Sliding Filament Theory

At the start of contraction, stored calcium ions are released into the cytoplasm. Calcium ions expose binding sites on the actin molecules. Globular heads on the myosin can bind to these sites, forming cross bridges. Repeated binding and release moves the filaments relative to one another and as this occurs simultaneously in many sarcomeres, the entire muscle shortens or contracts. This process requires energy in the form of ATP.
