Answer
The understanding of the process of muscle contraction is based on the sliding theory. According to this theory, of muscle contraction, the myofilaments do not shorten, but they slide over one another-- the thick myosin filaments pulling the actin filaments and the Z discs behind them. This causes the muscle to develop tension. If the sarcomeres shorten, the muscle shortens, but the muscle may develop tension without shortening.
To develop contraction and shorten the myosin head must bind to an ATP molecule. The ATPase in the myosin head then hydrolyses the ATP to ADP and Pi. The energy released by this reaction , activates the myosin head --"cocks' it or changes its conformation. In this new high energy position the myosin head holds on to the ADP and Pi molecule/ion for a while.
Work Step by Step
When the cocked myosin finds an exposed active site on an actin filament, it binds and the actin myosin complex forms a so-called cross-bridge. At this point, the myosin releases the ADP+Pi and bends into a low energy position pulling the actin filament with it. This is the power stroke.
When the myosin actin crossbridge finds a new ATP molecule, it binds with the ATP. This binding of the ATP to the crossbridge destabilizes the crossbridge and frees the myosin. This is the recovery stroke. The myosin head is now free to repeat the whole process: it hydrolyzes the new ATP and uses the energy released to recock. It is then able to attach to a new active actin site farther down the filament., and produce another power stroke.
