Muscle is a tissue in animal bodies. Their main purpose is to help us to move our body parts. They are one of the major systems of human and animal bodies. When a muscle is activated it contracts, making itself shorter and thicker, thereby pulling its ends closer.

Types of muscles 
There are three kinds of muscles:
Skeletal muscle, the muscle attached to bones. They pull on bones to make movements. 
Smooth muscle, for example, the muscle in blood vessels and the bladder
Cardiac muscle, the muscle of the heart

Muscle action can be classified as being either voluntary or involuntary.

The skeletal muscles move the limbs (arms and legs). They move the jaw up and down so that food can be chewed. Skeletal muscles are the only voluntary muscles, the only ones that we can choose to move.

The cardiac muscle is the muscle in the heart. When this muscle contracts it pushes blood through the circulatory system. The cardiac muscle is not voluntary. 

The smooth muscles are the other muscles in the body that are involuntary. Smooth muscles are in many places. They are in:
The gastrointestinal system – this includes the stomach and intestines. This is how food moves through us and we take energy from it.
Blood vessels – smooth muscles make blood vessels smaller or bigger. This controls blood pressure.
Hairs – smooth muscle in hair follicles makes your hair stand up when you are scared or get cold.

Muscle structure 
Muscles are made of many muscle cells. The cells contract together to make the muscle get shorter. The muscle cells know to do this together because many of them get information sent to them by nerves. The cells that get the message from nerves tell other cells that are near them. They tell the other cells by sending an electrical current.

Muscle cells are filled with proteins called actin and myosin. These are the proteins that make the muscle contract (get shorter.)

Muscle contraction 
When a nerve tells a muscle to contract, the muscle opens holes in its cell membrane. These holes are proteins that are called calcium channels. The calcium ions rush into the cell. Calcium also comes out of a special place in the cell called the sarcoplasmic reticulum. This calcium sticks to the specialized proteins actin and myosin. This triggers these proteins to contract the muscle.

Contraction also needs ATP. This is the energy that your cells use. It is made from using glucose in the cell. It takes a lot of energy to release contracted muscles. They use most of the energy for building muscles.

Exercise 
Exercise makes muscles get bigger (see hypertrophy). Exercise also makes muscles stronger.  If a person does not exercise, their muscles become smaller and weaker.  This is called muscle atrophy.

Diseases of muscles 
There are many different kinds of muscle diseases. There are three big groups of diseases:
Neuromuscular diseases – these are problems with how the nerves tell the muscles to move. Strokes, cerebral palsy, and Parkinson's disease are neuromuscular diseases.
Motor endplate diseases – these are problems with the place where the nerve tells the muscle to move. Tetanus and myasthenia gravis are motor endplate diseases.
Myopathies – these are problems with the structure of the muscle. Muscular dystrophy, cancers like Ewing's sarcoma, and cardiomyopathy are myopathies.

Related pages 
Bodybuilding
Strength training
Trapezius muscle

References 

Basic English 850 words
Animal tissues
Muscular system