Sliding Filament Model of Muscle Contraction
Notice that the binding of calcium to the troponin-tropomyosin complex causes the complex to change shape and unblock the actin binding site. The activated myosin immediately attaches to the actin binding site and bends at the hinge region as a stretched spring would release the energy that stretched it. Since myosin and actin are bound, myosin's change in shape slides the actin fiber. The myofibril remains contracted until myosin binds again with ATP.