Skeletal muscle fibers can be categorized into two types: slow-twitch (Type I) and fast-twitch (Type II). Type I muscle fibers are more efficient over long periods of time. They are mainly used for postural maintenance (such has holding the head upright), or endurance exercises (like marathon running). Type II muscle fibers use anaerobic respiration and are better for short bursts of speed than Type I fibers, although they fatigue more quickly.