Muscles work in pairs

The biceps can bend your elbow, but it cannot straighten it again. That takes the triceps.

Origin and insertion

To move a bone, a muscle has to be anchored to something that stays still. The end fixed to the stable bone is the origin, and the end attached to the bone that moves is the insertion. When the muscle contracts, it pulls the insertion toward the origin. The biceps brachii is a good example. The prefix bi means it has two origins, or heads. The long head starts on the supraglenoid tubercle just above the shoulder socket, and the short head on the coracoid process, a hook at the front of the shoulder blade. The two merge into one belly, and the tendon crosses the front of the elbow to insert on the radial tuberosity, a bump on the radius. A flat extension of it, the bicipital aponeurosis, blends into the fascia of the forearm.

The triceps brachii has three heads. The long head starts on the shoulder blade just below the socket, and the lateral and medial heads start on the back of the humerus. All three join one tendon that inserts on the olecranon, the point of your elbow.

Prime mover, antagonist, synergist

Most movements use several muscles at once, so anatomists give them roles. The muscle that does most of the work is the prime mover, or agonist. A muscle that helps it is a synergist, and a synergist that holds the bone of the origin steady is called a fixator. A muscle with the opposite action is the antagonist. For bending the elbow to lift a cup, textbooks name the biceps as the prime mover and the brachialis, which lies under it, as a synergist. Some anatomy reviews go further and call the brachialis the main bender of the elbow, with the biceps a powerful muscle for turning the palm up. Either way, the triceps is the antagonist of elbow bending.

The roles swap when the movement reverses. To straighten the elbow, the triceps is the agonist and the biceps is the antagonist. The thigh works the same way: the quadriceps femoris, a group of four muscles at the front, straightens the knee, and the hamstrings, a group of three at the back, bend it.

Why it takes two

A muscle can only pull. Once the biceps has shortened and bent the elbow, nothing in the biceps can push the forearm back. Gravity does it when the arm hangs down, but to straighten the elbow against resistance, as in pushing a heavy door open, the triceps has to pull from the other side. Antagonists also do a second job. They hold a limb in position, as when you keep your arm stretched out, and they check fast movements, braking a swing before the joint reaches its limit. Choose Bend, then straighten in the model. The biceps shortens while the triceps is stretched; then the triceps shortens, and near the end the biceps tightens briefly to stop the swing.

Shortening, lengthening, holding

A muscle does not have to get shorter to be working. When you lift a weight toward your shoulder, the biceps shortens while it pulls; this is a concentric contraction. When you lower the same weight slowly, the biceps is still pulling, but it lengthens as fewer cross-bridges are switched on; this is an eccentric contraction. When you hold the weight still, or try to lift something too heavy to move, the muscle makes tension without the joint angle changing; this is an isometric contraction. Isometric contractions work all day long to keep your posture. Choose Lift and lower a weight in the model and watch the label for the biceps change between all three.

Reading muscle names

Muscle names are clues. Bi and tri give the number of origins. A flexor decreases the angle at a joint and an extensor increases it, so the flexor digitorum muscles bend the fingers and the extensor digitorum straightens them. Size words turn up too: gluteus maximus, medius and minimus are the largest, the medium and the smallest of the buttock muscles.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.