Full Muscles of the Arm and Hand Description. The muscles of the upper arm are responsible for the flexion and extension of the forearm at the elbow joint. Flexion of the forearm is achieved by a group of three muscles – the brachialis, biceps brachii, and brachioradialis.uscles of the Arm and Hand. The muscles of the arm and hand are specifically designed to meet the body’s diverse needs of strength, speed, and precision while completing many complex daily tasks.