Signal for contraction increase $Ca^{++}$ level many folds in the sarcoplasm. $Ca^{++}$ now binds with sub-unit of troponin (troponin "C") which is masking the active site on actin filament and displaces the sub-unit of troponin. Once the active site is exposed, head of the myosin attaches and initiate contraction by sliding the actin over myosin.