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Muscles are tissue in your body that connects two bones. About 640 muscles are found in the human body. They make up about 40% of your total body weight. There are three types of muscle tissues, cardiac, smooth, and skeletal. Each of these muscles have a certain job in order to keep us functioning. The cardiac muscle is located in the heart it works by its self with no thought, which means its involuntary. It pumps blood from your heart into vessels of the circulatory system. The Skeletal is one of the major muscle types. The skeletal Produces movement, maintains posture, stabilizes joints, and generates heat. They are usually attached to bones by collagen fibers also known as tendons. Unlike the cardiac, the skeletal is voluntary control which means it is usually controlled by you. The smooth muscle is involuntary just like the cardiac and are found in the walls of hollow organs like the stomach and esophagus. They tighten up and relax to move food through the body. Muscle contractions can be divided into three categories, isotonic, isometric, and isokinetic. The isotonic is what causes the muscle to change length and can be concentric contractions or eccentric contractions. concentric contractions are what causes the muscle to shorten as it contracts. Like when you extend your leg your quad muscle contracts to straighten your knee. Eccentric contractions is lengthening when you move your arm down the muscle contracts and extends. The isometric contraction is when the muscle is still so it doesn’t extend nor does it contract. isokinetic muscle contraction produces movements of a constant speed. We don’t use this in our day to day life but if you swim it is used commonly when doing a breaststroke. In addition, we have two types of muscle fibers slow twitch and fast twitch. Slow twitch uses oxygen to generate fuel for longer extended muscle contractions. These slow twitch muscle fibers can go a nice amount of time until going fatigue, like when running long-distance or bicycling. Fast twitch can be used when you are sprinting or jumping because the fibers fire rapidly.