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. Activities such as lifting weights or heavy boxes require brute strength from the muscles of the arm. Writing, painting, and typing all require speed and precision from the same muscles. Complete athletic activities such as boxing or throwing a ball require arm and hand muscles to be strong, fast, and precise all at the same time.
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. These flexor muscles are all located on the anterior side of the upper arm and extend from the humerus and scapula to the ulna and radius of the forearm. Additionally, the biceps brachii operates as a supinator of the forearm by rotating the radius and moving the palm of the hand anteriorly. On the posterior side of the upper arm is the triceps brachii, which acts as an extensor of the forearm at the elbow and the humerus at the shoulder. The triceps brachii, as its name indicates, has three heads whose origins are on the scapula and humerus. These three heads merge to insert on the olecranon of the ulna.
Most of the muscles that move the wrist, hand, and fingers are located in the forearm. These thin, strap-like muscles extend from the humerus, ulna and radius and insert into the carpals, metacarpals, and phalanges via long tendons. The muscles on the anterior side of the forearm, such as the flexor carpi radialis and flexor digitorum superficialis, form the flexor group that flexes the hand at the wrist and each of the phalanges. The tendons of these muscles pass through a small corridor in the wrist known as the carpal tunnel. Inflammation of this region caused by repetitive stress or trauma may lead to pain and numbness known as carpal tunnel syndrome.
On the posterior side of the arm the extensor muscles, such as the extensor carpi ulnaris and extensor digitorum, act as antagonists to the flexor muscles by extending the hand and fingers. The extensor muscles run as long, thin straps from the humerus to the metacarpals and phalanges. The extensors are generally somewhat weaker than the flexor muscles that they work against, due to the relative ease in opening a hand compared to gripping something firmly.
Two special motions produced by the muscles of the forearm are the supination (anterior rotation) and pronation (posterior rotation) of the forearm and hand. Supination is produced by the biceps brachii of the upper arm and the supinator muscle of the forearm. Pronation is likewise produced by the pronator teres of the forearm. Both supinator and pronator teres muscles have their origins on the humerus and ulna and insert on opposite sides of the radius to roll the wrist in opposite directions.