Muscular contractions use ATP (“Adenosine Triphosphate) as the direct source of energy. Enzymes are catalysts and named after functions they perform. The ATPase enzyme is responsible for breaking the ATP bonds, and this occurs deep in the muscle fibers. Myosin, thick myofilaments holds the ATPase enzyme and binds with actin to form “cross bridges”. This allows the energy to be released which causes the myosin to detach from the actin. Then the fibers produce force and the ATP molecule splits.