Muscle makes up nearly half the body weight of an adult. More than 600 muscles cover the skeleton and give the body bulk and form. But their main task is to move limbs, push food through the gut, make the heart beat, and control blood flow around the body.

Atypical skeletal or voluntary muscle comprises thousands or millions of fibers whose coordinated contractions cause the whole muscle to contract. The energy for muscular contraction comes from chemical reactions involving fuel and oxygen brought to the muscles through a rich supply of blood vessels. Besides mechanical energy, muscle action produces heat and chemical wastes, which leave through capillaries leading to veins that carry blood from the muscle.

There are three main types of muscle: skeletal, smooth, and cardiac.

Muscles involved with movement are arranged symmetrically—like the bones to which they are attached—about a vertical, central plane. In addition to the surface muscles shown, there are many “deep” muscles, which play equally important roles.

Skeletal muscle

Skeletal muscles are joined to bones and make them move. Groups of skeletal muscles operate the arms, legs, torso, neck, and face. They range in size from tiny muscles rotating the eye to the large, powerful thigh muscles. Each skeletal muscle consists of long, slim muscle fibers from less than .25 inch (a few millimeters) to more than 1 inch long, in bundles bound together by connective tissue. Each fiber has a number of nuclei. Fiber bundles are organized according to the tasks they must perform. Thus, parallel bundles capable of strong contractions form the muscle felt at the side of the neck. And the deltoid muscle the topmost muscle on the outside of the arm has short fiber bundles arranged like the barbs of a feather that produce limited but powerful movements.
The basic units of each fiber are thick, dark filaments and thin pale ones. Seen under the microscope, these give skeletal muscle a striped appearance so that it is also called striped or striated muscle. Because we can usually move skeletal muscles at will, they are also known as voluntary muscles, although they are capable of involuntary reflex movements, too, as when a hand jerks away from a source of heat.

Smooth muscle (far left)— which consists of elongated cells, each with its own nucleus—is found in tissues such as the stomach and the intestines.
Skeletal muscles (left), also called striped or voluntary muscles, are controlled directly by the motor nerves of the central nervous system, and are chiefly concerned with movement They consist of long, thin, striped fibers, each with several nuclei.

Skeletal muscle contracts when it is stimulated by an electrochemical signal from the central nervous system. This acts on nerve endings linked to the muscle fibers. How much a whole muscle contracts depends on how many of its fibers have been activated.

In order to work, a typical skeletal muscle needs to have both its ends connected to the skeleton either directly or via tendons or fibrous sheets called aponeuroses. The fixed muscle end, closest to the body’s center, is called the origin. The other end called the insertion is attached to the bone that actually moves.

Many muscles have names that indicate their function. For instance, flexors bend joints, extensors straighten them. Abductors pull a part of the body away from its central axis, adductors do the opposite. The to-and-fro action of a limb or jaw or eyeball depends upon pairs of muscles acting in opposition to each other. Muscles have to work in opposing pairs because each muscle acts in just one direction. It can pull or squeeze, butthen simply relaxes, for muscles cannot push.
Skeletal muscles can be grouped in four main categories according to their general function: prime movers contract to cause active movement; antagonists act in opposition to prime movers; fixation muscles hold steady such parts as the shoulder blade to provide a base for movements involving other muscles; and synergists combine with prime movers to keep joints still.

Coordinated signals from the central nervous system ensure that opposing muscles do not contract simultaneously to cancel out one another. So when the triceps muscle of the upper arm contracts to straighten the arm, the opposing biceps muscle relaxes. The triceps is then acting as a prime mover and the biceps as the antagonist. But when the elbow is bent, the biceps contracts, the triceps relaxes, and their roles are reversed.
Powerful skeletal muscles moving long bones make these act as levers so that a short muscle movement produces a large limb movement. For example, a muscle contraction of less than 3 inches (8 centimeters) moves the fingers through an arc that is 12 times greater, although the force involved is correspondingly diminished.

A skeletal muscle consists of muscle fibers with shared nuclei, enclosed in a connective tissue membrane through which the fibers receive their blood and nerve supply. The fibers are composed of myofibrils, which contract or relax as filaments of myosin and actin that they contain move against each other.

Smooth muscle

Smooth muscle occurs in the digestive tract, urinary tract, blood vessels, bronchial tree, and other internal structures. Unlike skeletal muscle, a fiber of smooth muscle lacks stripes and has one nucleus instead of many nuclei. Also, it contracts more slowly than a skeletal muscle fiber, tends to contract rhythmically, and is not under direct control of the brain. By contracting or relaxing, smooth muscle narrows or enlarges the diameter of blood vessels to control the blood flow passing through. Similarly, alternate contraction and relaxation of smooth muscle forming stomach and intestine walls drives food through the gut in the action called peristalsis. The rate at which smooth muscle contracts depends on hormones and the autonomic nervous system. Because we cannot normally control its action, smooth muscle is also called involuntary muscle.

Muscles can only contract or relax, which enables them to pull or to be stretched. They cannot push. So to move bones at a joint, there must be opposing muscles or sets of muscles. In the arm, the biceps bends the elbow joint and is called the flexor. The triceps straightens the joint and is called the extensor.

Cardiac muscle

Cardiac muscle is striped like skeletal muscle, but contracts automatically like smooth muscle. Its fibers surround the ventricles the heart’s main pumping chambers and contract rhythmically at intervals regulated by the sinoatrial node, the “pacemaker,” and by the autonomic nervous system.