An estimated 850 muscles exist in the human body, although scientists disagree about the exact number. Your muscles make up approximately 40 percent of your total body weight. Major activities such as running or jumping, as well as minor ones such as blinking, talking and swallowing, depend on muscle contraction and relaxation. Muscles also allow your inner organs, including your blood vessels and digestive and respiratory systems, to function.
Skeletal Muscle
Skeletal muscles are the most visible of the body's muscles, since they are attached to your skeleton and allow your body's movement. These muscles are voluntary, meaning you can control their movement. They have a striated or striped appearance because of the light and dark pattern of proteins inside each muscle cell. Skeletal muscles work in pairs, one contracting while the other relaxes to facilitate movement, according to the Best Health website.
Smooth Muscle
Internal organs, such as the digestive system, bladder, lungs, glands, blood vessels and uterus are also composed of muscle, called smooth muscle. Smooth muscle is capable of stretching and maintaining tension, and is involuntary, performing actions such as constricting the pupils of the eyes or forming goosebumps on the skin. Smooth muscle differs from skeletal and cardiac muscle in that it is not striated. Peristalsis, the wave-like process by which food is moved through the body, is carried out by the smooth muscles of the digestive tract walls.
Cardiac Muscle
Cardiac muscles, or myocardium, are found only in the heart, and their contraction is responsible for the pumping of blood throughout the body. Muscles of the heart work involuntarily. You cannot consciously control them since they are regulated automatically by your nervous system. Like skeletal muscle, heart muscles have striated fibers. Specialized cells within the cardiac muscles keep your heart beating regularly.
Muscle Function
Muscles have the job of producing the body's movement, maintaining its posture, and generating heat, according to the FAQs.com website. Your muscle fibers contract, maintaining muscle tone, even as you sleep or rest. Muscles also serve to help maintain body temperature, since heat is a byproduct of any muscle activity.
Muscle Health
Aging causes a natural decline in the size and strength of your muscles, resulting in less flexibility and more limited movement. Counteract some of the effects of aging on your muscles by following a regular program of exercise, both for aerobic conditioning and for building strength. Aerobic activities such as walking, biking and jogging help condition the heart and lungs. To actually increase the size and power of your muscles requires that you also perform anaerobic exercise, such as weight lifting. Such strength training forces the body to work harder, building the capacity of the muscles.
