The answer is sliding filament.
Muscle fibers are made up of myosin and actin filaments that straddle one another in alternating sequence. The myosin has myosin heads that act on the actin and enable the myofibrils to slide across each other in opposite directions. This causes the muscle to shorten hence contract. This is an energy-requiring process (hence hydrolyses ATP) and also requires calcium ions. In an animation, the myosin would seem to ‘walk’ on the actin filament.
