The presence of calcium in the sarcoplasm is directly responsible for __________. Multiple choice 1 decreasing the length of the myosin myofilament energizing the myosin heads degrading the actin binding sites on the myosin the movement of actin towards the M line exposing the binding sites on actin 2. Rigor mortis occurs after death because __________. Multiple choice 2 actin binding sites become permanently blocked so that crossbridge formation cannot occur no calcium is present to bind to the regulatory protein tropomyosin detachment of crossbridges does not occur due to the lack of ATP the power stroke becomes stronger after death the muscle fibers release sodium into the extracellular fluid