The muscles in the human body that are connected to bones and aid in the movement of the skeleton are called skeletal muscles. These muscles are connected to bones by tendons. Within each of the skeletal muscles are bundles of muscle fibers. These bundles are also called fasciles. Each skeletal muscle fiber contains many myofibrils, which are rod-shaped and contains many sarcomeres (the functioning unit of muscle tissue.) Sarcomeres are made up of thin filaments (composed mainly of the protein ) and thick filaments (composed mainly of the protein .) The actin of the outermost myofibril is connected to the cell membrane by a protein called .