Muscles are special fibers that contract (tighten) and relax to move different parts of the body.
Voluntary muscles are all the muscles you can control by will or thinking, such as your arm muscles.
Involuntary muscles are the muscles you cannot control at will, but work automatically, such as the muscles that move food through your intestine.
Most voluntary muscles cover the skeleton and are therefore called skeletal muscles. They are also called striated (striped) muscle because there are dark bands on the bundles of fiber that form them.
Most involuntary muscles form sacs or tubes such as the intestine or the blood vessels. They are called smooth muscle because they lack the bands or stripes of voluntary muscles.
Most muscles are arranged in pairs, because although muscles can shorten themselves, they cannot make themselves longer. So the flexor muscle that bends a joint is paired with an extensor muscle to straighten it again.
This microscopic cross-section shows striated, or striped, skeletal muscle. It is so-called because its fibers are made of light and dark stripes.
The heart muscle is a unique combination of skeletal and smooth muscle. It has its own built-in contraction rhythm of 70 beats per minute and special muscle cells that work like nerve cells for transmitting the signals for waves of muscle contraction to sweep through the heart.
Your body’s longest muscle is the sartorius on the inner thigh.
Your body’s widest muscle is the external oblique which runs around the side of the upper body.
Your body’s biggest muscle is the gluteus maximus in your buttocks (bottom).
You have more than 640 skeletal muscles and they make up over 40% of your body’s entire weight, covering your skeleton like a bulky blanket. This illustration shows only the main surface muscles of the back, but your body has at least two, and sometimes three, layers of muscle beneath its surface muscles. Most muscles are firmly anchored at both ends and attached to the bones either side of a joint, either directly or by tough fibers called tendons.
Most muscles are long and thin and they work by pulling themselves shorter – sometimes contracting by up to half their length.
Skeletal muscles, the muscles that make you move, are made of special cells which have not just one nucleus like other cells do, but many nuclei in a long fiber, called a myofiber.
Muscles are made from hundreds or thousands of these fibers bound together like fibers in string.
Muscle fibers are made from tiny strands called myofibrils, each marked with dark bands, giving the muscle its name of stripcy or ‘striated’ muscle.
The stripes in muscle are alternate bands of filaments of two substances: actin and myosin.
The actin and myosin interlock, like teeth on a zip.
When a nerve signal comes from the brain, chemical ‘hooks’ on the myosin twist and yank the actin filaments along, shortening the muscle.
The chemical hooks on myosin are made from a stem called a cross-bridge and a head made of a chemical called adenosine triphosphate or ATP.
ATP is sensitive to calcium, and the nerve signal transmitted from the brain that tells the muscle to contract does its work by releasing a flood of calcium to trigger the ATP.
Muscles, such as the biceps and triceps in your upper arm, work in pairs, pulling in opposite directions to one another.