Explain the sliding filament model of muscular contraction

The presence of calcium ions causes tropnonin to detach from tropomyosin-this exposes the mysoin head binding sites. Myosin head groups attach to the surrounding actin filaments forming a cross bridge. The head group then bends, allowing the myosin (thin filament) to be pulled along along and so overlap more with the  action (thick filament). This is the power stroke. ADP and Pi are released. The cross bridge is broken as new ATP attaches to the myosin head. The head group moves backwards as ATP is hydrolysed to ADP and Pi. This occurs as energy is required for the myosin heads to return to its original state. It can then form a cross bridge with the actin and bend again.