2. Sliding Filament Theory Muscle contraction occurs by a sliding filament mechanism whereby the sarcomeres shorten (the Z-lines come closer together) by the action of the actin filaments sliding over the myosin filaments. Myosin filaments may look somewhat like a golf club but they are not inflexible. In fact, muscle contraction would be impossible if the myosin molecules did not have a "hinge" along the shaft that allows for a ratchet movement of the head. The force behind muscle contraction is the ratchet movement of these tiny myosin heads toward the center of their sarcomere. This ratchet movement occurs many times during a muscle contraction. Electron microscopy combined with chemical experiments show that muscle is composed of 2 contractile proteins: a) Thin filaments: actin, attached to Z line, found in both A and I bands b) Thick filaments: myosin, found in A band
