Muscles are tissues or organs of the human body, of the musculoskeletal system characterized by their ability to contract and generally by a response to a nervous stimulus; The basic unit of the muscle is the myofibril, a very small thread-like structure that in turn is made up of complex proteins, thus being the organ with the greatest adaptation and undergoing modifications that no other organ in the body can undergo through its content and its shape. Being thus damaged by some atrophy, it recovers by reinforcing itself over time, if it is abused or if it is not used, it is damaged in the same way, decreasing the size of the same muscle and its strength, as well as the cells that maintain it. stuck to the bones.

There are three types of muscle tissue: smooth, skeletal, and cardiac tissue. Smooth muscle is visceral or involuntary muscle, composed of spindle-shaped cells with a central nucleus; This muscle is located in the skin, internal organs, reproductive system, large blood vessels and excretory system, being found in other organs that are formed by other tissues such as the heart and intestine, which contain layers of connective tissue.

Muscle tissue is made up of long fibers and surrounded by a cell membrane called the sarcolemma, most of which are attached to various areas of the skeleton by tendons, hence their name, forming most of the body mass of vertebrates; the action of retracting is often visibly noticeable on the skin. Cardiac muscle forms most of the heart and lacks voluntary control, since it is innervated by the autonomic nervous system, and its cardiac contraction mechanism is based on the automatic generation and transmission of impulses.

The functions of the muscles are to produce movement, generate mechanical energy, joint stability, protect against involuntary stimuli, maintain posture, provide heat, stimulate blood and lymphatic vessels, and provide information on any damage that has occurred in any part of the body. The main muscles of the human body are: arms, legs, abdomen, back and buttocks.