 Hey everybody, Dr. O here. In this video we're going to cover the different muscle fiber types and you're going to see that I'm going to use different types of terminology because there's lots of ways to describe them. I generally think of a muscle fiber as either being built for power or built for endurance but then there's going to be some gray area in the middle. The two simplest ways or two most important things to look at when you're trying to determine what a muscle fiber does or how it functions is number one, how quickly it contracts. So you're going to see we have fast and slow twitch and then some variation. And then how they produce ATP because some are going to be oxidative meaning they use aerobic oxidative metabolism and others are going to be glycolytic which means they rely on anaerobic glycolysis. The oxidative metabolizing muscle fibers are going to be built for more endurance and the glycolytic ones are going to be powerful but they're going to fatigue very quickly. So let's go ahead and look at the three main muscle fiber types and then I'll throw a fourth one in. So here we, let's start with our slow fibers also known as slow oxidative fibers or type one fibers and I'll go through the column on the right there in just a second. So these slow twitch fibers are going to be slow to contract but also slow to fatigue. So these are built for more endurance based activities. They have a small diameter so they're not very powerful. So they don't, they don't contract powerfully but they have a huge amount of oxygen, tons of mitochondria to use that oxygen and produce ATP, tons of blood supply and they have myoglobin which myoglobin is, is similar to hemoglobin. And that's why these slow fibers are going to be red. So they're slow to contract, not very powerful but they are resistant to fatigue. So they're also called slow oxidative fibers because they use aerobic respiration primarily of, of glucose and triglycerides. So you see here on the right hand side, slow contraction time but their resistance to fatigue is high. We've already said that they're aerobic. They can be used for hours so you think endurance activities like long, long running marathons, et cetera, not very powerful. Mention they already have, they have a lot of mitochondria, a lot of, a lot of blood vessels and major fuel is going to be triglycerides as far as what's stored there but when you're physically active they will rely on aerobic respiration using glucose as well. Okay, so that's the slow fibers or your slow twitch fibers. Next we have your quick twitch fibers or your fast fibers also known as fast glycolytic fibers or type 2B fibers. These contract very quickly, very explosively. They have a large diameter so they can generate a ton of power. Their energy reserves are going to be glycogen so they can, they can break glucose down anaerobically but these are going to, they don't have a big blood supply, don't have a lot of mitochondria. So these are going to be very pale looking or white and they're going to run out of fuel very quickly. So these are the ones that are very explosive and powerful but they are, they are going to fatigue very quickly. So these are going to be for short term boosts of activity, activity. Also called fast glycolytic fibers because they rely on anaerobic glycolysis and if you haven't covered metabolism yet, aerobic respiration of glucose can get you 36 ATP. Anaerobic glycolysis can only get you a net gain of two. So as you can see running out of fuel very quickly. All right, so that's your fast twitch fibers. So we covered slow twitch and quick twitch or fast twitch fibers, the type one and type 2Bs but then we have in between we have these intermediate fibers and these are called fast oxidative fibers. So they're fast, they're quick twitch, but they all, they're oxidative. So as you can see here, they're going to have more mitochondria and more blood supply than your truly quick twitch fibers. So they're somewhere in the middle, so they're mid-sized as well. So they can generate a decent amount of power. They do have some myoglobin and they have more capillaries in mitochondria than the truly quick twitch explosive fibers. So they're going to be this kind of intermediate fiber in between. So they're going to last longer than a quick twitch fiber but not as long as a slow twitch fiber. So they're right there in the middle. What's kind of cool about these is there are two different types. You see here there's type 2A and type 2X. So the type 2A fibers are the ones that you can get a lot more of with training, with physical activity. So let's look at the type 2X though. Type 2X fibers are actually, they're very fast and they're very powerful. They're actually faster and more powerful than your type 2A fibers. These are the ones that they fatigue really quickly though. So these are the ones that are often called couch potato muscle fibers because everyone has some of them. Even if you're not a well-trained athlete, you need to move quickly and explosively occasionally, right? You've got to climb a ladder, you've got to jump by the way of a car, whatever it might be. So these are very efficient at rest and they can generate a bunch of power quickly when needed but they fatigue really quickly. So if you're not well-trained, you can still do all these things over very short periods of time. But if you work out, if you start to train, these type 2X fibers will actually convert and become the more useful, longer lasting type 2A fibers. But then during periods of inactivity, if you stop working out, they'll revert back to being the 2X fibers. So basically the interesting about these is they're designed to use very little fuel at rest but they're there in case you need it if you're a couch potato or don't need to use these muscle fibers very often. All right, so the key things that I look at, I probably think about the slow twitch versus quick twitch. So slow twitch fibers think endurance but not very much power. Quick twitch fibers think power but not very much endurance. Now different muscles are going to have different percentages of all of these. We don't really have purely quick twitch or purely slow twitch. But think of, so muscle color is going to matter. So if you have a muscle fiber that's mostly quick twitch fibers, it's going to be white. It's about like chicken breast, right? A chicken, the longest recorded flight of a chicken I've ever seen is 13 seconds. So they can use their breasts for explosive contractions but they fatigue really quickly, whereas the muscles that have a lot of your slow twitch fibers are going to be red. So think dark meat on a chicken. So a chicken can walk and run all day but it can only fly for a few seconds. So your muscles that are built for endurance are going to have more slow twitch fibers. Muscles built for power are going to have more quick twitch fibers. I think a couple more terms and I'll be done here, postural muscles versus phasic muscles. So postural muscles are muscles that are built for endurance. Like when you're, if you're going to stand all day, the soleus muscle in your calf is going to be a postural muscle. It can hold you up for hours at a time. So that's going to be dark meat. It's going to have more of these red slow twitch fibers in it. The gastrocnemius muscles, though the big bellies right outside of are superficial to that soleus, that's a phasic muscle. So if you're going to climb stairs or jump, these explosive muscles will contract, but they can only do their job for a few seconds. I know that's a lot there, but that's pretty much everything you need to know about the different muscle fiber types. I hope this helps. Have a wonderful day. Be blessed.