Skeletal muscle fiber is made up of myofibrils, which are long contractile fibers composed of two essential myofilaments – thin filament and thick filament. Each skeletal muscle fiber contains anywhere from several hundred to several thousand myofibrils. 

The thin filament is composed of the protein actin and a thick filament is composed of the protein myosin. The thin and thick filaments are arranged in an alternating pattern of dark and light bands, forming the sarcomere, which is the fundamental unit of contraction. 

The alternating dark and light bands give the skeletal muscle its distinctively striated appearance.