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Not All Muscles are The Same. Muscle Fiber Types 101

Muscles drive the movement of the human body. They also help serve as a protective shock absorber for the skeleton and serve as the biggest repository of glucose in the body (as glycogen).


There are about 700 named muscles that constitute approximately half of an individual's body weight. Each of these muscles are made of muscle fibers.


Given the tremendous number of muscles within the human body, it stands to reason that not all muscles are the same.


Today, we discuss the different types of muscle fibers in the body along with everything else you need to know about muscle fiber types.


Muscle Fibers 101


Within a single muscle (such as your biceps), there are thousands of individual muscle fibers composed of both fast and slow twitch muscle fibers.


Now, the percentage of slow vs fast twitch muscle fibers within a muscle depends on several factors including genetics, style of training, and which muscle group you’re analyzing.


For instance, one muscle group in the body could have an 80/20 split of slow vs fast twitch muscle fibers while another group could be more evenly split, such as 60/40 or 50/50. A prime example of this is the quadriceps which is pretty evenly split between fast and slow twitch fibers while the soleus (one of the two calf muscles) is predominantly composed of slow-twitch fibers.


You may have recognized this in your own training as some muscle groups seem to fatigue very quickly when put under tension while others seem to be able to go and go and go like the Energizer bunny.


Before we go any further, let’s briefly discuss the three main types of muscle fibers that can be found within a single muscle.


Type I


Also known as “slow-twitch” muscle fibers, Type 1 muscle fibers develop force and relax more slowly compared to type II (“fast twitch”) muscle fibers. They also use aerobic respiration (oxygen and glucose) to generate ATP.


Type I fibers can function for considerable lengths of time before fatiguing, which makes these fibers ideally suited to endurance-based activities such as producing isometric contractions, maintaining posture, and stabilizing joints. 


Since Type 1 fibers cannot generate high levels of tension, they are not involved in powerful, explosive movements that require great amounts of energy.


Type IIA


Known as “fast-twitch” muscle fibers, type IIA fibers contract and relax rapidly, allowing them to generate powerful, explosive forces. Fast-twitch fibers are glycolytic fibers that primarily use anaerobic glycolysis to generate ATP.


While type IIA fibers are capable of generating considerable amounts of force, they fatigue quickly, making them ideally suited to high-intensity efforts lasting short durations.


Type IIX


Type IIX muscle fibers are another type of fast-twitch muscle fiber, but these are even faster than Type IIA muscle fibers. However, these fibers are extremely rare to find, even in trained individuals.


Complicating the matters a bit is the fact that a single muscle fiber can have elements of both fast and slow-twitch, creating a hybrid fiber of sorts called Type I/IIA or Type IIA/IIX.


Researchers are still in the process of fully understanding these hybrid fibers, but one thing they do know is that the more sedentary an individual is, the greater proportion of these hybrid fibers their muscles contain.


Can Muscle Fiber Types Change?


This has been an area of great debate amongst researchers, but the evidence is becoming increasingly clear that yes, muscle fibers can change.


They are dynamic and flexible.


This is especially pertinent to the hybrid muscle fibers like Type I/IIA and Type IIA/IIX. Depending on your style of training, these fibers will shift from Type IIA/IIX to Type IIA and in the case of Type I/IIA to Type I or Type IIA (again depending on which method of training you engage in the most for that particular muscle group).


Knowing Your Personal Fiber Types


Short of getting a muscle biopsy, there’s no definitive way to know the composition of your muscles, but you can gather some empirical data by tracking your workouts. For instance, if you find that your chest responds best to low volume high intensity training with heavy weights, then it’s likely your chest has a heavier portion of fast-twitch fibers than slow twitch.


Additionally, if you find that you excel at endurance-based activities and struggle with more explosive movements, then it’s a pretty fair assessment that you have more slow-twitch fibers.


Does this mean you still can’t get results or make improvements?


Absolutely not!


While you may not have the type II fibers of a top-tier field sports athlete, you can still make improvements and train the type II fibers by performing powerful, explosive movements like sprints, power cleans, plyometrics, or lifting very heavy weights.


Conversely, if you want to work on your endurance-focused fibers, you can engage in activities that preferentially “target” those fibers, such as high-rep resistance training (15+ reps per set), isometric exercises, and steady-state cardio.


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