Muscles are absolutely crucial to us. They four main things in the body. They cause motion, control movement of substances within the body, keep the posture of a body, and produce heat as a byproduct of contraction. When most people think about muscles, they just think about the muscles you can see, but really, there are three types of muscle. These are skeletal muscle, smooth muscle, and cardiac muscle. Skeletal muscle is the type of muscle that we can see and feel. When a body builder works out to increase muscle mass, skeletal muscle is what is being exercised. Skeletal muscles attach to the skeleton and come in pairs, one muscle to move the bone in one direction and another to move it back the other way. These muscles usually contract voluntarily, meaning that you think about contracting them and your nervous system tells them to do so. They can do a short, single contraction (twitch) or a long, sustained contraction (tetanus). Skeletal muscle can also atrophy, meaning that if I just sit around at the computer all day, my muscles will slowly shrink. Smooth muscle is found in your digestive system, blood vessels, bladder, and airways. Smooth muscle has the ability to stretch and maintain tension for long periods of time. It contracts involuntarily, meaning that you do not have to think about contracting it because your nervous system controls it automatically. An example would be how after I eat a large meal, I don't have to actively think about moving it through my digestive system. It just happens automatically. Cardiac muscle is found only in your heart, and its big features are endurance and consistency. It can stretch in a limited way, like smooth muscle, and contract with the force of a skeletal muscle. It is a twitch muscle only and contracts involuntarily. The human body could not go on just as a marionette can't move without strings, and we will always rely on them.