Muscle is comprised of thick and thin myofilaments that slide across each other during contraction. Activation of a muscle fiber causes release of calcium into the environment of thick myofilaments, revealing the binding sites on the thin filaments that permit cross bridging from the thick filaments. Muscle shortening is the product of repeated contraction of the cross bridges. Slow twitch muscle fibers remain contracted longer than fast twitch fibers, with the former being involved in maintenance of posture and the latter in fine and rapid motor function.