Why is ATP necessary for muscle contraction?

ATP provides the energy that muscles need to contract and relax in a continuous repetitive pattern. 

During muscle contraction, ATP binds to myosin. This moves the myosin to a high-energy state and activates the myosin head. As soon as actin-binding sites are available, the high-energy myosin head attaches to actin, forming a cross-bridge and pulling the actin filament toward the center of the sarcomere. This shortens the sarcomere and generates tension, resulting in muscle contraction.   

Once the myosin head binds to actin, ATP is hydrolyzed, releasing ADP and a phosphate molecule. With this, the myosin head moves to a low-energy state, which detaches it from the actin filament and breaks the myosin-actin cross-bridge. This allows the sarcomere to return to its original length, resulting in muscle relaxation. 

The myosin releases ADP, which binds to the free phosphate molecule, forming ATP, which powers the next muscle contraction.