The body's muscle fibers are simple, yet complex in the ways of how we understand and train them. How we train, whether it be strength or aerobic based, will influence our muscular fiber composition. This is due to muscular fibers adapting to the stress we put on them through our training.
Stress caused from various modes of training will influence a muscle's myofibril composition (a myofibril is composed of actin & myosin filaments), which is what helps fibers earn their titles of Type I, IIA, and IIX. Our body's muscle's will be composed of varied amount of each fiber type.
There are multiple physiological differences between the fibers that earn them their names of fast and slow twitch. At the end of each contraction, force output is very similar between types, but the rate of contraction varies, hence their fast and slow twitch names.
A few of these differences involve how the muscle types utilize energy, the order in which they're recruited, and how fast they produce force.
There are three methods in describing or discovering what type of fiber a muscle fiber is (aka the methods researchers use when classifying a muscle as slow or fast twitch). These include: myosin histiological staining, MHC isoform identification, and identification from metabolic enzymes.
Type I (slow twitch): This muscle fiber is efficient at utilizing oxygen to produce ATP, which is our muscle's form of energy. They contract slowly, and are more resistant to fatigue due to their ability to utilize oxygen at a higher rate.
Type IIA (fast twitch): This muscle fiber produces energy anaerobically (aka without the use of free oxygen), and contracts much faster than Type I fibers. Due to their lack of aerobic energy metabolism they fatigue quickly. The IIA fiber is often considered the intermediate fiber.
Type IIX (fast(er) twitch): Similar to the type IIA muscle fiber, this fiber also creates energy anaerobically and will contract quickly with the same (but somewhat faster) rate of fatigue. This muscle fiber contracts the quickest of the three, which makes it the most prone to early fatigue.
The body's energy systems will play a role in how each muscle fiber contracts and is used. The body has three types of energy systems and they include: ATP-PC, Glycolytic, and Oxidative. Each energy system will play a role in different activities and their rough time estimates based off of maximal energy demands are shared below.
If you have an understanding of the energy systems, then you can see how they'll influence a muscle fibers ability to contract. Check out the table below, which provides a brief visual of the different energy systems and muscle fibers they'll most heavily influence.
| Muscle Fiber Type | Type of Training Influenced |
| Type I - Slow Twitch | Endurance & High-Rep Movements |
| Type IIA - Fast Twitch | Hard Runs & Relative Strength Training |
| Type IIX - Fast(er) Twitch | Sprints & Explosive movements |
Keep in mind, the above table isn't a completely perfect, cut-and-dry example of which fibers will be present in certain activities. In every activity you perform there will be a combination of almost all motor neurons and muscle fiber types, but the amount will differ.
The table below is a rough example of which elite athletes will have the highest composition of each muscle fiber. This table is obviously not meant to be taken as an “every case” scenario, but to provide a visual into how each elite athlete will most often train to influence certain fibers. There will always be crossover for strength athletes.
| Muscle Fiber Type | Elite (strength) Athletes |
| Type I - Slow Twitch | Triathletes & Marathon Runners |
| Type IIA - Fast Twitch | Functional Fitness Athletes & Bodybuilders |
| Type IIX - Fast(er) Twitch | Weightlifters & Powerlifters |
These tables are illustrations to help provide context of the fibers you'll see most present with certain activities and athletes, which can be further explained by the Size Principle.
This principle is a concept in science that states our nervous system recruits motor neurons with muscular fibers in a predicted order. The order typically involves the smaller neurons and slower muscles being recruited first, then the large and faster muscle fibers recruited last.
For example, think about performing a chin-up for 10-reps. If this movement is easy for you, then you may find that your first 4-5 reps are heavily influenced by smaller motor neurons/slow twitch muscles, and then the large neurons/fast twitch fibers kick in around rep 6+. On the contrary, if this movement is difficult, then your larger neurons/fast twitch fibers will kick in much sooner.
In a lot of research settings this principle stands true, but there's one issue. A study from 2014 brings up the point that it's much more difficult to gauge recruitment patterns with various real life force changes. For example, an athlete that's performing multiple movements in one sequence may experience a fluctuation in different recruitment patterns. This concept makes it difficult to define the Size Principle with a clear definition of how our body recruits the different muscle fiber types.
In short, yes and no. We can to an extent.
It's generally agreed upon that we're born with an equal 50/50 slow and fast twitch fiber count. As we train, we influence the efficiency of each muscle type targeted. This will lead to an increase in the size and the rate at which we can produce force with that muscle (think, progressive overload in our training). We know we can increase a fiber's size, but can we change something like a Type I fiber into a Type II?
Again, science is conflicted on this answer. A lot of research suggests we can influence about 10% of our muscle type makeup, but we can't fully change certain fibers into others. For example, we can't change our body into a lean mean 100% Type IIX fast twitch machine (… if only).
Before losing hope, a study from 2012 concluded that we have the ability to influence intermediate fibers. For example, we can change some Type IIA fibers to Type IIX (this is how we become more powerful with training over time). The body is able to adapt and change an intermediate fast twitch fiber (Type IIA) to produce an even quicker contraction, which made it a Type IIX. Also, researchers point out that in longer duration events, the body may be able to convert Type IIA fibers into Type I.
Intermediate fibers demonstrate the highest ability to change from training (it's suggested this is due to their higher oxygen capacity). The hangup involves the change between true Type I and Type IIX fibers. This area of research is still lacking.
We're composed of multiple types of muscle fibers, and when we're born they're pretty evenly distributed. As we progress through out lifting careers, we influence how our body adapts and responds to stress by how quickly we can produce force. Certain athletes will have naturally higher amounts of certain fibers than others (or the ability to adapt better).
At the end of the day, it's most important to train in a fashion that will cause you to optimally develop and excel in your sport.
Editor's note: Maria Dalzot, Registered Dietitian, Mountain/Trail Runner, and BarBend reader, had this to say after reading this article:
As an endurance athlete, it seems the only time my fast twitch muscles are activated is when reaching for food after a 4-hour run in the mountains. Speaking of food, the body's energy systems (ATP-PC, Glycolytic, and Oxidative) will not only play a role in how each muscle fiber contracts and is used, but will also determine which fuel is used for oxidation. Carbohydrates are an efficient fuel source during anaerobic exercise when the body cannot process enough oxygen to meet its needs. As more oxygen becomes available during less intense exercise, fat becomes the predominant fuel source, sparing muscle glycogen stores. But just as in how every activity you perform there will be a combination of
motor neurons and muscle fiber types involved, your body will use a combination of carbohydrates and fat for fuel. There is no switch that turns carbohydrate or fat oxidation completely on or off. Nothing in the body happens in isolation.
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