Computer graphics image of the protein molecule actin, which together with myosin, forms the contractile mechanism of muscle fibres. In skeletal muscle the sarcomere (the functional unit of the myofibril, the muscle fibre) is composed of 2 types of myofilaments, arranged in interlocking stacks parallel to axis of myofibril. Thick filaments are composed mainly of myosin, thin filaments being composed mainly of actin. Both types remain constant in length during contraction, which occurs by the thick & thin filaments sliding over one another & causing a reduction in overall length of the sarcomere.