The biceps and triceps muscles are used to bend and extend the arm, while the deltoids are the muscles that lift the arm. When the bicep muscle contracts or flexes, it bends the forearm toward the shoulder. When the tricep flexes, it extends the forearm, straightening the arm. The bicep and tricep do not contract simultaneously. When one muscle is contracted, the other relaxes.
The deltoid muscles consists of three heads: the anterior, lateral, and the posterior heads. The anterior head lifts the arm up to and above the front of the body. The lateral head lifts the arm up to and above the side and the posterior head raises the arm toward the rear of the body, past the back. The three deltoid heads attach to the humerus, which is the long bone of the arm, extending from shoulder to elbow. The anterior deltoid contributes to many chest-related movements, while the lateral deltoid is dedicated to lateral or side movements. Finally, the posterior deltoid works in conjunction with the rhomboid and teres major muscles in back-related movements. While the bicep and tricep muscles can bend or extend the arm after it is raised by the deltoid, they do not raise actually the arm.