Muscles contracts and extends because of muscle fibers. Each muscle fiber is made up of subunits referred to as sarcomeres which are connected end to end forming a long chain. These think and thick filaments or sarcomeres are the one that does the actual work in contracting of muscles. According McGuire and Beerman (2007) during contraction, the ends of the sarcomeres will pull together towards each other in a way that the sarcomere will shorten causing a contraction. This means that alignment of sarcomere end to end allow muscles to contract in a specific direction. However, shortening of sarcomeres is attributed to internal process of actin and myosin. Actin and myosin are the two main proteins that open inside the muscle. It is the interaction of the two proteins inside the sarcomere that causes shortening and relaxation process. Actin, a round shaped protein which looks like s mall ball, are linked together to form a long chain that appears like a filament referred to as a thin filament. On the other hand, myosin appears like a long thin protein and each has a head. Myosins are linked together for form a bundle into a filament with their heads pointing outward. In a sarcomere, the thin and thin filaments made up of myosin and actin are arranged consequentially next to each other in a way that they always interaction in an organized manner causing a contraction of the muscle. Thin filaments are made up of actin and are attached to the walls of sarcomere. On the other hand, thick filaments are composed of myosin, but they do not run in the entire length of the sarcomere. In their interaction during a contraction, the heads of myosin in the thick filaments bind actin thin filaments pulling the thin filaments towards the center. In the entire length of the sarcomere, myosin binds actin at different sites. In addition, there are two small proteins that help in regulation of muscle contraction. These are tropomysin and troponin. Troponin is associated with regulating thin filaments and usually binds to actin molecule. Tropomysin on the other hand is long thin protei that binds troponin molecule. When troponin bind actin, tropomysoin prevents myosin heads from pulling actin and therefore prevents a contraction.