. PEC stands for pectoralis, which is a large muscle located in the chest and upper arms. It is responsible for the movement of the shoulder joint and is used in activities such as pushing, pulling, and lifting. The pectoralis is one of the most important muscles in the upper body and is essential for maintaining good posture and strength.