Role of calcium in muscle contraction When the muscle fibre is depolarized then Sarcoplasmic Reticulum releases calcium ions which bind with specific site present on troponin component of actin filament. Increase in Ca++ level leads to the binding of calcium with a subunit of Troponin on actin filaments and thereby remove the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. Thus, sarcomere contracts .