The muscles in the human body that are connected to bones and aid in the movement of the are called skeletal muscles. These muscles are connected to bones by . Within each of the skeletal muscles are bundles of muscle fibers. These bundles are also called . Each skeletal muscle fiber contains many , 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 .