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Home Training Build Muscle The Difference of Muscle Fiber Types

The Difference of Muscle Fiber Types

The Difference of Muscle Fiber Types


Skeletal muscle is made up of individual components known as muscle fibers. The difference is from the type of myosin present and the degree of oxidative phosphorylation that the fiber undergoes. There are 4 different types of muscle fibers including Type I fibers, Type IIa fibers, Type IIx fibers, and Type IIb fibers. Take a look at the chart below. 

  Type I fibers Type II a fibers Type II x fibers Type II b fibers
Contraction time Slow Moderately Fast Fast Very fast
Size of motor neuron Small Medium Large Very large
Resistance to fatigue High Fairly high Intermediate Low
Activity Used for Aerobic Long-term anaerobic Short-term anaerobic Short-term anaerobic
Maximum duration of use Hours <30 minutes <5 minutes <1 minute
Power produced Low Medium High Very high
Mitochondrial density Very High High Medium Low
Capillary density High Intermediate Low Low
Oxidative capacity High High Intermediate Low
Glycolytic capacity Low High High High
Major storage fuel Triglycerides Creatine phosphateglycogen ATP, Creatine phosphate, glycogen (little) ATP, Creatine phosphate
Note Consume lactic acid Produce lactic acid and Creatine phosphate Consume Creatine phosphate Consume Creatine phosphate
Myosin heavy chain,
human genes

Adapted from



Every muscle group is made up of these slow (Type I), intermediate (Type IIa- fast oxidative), and fast twitch (Type IIX/IIb- fast glycolytic) muscle fibers. Slow twitch muscle fibers are very efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long period of time before they fatigue. Intermediate fast-twitch muscle fibers can use both aerobic and anaerobic metabolism almost equally to create energy. Fast-twitch muscle fibers get their name by being able to fire rapidly (contracting in under .01 seconds or less after stimulation) and work only under anaerobic conditions. Since they are without oxygen, that means they fatigue very easily.

So an all-out sprint on the track or set in the weight room under maximal loads incorporates the fast glycolytic muscle fibers while a long run around the park for 45 minutes will incorporate the slow twitch muscle fibers. So now that we have an understanding of the structure and components of the muscle fibers, we can better understand which type of exercise is more beneficial to getting ripped up.

Have you ever looked at a chicken before? Sure you have.


Notice how the chicken has a big upper body and small legs? Why is this? A chicken stands and walks on his feet all day long, yet he has these tiny legs. That is because their legs are completely comprised of slow twitch muscle fibers, since it never is required to perform a quick burst of speed through its legs. This leads us to the next question: Why is their upper body big? When a chicken takes off for flight, it does so with a short quick burst of energy. These muscles are mainly comprised of fast twitch muscle fibers. However, we can explore this even deeper. Have you ever noticed how a drumstick or thigh is dark meat while the breast consists of white meat? The chicken breast is what provides for flight and requires an exorbitant amount of energy in a short period of time. This is similar to an all-out sprint on the track or a very heavy set in the weight room, where we must recruit fast-twitch muscle fibers in order to perform the bout. Dark meat represents the slow twitch muscle fibers, because there is more hemoglobin providing it with a reddish tinge. Dark meat is also the fattier of the two kinds. 

Luckily for us humans, we can practically transform which muscle fibers our body will be comprised of through training (whether we want to run for distance or be explosive). Consequently, we can conclude that working these fast twitch muscle fibers in preference to your slow twitch muscle fibers is what will make or break your physique. Slow twitch fibers do not produce any form of muscle growth.  In order to activate your slow twitch muscle fibers, you need to work with incredibly submaximal loads. Have you ever known of someone that gained muscle mass by just running on a treadmill? Steady-paced cardio actually atrophies the muscle, instead of building it. Rather, in order to build muscle mass, your body has to experience resistance at near maximal loads in the presence of hypertrophy.

Activating your fast twitch muscle fibers means that your muscles are firing so rapidly because the intensity, or weight, you are working with is near maximal to present hypertrophy. Remember, the more muscle you have, the more fat you burn. That goes for you too ladies. An all-out sprint or heavy squat/deadlift/bench press is what damages the muscle fibers completely and provides the stimulus for the muscle to gorge itself with blood and ultimately grow back stronger. The reason why this type of training is most effective to your goals is because of the particular type of muscle fibers that it damages. When you damage the fast twitch muscle fibers, it takes a very long time and a great deal of energy for it to recover. The more energy it takes, the more calories you burn. So, long after the exercise is over, your body is burning calories at an extremely elevated rate. This is drastically different from the way that slow twitch fibers repair themselves. Biologically speaking, we were always meant to be able to walk/run miles and miles a day until we were tired, and then be able to do that again the following day. So any kind of exercise you do o involving your slow twitch muscle fibers will not take much time to recover, meaning that the only calories you will really burn from that type of exercise will be during the actual session, rather than after the exercise in addition.  That is largely in part due to the size of the muscle fibers.

Slow twitch fibers are about half the diameter of fast twitch muscle fibers and take three times as long to contract. By implementing your fast twitch muscle fibers you are activating a much larger muscle, indicating that it requires much more energy (or calories) in order to activate that muscle, and are working much harder during the actual process since they are firing three times as fast as slow-twitch fibers. It requires an exorbitant amount of energy to allow your fast twitch muscle fibers to respond with such vigor. Moreover, fast twitch muscle fibers hold significantly more glycogen than slow twitch muscle fibers. By breaking down more glycogen, you more easily tap into your fat reserves at a quicker rate than if you were just working your slow twitch fibers. That is why you rarely see a fat athlete who participates in a sport that requires short bursts of energy for 5-10 seconds that require activation of your fast twitch muscle fibers: football, track and field, boxing, basketball, etc.

Intensity is the most efficient way to burn fat not only because of the calories that it burns during exercise though, but also because of the recovery process that takes place after. First off, it takes a lot more energy to recover and repair a larger muscle fiber than a smaller one. More importantly, high intensity exercise elevates your metabolism by around 20% long after the exercise is done. Hormonal changes occur after the exercise that increases fat oxidation following the anaerobic workout. You will have an increased metabolism throughout the rest of the week from this type of central nervous system stimulation. Studies show that oxygen consumption and fat oxidation increases considerably, which requires a lot of energy expenditure to maintain homeostasis. For instance, when comparing individuals performing cardio, studies show that there is a 9x greater decrease in subcutaneous skinfolds (fat) when doing High Intensity Interval Training than traditional endurance based workouts.

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