There are several different sets of muscle working in the arm. The biceps brachii and triceps brachii, are more commonly known as the biceps and triceps, respectively. The biceps in the front of the upper arm are responsible for flexing or bending the forearm at the elbow. The triceps in the back of the upper arm are responsible for extending or straightening the arm at the elbow. The triceps are also responsible for extending the arm at the shoulder.
The forearm also has anterior (front) and posterior (back) muscles. Some of these muscles attach from the forearm to wrist bones and are used to move the wrist. Other muscles from the forearm attach to the fingers, and are used to move the fingers. Muscles in the front of the forearm bend the wrist and fingers. Muscles in the back of the foreard straighten the wrist and fingers.
The trapezius muscles are back muscles that connect between the neck, shoulders, and the thoracic vertebra. They help move the scapula and support the arm. The latissimus dorsi, or lats, connect from the shoulder to the hip, and help pull the arm down towards the pelvis, or pull the body up when doing chin-up exercises.
Other muscles, such as the deltoid muscles in the shoulders and pectoralis muscles in the chest, are also involved in shoulder and arm movement.