Muscles and nerve fibers allow us to move our bodies. They enable our internal organs to function. The human body has over 600 muscles, which make up around 40 percent of our bodyweight.

All muscles are made of a kind of elastic tissue.

Each muscle consists of thousands, or tens of thousands, of small musculus fibers. Each muscle fiber is about 40 millimeters long. It consists of tiny strands of fibrils.

Each muscle fiber is commanded by a nerve, which makes it contract. A muscle’s strength depends mainly on how many fibers are present.

To fuel a muscle, the body metabolizes food to make adenosine triphosphate (ATP), and muscle cells turn ATP into mechanical energy.

Types of muscle

Humans and other vertebrates have skeletal, smooth, and cardiac muscles.

Skeletal muscles

Muscles are key to movement, posture, and internal bodily functions.

Skeletal muscles move the external parts of the body and the limbs. Skeletal muscles cover the bones and give our bodies their shape.

For every skeletal muscle in the human body there is an identical one on the other side.

There are about 320 pairs of identical bilateral muscles. When one muscle contracts, the other expands, and this allows movement.

The muscles are attached to strong tendons, and the tendons are either attached to or directly connected to the bones. The tendons extend over the joints, and this helps to keep the joints stable. We consciously control skeletal muscle.

Most of the movements that we can see occur when the skeletal muscles contract. These include moving our eyes, head, arms, fingers, running, walking, and talking.

Facial expressions, such as smiles, frowns, mouth, and tongue movements are all controlled by the skeletal muscles.

Skeletal muscles are continuously making tiny adjustments to maintain posture. They keep a person’s back straight or hold their head in one position. The bones need to be kept in the right position so that the joints do not dislocate. The skeletal muscles and tendons do this.

Skeletal muscles also generate heat when they contract and release. This helps maintain body temperature. Nearly 85 percent of the heat produced by the body is due to muscle contraction.

Types of skeletal muscle

Skeletal muscles are divided into different types.

The two main types are slow twitch or fast twitch muscles.

Type I, red, or slow twitch muscles: These are dense and have capillaries. They are rich in myoglobin and mitochondria. This gives them their red color. This type of muscle can contract for a long time without much effort. Type I muscles can sustain aerobic activity using carbohydrates and fats as fuel.

Type II fast twitch muscles: These muscles can contract rapidly and with a lot of force. Contraction is strong but short-lived. This type of muscle is responsible for most of our muscle-strength, and our increase in mass after periods of weight training. It is the least dense in myoglobin and mitochondria.

Striated muscles

Skeletal muscles are striated muscles. They consist of thousands of sarcomeres, or muscle units. Smooth muscles are not striated.

A striated muscle looks striped under a microscope, because each sarcomere is made up of parallel bands of different materials.

When the bands in the sarcomeres relax or contract, the whole muscle extends or relaxes.

Different bands within each muscle interact, allowing the muscle to move powerfully and smoothly.

Cardiac muscles

Cardiac muscles are responsible for heartbeat. They only exist only in the heart.

The cardiac muscles work without stopping, day and night. They work automatically, but they are similar in structure to the skeletal muscles. They are sometimes classified as striated muscles.

They make the heart contract so that the heart can squeeze our blood, and release so that the heart can fill up with blood again.

Smooth muscles

Smooth muscles are responsible for movements in the stomach, intestines, heart, arteries, and hollow organs. The smooth muscles in the bowel are also called visceral muscles.

These muscles are activated automatically. We are not aware that we are using them. Unlike skeletal muscles, they do not depend on conscious thought.

The smooth muscles in the walls of the intestines contract and push food forward. During childbirth, the smooth muscles in a woman’s uterus contract. Our pupils shrink and expand, depending on how much light there. These movements depend on smooth muscle movements.

Smooth muscles are also present within the walls of the bladder, the bronchi, and the arrector pili in the skin, which makes the hair stand up.

What can go wrong with muscles?

A wide range of problems can arise with muscles.

Some common ones are:

Stretching before exercise can help prevent muscle cramps.

Muscle cramps, or Charley horses: these can result from dehydration, tight calf muscles, low levels of potassium or magnesium, neurologic or metabolic disorders, or some drugs.

Congenital muscle abnormalities: some people are born with muscles or groups of muscles that are not properly developed. This can be an isolated problem or part of a syndrome.

Muscle weakness: problems with the nervous system can mean that messages are not transmitted effectively between the brain and the muscles.

This can happen in upper or lower motor neuron dysfunction, or conditions such as myasthenia gravis, which affect the area where the nerves join the muscles. Stroke, spinal cord compression, and multiple sclerosis can all lead to muscle weakness.

If a patient seeks medical help for muscle weakness, the physician will carry out a physical examination and grade the strength of the patient’s muscles before deciding whether additional tests are needed.

A universal scale for testing muscle strength is as follows:

0: No visible muscle contraction

1: Visible muscle contraction with no or trace movement

2: Limb movement, but not against gravity

3: Movement against gravity but not resistance

4: Movement against at least some resistance supplied by the examiner

5: Full strength

If a physician finds evidence of muscle weakness, they may order tests to find out what the underlying problem is. The treatment will depend on the cause.

Muscle pain can be a sign of infection or injury.