Muscle Types, Antagonistic Pairs, and Movement

Muscles are tissues that produce movement by contracting, and the body has three types: skeletal muscle, smooth muscle and cardiac muscle. In brief: skeletal muscle moves the bones and is under voluntary control, smooth muscle moves materials through organs such as the gut and blood vessels, and cardiac muscle pumps the heart. Skeletal muscles work in antagonistic pairs, such as the biceps and triceps, because a muscle can only pull when it contracts and cannot push.

Every movement, from a heartbeat to a sprint, results from muscle contraction. This guide describes the three muscle types, how a muscle contracts, how antagonistic pairs move a joint, compares the types in a table, works through the movement of the elbow and corrects common mistakes.

What are muscles?

Muscle tissue is specialized to shorten, or contract, when stimulated, using energy from ATP. About 40 percent of the mass of a typical adult is skeletal muscle. All muscles have protein filaments, actin and myosin, that are responsible for contraction. Muscles can only contract actively and relax. They return to their original length passively, by the action of another muscle or by elastic recoil.

What is skeletal muscle?

Skeletal muscle is attached to bones by tendons. Its cells are long fibers with several nuclei and a striped (striated) appearance under the microscope, caused by the regular arrangement of actin and myosin filaments. It is under voluntary control, which means you decide when to move it, and it can contract strongly and quickly, but it tires. There are about 600 skeletal muscles in the human body. Examples include the biceps, quadriceps and the diaphragm.

What is smooth muscle?

Smooth muscle forms the walls of hollow organs such as the stomach, intestines, bladder and blood vessels, and the muscles of the iris of the eye. Its cells are short and spindle-shaped, with one nucleus and no stripes. It is under involuntary control, which means it works automatically, controlled by the autonomic nervous system and hormones. It contracts slowly and can stay contracted for long periods without tiring. Peristalsis in the gut is an example of smooth muscle action.

What is cardiac muscle?

Cardiac muscle is found only in the walls of the heart. It is striated like skeletal muscle but involuntary like smooth muscle. Its cells are branched and joined end to end by intercalated discs, which allow electrical signals to pass quickly from cell to cell, so the cells contract together. It has many mitochondria and a rich blood supply, and it does not fatigue under normal conditions. It contracts rhythmically without any nerve signal, controlled by the pacemaker cells of the heart.

Muscle types compared

How does a muscle contract?

A skeletal muscle fiber is packed with units called sarcomeres, which contain overlapping thin filaments of actin and thick filaments of myosin. When a nerve signal arrives, calcium ions are released inside the fiber. The myosin heads attach to the actin, bend and pull the actin filaments toward the center of the sarcomere, then detach and re-attach further along. This process uses ATP and is described by the sliding filament model. The filaments themselves do not shorten, but the sarcomeres do, so the whole muscle becomes shorter.

Muscles need a lot of energy. They obtain it by aerobic respiration. During intense exercise, when oxygen is insufficient, muscle cells also make ATP by anaerobic respiration, producing lactic acid, and this is one reason for fatigue.

What are antagonistic muscle pairs?

A muscle pulls but cannot push, so to move a joint in two directions two muscles are needed, which work in opposite ways. They are called an antagonistic pair. When one contracts (the agonist or prime mover), the other relaxes (the antagonist) and is stretched. The roles then reverse for the opposite movement.

• Biceps and triceps in the upper arm: the biceps bends (flexes) the elbow, and the triceps straightens (extends) it.
• Quadriceps and hamstrings in the thigh: the quadriceps straighten the knee, and the hamstrings bend it.
• Tibialis anterior and gastrocnemius in the lower leg: one lifts the foot, and the other points the toes downward.

Worked example: bending the elbow

Describe how the arm lifts a book from the table to the shoulder.

1. The brain sends nerve signals to the biceps. The biceps (flexor) contracts and shortens.
2. The biceps is connected by tendons to the shoulder blade at one end and to the radius in the forearm at the other. When it shortens, it pulls on the radius.
3. The elbow is a hinge joint. The forearm acts as a lever around the elbow, and it rotates upward.
4. While the biceps contracts, the triceps (extensor) relaxes and lengthens, so that it does not oppose the movement.
5. To lower the arm, the triceps contracts and the biceps relaxes. Lowering the book slowly also involves controlled lengthening of the biceps.

In this lever, the biceps attaches close to the joint, a few centimeters from the elbow, while the book is about 30 centimeters away. The muscle therefore must produce a force several times greater than the weight of the book. For example, with the muscle attached 4 cm from the elbow and a 2 kg book 32 cm away, the muscle force is about 2 × 32 ÷ 4 = 16 kg of force (ignoring the weight of the forearm), which shows that the system trades force for speed and range of movement.

How was muscle contraction explained?

In the 1950s Hugh Huxley and Andrew Huxley (who were not related), together with Jean Hanson and Rolf Niedergerke, independently proposed the sliding filament theory, based on observations of the striped pattern during contraction. Their work was supported by electron microscopy. Later research described the molecular movements of the myosin heads in great detail.

Which mistakes are common?

Mistake 1: Saying that muscles push

Muscles only pull when they contract. Opposing muscles are needed to reverse the movement.

Mistake 2: Thinking the antagonist is inactive

It relaxes, but it also controls the movement and helps stabilize the joint.

Mistake 3: Saying that smooth muscle is weak

It is slower but can generate strong, sustained force, as in the uterus during labor.

Mistake 4: Assuming that the heart is smooth muscle

Cardiac muscle is striated and has intercalated discs. It is a separate type.