This is because sacromere is the basic unit of cardiac muscles which is made up of thin and thick filaments. Cardiac muscle is consist of the contractile cells of the heart which it's appearance is striated as a result of alternating thick and thin filaments which is composed of myosin and actin. Actin and myosin are both firm contractile protein filaments, the actin form the thin filaments, and myosin form thick filaments.
