Muscular System:

While most people associate muscles with strength, they do more than assist in lifting heavy objects. The 650 muscles in the body not only support movement - controlling walking, talking, sitting, standing, eating and other daily functions that people consciously perform - but also help to maintain posture and circulate blood and other substances throughout the body, among other functions.

Muscles are often associated with activities of the legs, arms and other appendages, but muscles also produce more subtle movements, such as facial expressions, eye movements and respiration, according to the National Institutes of Health (NIH).

Three types of muscles

The muscular system can be broken down into three types of muscles: skeletal, smooth and cardiac, according to the NIH.

Skeletal muscles are the only voluntary muscle tissue in the human body and control every action that a person consciously performs. Most skeletal muscles are attached to two bones across a joint, so the muscle serves to move parts of those bones closer to each other, according to The Merck Manual.

Visceral, or smooth, muscle is found inside organs such as the stomach and intestines, as well as in blood vessels. It is called a smooth muscle because, unlike skeletal muscle, it does not have the banded appearance of skeletal or cardiac muscle. The weakest of all muscle tissues, visceral muscles contract to move substances through the organ, according to The Merck Manual. Because visceral muscle is controlled by the unconscious part of the brain, it is known as involuntary muscle, as it cannot be controlled by the conscious mind.

Found only in the heart, cardiac muscle is an involuntary muscle responsible for pumping blood throughout the body, according to The Merck Manual. The heart's natural pacemaker is made of cardiac muscle that signals other cardiac muscles to contract. Like visceral muscles, cardiac muscle tissue is controlled involuntarily. While hormones and signals from the brain adjust the rate of contraction, cardiac muscle stimulates itself to contract.

Muscle shapes

Muscles are further classified by their shape, size and direction, according to the NIH. The deltoids, or shoulder muscles, have a triangular shape. The serratus muscle, which originates on the surface of the second to ninth ribs at the side of the chest, and runs along the entire anterior length of the scapula (shoulder blades), has a distinctive sawlike shape. The rhomboid major, which attaches the scapula to the spinal column, is a diamond shape.

Size can be used to differentiate similar muscles in the same region. The gluteal region (the buttocks) contains three muscles differentiated by size: the gluteus maximus (large), gluteus medius (medium) and gluteus minimus (smallest), the NIH noted.

The direction in which the muscle fibers run can be used to identify a muscle. In the abdominal region, there are several sets of wide, flat muscles, according to the NIH. The muscles whose fibers run straight up and down are the rectus abdominis, the ones running transversely (left to right) are the transverse abdominis and the ones running at an angle are the obliques. As any exercise enthusiast knows, obliques are among the hardest muscles to develop to achieve "six-pack" abs.

Muscles also can be identified by their function. The flexor group of the forearm flexes the wrist and the fingers. The supinator is a muscle that allows you to roll your wrist over to face palm up. Adductor muscles in the legs adduct, or pull together, the limbs, according to the NIH.

Information Taken from: https://www.livescience.com/26854-muscular-system-facts-functions-diseases.html