In the center of the sarcomere is the A band - the myosin filaments. The dark stripe within the H zone is called the M band - proteins that help hold the myosin filaments. When the muscle contracts, the sarcomere shortens. The H zone and the I band become much narrower, and the Z lines move towards the A band. Contraction occurs after an action potential.
Muscles can only contract and relax.
Hydrostatic skeletons 2. When muscles oriented in a certain direction contract, the fluid-filled body cavity bulges out in the opposite direction. Exoskeleton - a hardened outer surface to which internal muscles can be attached. Contractions of these muscles cause jointed segments of the exoskeleton to move relative to each other. Muscles and exo or endoskeletons work around joints - places where the limb can bend so the animal can move the limb.