MUSCLES ENGAGED Chest Strengthening the chest muscles will help you push things easily and effortlessly. The muscles in your chest help stabilize your shoulder joint, which reduces your vulnerability shoulder joint injuries. Working your chest muscles also has an aesthetic factor (body contouring). Equipment Used Pec Deck The pec dec (butterfly press or lateral chest press machine) is a fixed weight machine commonly found in your local gym or health club. This fixed weight station isolates the pectoral (chest) muscles and strengthens the upper body. Exercise Instructions Step 1 Start in a seated position with your hands placed at chest height holding the handles to your side. Step 2 Pull the two handles together and fully squeeze your pecs at the end of the motion. Step 3 Open your arms back up to the starting position and repeat.