Types of muscle fibers: Slow-twitch Vs. Fast-twitch


Muscle fiber types and how they affect your workouts

In running, you’ve all seen the sprinter and the marathoner. One looks like an 80’s movie character and the other like he has had too many crash course diets. They are both runners so how come they don’t look alike? It’s quite simple – they practice different sports. In other words, the answer is in the way these athletes train ther muscles. It’s explosive strength versus endurance. Also, the answer depends on the types of muscle fibers that are predominant in their bodies. In this post you’ll find out the main characteristics of each type and how muscle fiber types affect your workouts.

sprinter vs long distance runner muscles

What is a muscle fiber?

A muscle fiber is the scientific term for a muscle cell (i.e.. both terms mean the same thing). Muscle fi­bers are shaped like cylinders and grouped together in columns called fascicles. Think of packaged spaghetti, and you’ll have an idea of how muscle fibers are stacked within fascicles. Fascicles are then banded together to form skeletal muscles – see fig 1.

structure of skeletal muscle anatomy

Fiber vs. Fibre?

There is no difference in meaning between fiber and fibre. Fiber is the preferred spelling in American English, and fibre is preferred in all the other main varieties of English. They all mean the same thing.

Types of muscle fibers 

Not all muscle fibers within a skeletal muscle are the same. Two criteria to consider when classifying the types of muscle fibers are how fast some fibers contract relative to others, and how fibers produce ATP. Quite simply, adenosine triphosphate (ATP) is the energetic currency of a cell, and it is required for the cell to perform work of any kind, ranging from the synthesis of DNA to sending chemical signals and nerve impulses to the brain.

There exist two main types of muscular fibers, both possessing very different chemical and physical features.

  • Slow twitch fibers – type 1
  • Fast twitch fibers – type 2

All fiber types exist in skeletal muscles, but some muscles within the body—such as postural trunk muscles—have more slow-twitch fibers, while other muscles (such as those in the calves) have more
fast-twitch fibers. The proportion of muscle fiber types varies from person to person based on both genetics and training which explains in part why some people are highly suited to endurance training and others to strength training. According to some theories, muscle fibers can be adapted or changed by training counter to their natural disposition, but other studies suggest the contrary.

Types of muscle fibers: Slow-twitch Vs. Fast-twitch

Slow twitch fibers (ST) – type 1

These are suitable for aerobic exercise and use oxygen to produce a small amount of tension over a long period of time, as they are resistant to fatigue.

Performers in endurance events (e.g. marathon running) tend to have a high percentage of slow twitch muscle fibres.

ST fibers do not tire easily so are used for low-intensity, long-duration aerobic activities such as walking and jogging. They are not as reactive and speedy as their fast twitch brothers but they can work non-stop for hours. Even when trained these fibers remain slim and light.

Fast twitch fibers (FT) – type 2

FT fibers are essentially the opposite. A type of muscle fibers associated with anaerobic work. They produce a large force over a short period of time: low resistance to fatigue (they fatigue easily).

Performers in power events (e.g. sprinting, power lifting) tend to have a high percentage of fast twitch muscle fibres.

FT fibers can generate high levels of tension, contract (react) very rapidly and work super fast, but have poor endurance (they quickly run of power). When trained they naturally become big and heavy.

Fast twitch fibers can be further subdivided into FTa (type IIa) and FTb (type IIb) fibers, based on their ability to produce energy under aerobic conditions. You can see all the differences between those two types of fast twitch fibers in the table below. However the main difference is in the following.

Fast oxidative glycolytic fibers (type 2a) are more resistant to fatigue than type 2b. This is because they have more capillaries surrounding them, more mitochondria and a greater number of aerobic enzymes than type 2b fibers. However, they do generate slightly less force.

Fast glycolytic fibers (type 2b) have the greatest anaerobic capacity and therefore generate the largest amount of force. Also, they have the lowest endurance capacity of all fiber types. They tire very quickly and we use them almost exclusively for explosive power activities such as sprinting and jumping.

Structural differences

The three types of muscle fiber mentioned above vary in structure and function as summarised in the table below:

muscle fiber differences

Muscle fiber types in relation to choice of physical activity

Some people are simply born with more slow-twitch fibers, which makes them genetically suited for endurance sports (long-duration, low-intensity activity).

Those born with a predominance of fast-twitch fibers have a huge advantage in sports involving quick bursts of speed and power that don’t have to be sustained for more than a minute (e.g. weight lifting).

Types of muscle fibers and bodybuilding

Your maximum number of muscle fibers is the first key genetic factor that will determine your genetic potential. The ratio between a fast-twitch and a slow-twitch muscle fibers is the second key hereditary facor that affects your success in strength training. Therefore, genetics determines the type of muscle fiber that is predominant in your muscle cell: white (fast twitch) or red (slow twitch).

Some people are born with a predominance of FT fibers, which makes them better suited to activities requiring speed, strength or power. Therefore, people with greater percentage of fast-twitch muscle fibers have better potential to be successful in strength training/bodybuilding.

Can you change your muscle fibres?

Unfortunately, a fast-twitch fiber can’t turn into a slow-twitch fiber, and a slow-twitch fiber can’t become fast-twitch. However, it is possible to change the function of certain muscle fiber types through specific kinds of training.

With aerobic training, FTa fibers can learn to use more oxygen and so assume some of the characteristics
of ST fibers – i.e. they become more aerobic and gain greater endurance.

So, endurance training does not change the fiber type but will increase the muscles’ aerobic capacity. It is not, unfortunately, possible for changes to occur in the opposite direction – i.e. for ST fibers to assume the characteristics of FT fibers.

Training by fiber type

So, if type I fibers tend to be more enduring, and type II’s stronger and faster, common sense would suggest:

  • People who are most interested in developing power, speed, and strength need to train for type 2 muscle fibers.
  • Most people should train for all muscle fibers. However, dedicated power lifting, weight training, or sprinting athletes can improve by targeting type 2.
  • Evidence is not clear, but generally, performing more high-intensity, fast exercises with bigger weights will lead to more developed type 2a fibers.
  • This in turn will lead to greater performance in power and speed sports.

To hit your slow-twitch, type I fibers: Train with higher reps and less weight (say, three to four sets of 12 to 15 reps using a weight that results in failure at the top end of the range).

To hit your fast-twitch, type II fibers: Train with lower reps and more weight (say, two to four sets of three to 12 reps).

Closing thoughts: Types of muscle fibers 

Every muscle fiber can be categorized as either slow or fast, depending on how quickly it can contract.

Slow-twitch muscle fibers (Type I) are oxygen-dependent and contract relatively slowly, but can contract for longer periods of time without fatigue.

Fast-twitch muscle fibers (Type II) are not oxygen-dependent and contract more rapidly than slow-twitch fibers, but tire relatively quickly (they also produce greater muscle power).

Elite athletes have muscle fiber compositions that complement their sport. Put simply, a top sprinter or bodybuilder would probably have a high percentage of explosive FT fibers and fewer ST fibers, while an endurance athlete is more likely to have a high percentage of ST fibers and fewer FT fibers.

Regardless of your genetically determined fibre mix, you can still increase muscle size and strength through intensive training and good nutrition.

Related articles:

  1. What Predominant Muscle Fiber Type You Are? Methods & Tests!
  2. Thorstensson Fatigue Test: Determining Muscle Fiber Composition
  3. Vertical Jump Test/Power test: What Does It Show Us?

About Author

Hey! My name is Kruno, and I'm the owner and author of Bodybuilding Wizard. I started this website back in late 2014, and it has been my pet project ever since. My goal is to help you learn proper weight training and nutrition principles so that you can get strong and build the physique of your dreams!

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