Answer
Generally, contraction means that the muscle fiber develops tension. However, in some kinds of contraction, shortening does not not take place.
The sliding filament theory of muscle contraction holds that the exposure of active actin sites leads to repetitive binding of myosin to actin and resultant the sliding of thick filaments over thin filaments. The myofilaments do not get any shorter during contraction, but the thin filaments slide over the thick filaments pulling the Z-discs behind them; this causes each sarcomere as a whole to shorten.
Energy is needed for the initiation of the process of contraction. This energy is supplied by ATP bound to the myosin heads. The energy is released when the ATPase enzyme ( also in the myosin head) hydrolyses the ATP to give ADP+Pi. Energy is made available and the myosin keeps the ADP and Pi still bound to it.
The energy released changes the myosin head to its active high energy state (cocks the myosin head), in which conformation it is ready to bind actin. The binding of myosin to an exposed actin site on a thin filament forms a cross bridge between myosin and actin, and the ADP and Pi molecules are released. The myosin head then flexes /bends to a low energy position, pulling the thin filament with it. This is called the "power stroke." The myosin head remains bound to actin until it encounters another ATP molecule.
Work Step by Step
When the myosin head encounters and binds to a new ATP molecule, this destabilizes the actin-myosin bond and breaks the cross-bridge and the myosin begins the preparation for the execution of a new power stroke.
