MUSCLE, DRAWING. MUSCLE, DRAWING Contracted myofibril. Myofibril is composed of thin myofilaments actin in red and thick myofilaments myosin in blue, as well as accessory proteins. The sarcomere is limited by the two darker pink strips lines Z on the section. The lines M, sites of connection between the filaments of myosin, are reinforced by the myomesin and the protein M. See image 1212207 for the myofibril at rest.