There are more than 600 muscles in the human body. A type of elastic tissue makes up each muscle, consisting of thousands, or tens of thousands, of tiny muscle fibers. Each fiber consists of many small strands called fibrils.
Impulses from nerve cells control the contraction of each muscle fiber. Muscle strength depends primarily on how many fibers are present.
To fuel a muscle, the body makes adenosine triphosphate (ATP), which muscle cells convert into mechanical energy.
Types of Muscle
Humans and other vertebrates have three types of muscle: skeletal, smooth, and cardiac.
1- Skeletal Muscles
Skeletal muscles move the external body parts and organs. They cover the bones and give the body its shape.
As skeletal muscles only pull in one direction, they work in joints. When one of the muscles in the pair contracts, the other expands, facilitating movement.
Muscles are attached by strong tendons, which are either attached to bones or attached directly. Tendons span the joints, and help stabilize the joint. A person in good health can consciously control their skeletal muscles.
Most visible body movements—such as running, walking, talking, and moving the eyes, head, limbs, or digits—occur when skeletal muscles contract.
Skeletal muscles also control all facial expressions, including smiles, frowns, and movements of the mouth and tongue.
Skeletal muscles are constantly making small adjustments to maintain body posture. They keep a person’s back straight or keep their head in one position. Together with the tendons, they hold the bones in the correct position so that the joints do not dislocate.
Skeletal muscles also generate heat as they contract and release, and this helps maintain body temperature. About 85 percent of the heat the body generates comes from muscle contraction.
Types of Skeletal Muscles
The two main types of skeletal muscle are slow twitch and fast twitch.
Type I, Red, or Slow-twitch Muscles
They are dense and rich in myoglobin and mitochondria. They have capillaries, which give them a red color. This type of muscle can contract for long periods of time without any effort. Type I muscles can sustain aerobic activity using carbohydrates and fat as fuel.
Type II, White, or Fast-twitch Muscles
These muscles can contract quickly and with great force. Contractions are strong but short-lived. These muscle types are responsible for most of the body’s muscle strength and mass gain after weight training intervals. Compared to slow-twitch muscle, it is less dense in myoglobin and mitochondria.
2- Smooth Muscle
Smooth muscles are responsible for movement in the stomach, intestines, blood vessels, and hollow organs. The smooth muscles in the intestines are also called visceral muscles.
These muscles work automatically, without a person being aware that they are using them. Unlike skeletal muscles, they do not rely on conscious thought.
Many different body movements depend on smooth muscle contraction. These include the intestinal walls that push food through, the contraction of the uterus during childbirth, and the pupils contracting and dilating to accommodate the amount of available light.
Smooth muscle is also present within the walls of the bladder and bronchi. The erector spinae muscles in the skin, which erect hairs, are composed of smooth muscle fibers.
3- Cardiac Muscle
Cardiac muscles are responsible for beating the heart and are present only in the heart.
These muscles work automatically, day and night, without stopping. They are similar in structure to skeletal muscles, so doctors sometimes classify them as striated muscles.
The heart muscle contracts so the heart can squeeze blood and then relaxes so it can fill with blood again.
What Can Go Wrong with Muscles?
A wide range of problems can develop with the muscles.
Some common ones are:
- Muscle Cramps, or Charley Horses: These can result from dehydration, low potassium or magnesium levels, certain neurologic or metabolic disorders, and certain medications.
- Congenital Muscle Abnormalities: Some people are born with muscles or muscle groups that don’t develop properly. These abnormalities may be an isolated problem or part of a syndrome.
- Muscle Weakness: Problems with the nervous system can affect the transmission of messages between the brain and muscles.
Muscular Weakness
Muscle weakness can affect people with upper or lower motor neuron disorders or conditions such as myasthenia gravis that affect the area where nerves join muscles. Stroke, spinal cord compression, and multiple sclerosis can also cause muscle weakness.
If a person seeks medical help for muscle weakness, the doctor will perform a physical exam and decide whether additional tests are necessary.
They are likely to use a universal scale for testing muscle strength:
- There is no visible muscle contraction.
- Visible muscle contraction with no or trace movement
- Movement with full range of motion, but not against gravity.
- Movement through full range of motion against gravity but not resistance.
- Move through a full range of motion against at least some resistance that the examiner provides.
- Full power
If a doctor finds evidence of muscle weakness, he or she may order tests to identify the underlying problem. Treatment will depend on the cause.
If the muscles are sore, it could be a sign of infection or injury.
