However, they actually break down ATP at a high rate and that's exactly why they contract quickly and that's exactly why we call them fast twitch fibers, fast twitch muscles. Now although they are still slow to fatigue, they have a low resistance to fatigue than our type one. So type one muscles are capable of resisting fatigue at a very high percentage while type two A basically resist fatigue at a lower rate than type one. Now what about type two B? Well, type two B muscles are basically those muscles that appear white and that's because they contain a relatively low concentration of myoglobin. Now they break down ATP quickly but at the same time they actually fatigue very quickly and they contain a very high concentration of glycogen, our glucose stored inside our muscles.