Non human fiber types include true IIb fibers, IIc, IId, etc. Further fiber typing methods are less formally delineated, and exist on more of a spectrum. They tend to be focused more on metabolic and functional capacities (i.e., oxidative vs. glycolytic, fast vs. slow contraction time). As noted above, fiber typing by ATPase or MHC does not directly measure or dictate these parameters. However, many of the various methods are mechanistically linked, while others are correlated "in vivo". For instance, ATPase fiber type is related to contraction speed, because high ATPase activity allows faster crossbridge cycling. While ATPase activity is only one component of contraction speed, type I fibers are "slow", in part, because they have low speeds of ATPase activity in comparison to type II fibers. However, measuring contraction speed is not the same as ATPase fiber typing. Because of these types of relationships, Type I and Type II fibers have relatively distinct metabolic, contractile, and motor-unit properties. The table below differentiates these types of properties. These types of properties—while they are partly dependent on the properties of individual fibers—tend to be relevant and measured at the level of the motor unit, rather than individual fiber. Traditionally, fibers were categorized depending on their varying color, which is a reflection of myoglobin content. Type I fibers appear red due to the high levels of myoglobin. Red muscle fibers tend to have more mitochondria and greater local capillary density. These fibers are more suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP (adenosine triphosphate). Less oxidative type II fibers are white due to relatively low myoglobin and a reliance on glycolytic enzymes. Fibers can also be classified on their twitch capabilities, into fast and slow twitch. These traits largely, but not completely, overlap the classifications based on color, ATPase, or MHC. Some authors define a fast twitch fiber as one in which the myosin can split ATP very quickly. These mainly include the ATPase type II and MHC type II fibers. However, fast twitch fibers also demonstrate a higher capability for electrochemical transmission of action potentials and a rapid level of calcium release and uptake by the sarcoplasmic reticulum. The fast twitch fibers rely on a well-developed, anaerobic, short term, glycolytic system for energy transfer and can contract and develop tension at 2–3 times the rate of slow twitch fibers.