What is the sliding filament theory of muscle contraction?

There are two main muscle filaments in muscle fibres: Actin and myosin filaments. Actin filaments are thin and have binding sites for the globular heads of myosin filaments. Myosin filaments are thick and formed of a fibrous "tail" and globular "head". These two filaments overlap and interact to form muscle fibres. The functional unit of a muscle fibre is called a sarcomere.

During the contraction of a muscle fibre, calcium moves into the sarcomere and binds to the protein troponin on actin filaments revealing myosin head binding sites on actin filaments. Myosin heads containing ADP form crossbridges with the actin filaments by binding to the myosin head binding site on actin. The myosin head then releases the ADP molecule causing the myosin filament to move to the cocked position, pulling the actin fibre towards the middle of the sarcomere in a "power stoke" . ATP then attaches to the bound myosin head, breaking the myosin head - actin cross bridge. The enzyme ATPase then hydrolyses ATP to ADP + inorganic phosphate (Pi) providing energy for the myosin head to return to it's original uncocked position. As the myosin head now contains ADP it is ready to form another crossbridge, starting the cycle again.