Briefly describe the mechanism of muscle contraction.

In muscle, myosin protein has 'heads' that act as cross-bridges as they bind to actin, another protein. When ATP binds to myosin, its head detaches from actin, swings forward and rebinds. Rebinding releases ADP and a phosphate ion, and rage the actin filament alongside the replacement of the myosin head, causing the contraction of muscle.