All About Muscle: Muscle Fibers and How to Train Accordingly

To look further into muscle, our body is composed of a diverse array of different muscle groups.

Everyone has a unique combination of muscle. Muscle composes of three specific types of fibers. Dependant on the athlete, they specifically have one primary muscle fiber that has been trained over the others, to suit their sport.

There are two types: Slow twitch fibers (Type I) and fast twitch fibers (Type II).

Within, there are one slow twitch fiber and two fast twitch fibers. These fibers are reflected enzyme activity and have specific metabolism properties. These include:

Slow Twitch Fibers

  • Type I Fibers

Fast Twitch Fibers 

  • Type IIa Fibers
  • Type IIb Fibers

Each type of muscle fiber has their own, unique function.

Slow twitch, Type I Fibers

Type I fibers specialize in long-duration contractile activities and are found in abundance in elite endurance athletes. ¹

These specific fiber types high amounts of mitochondria and myoglobin.

What that means, is that it supports the capacity for aerobic metabolism and fatigue resistance, which is specifically used for endurance athletes. These fibers produce less force, are slower to make maximal tension, BUT are able to maintain long-term contractions.²

Example athlete: Distance Runner

Fast Twitch, Type II Fibers

Conversely, type IIa and IIb fibers facilitate short-duration anaerobic activities and are proportionally higher in elite strength and power athletes.³

These two types of fibers have lower amounts of mitochondria and myoglobin and fatigue faster than it’s type I counterpart. These fibers are larger, quicker, and produce a greater and powerful force.³

Type IIa

These are hybrids of both Types I and Type II fibers. Still, with a significant amount of mitochondria and myoglobin, these fibers split ATP to produce fast muscular contractions, even though they are more prone to fatigue. Strength/Resistance training turn type IIb to IIa due to a more efficient utilization of the oxidative cycle. 

Type IIb

These have low levels of mitochondria and myoglobin and produce ATP even slower. These breakdowns very easily, and it resorts to short, fast bursts of power.

Example athletes: Sprinters, Throwers, Powerlifters, Strength Athletes, Bodybuilders


Everybody is born with a genetic preposition of muscle fibers that are different.

Some genotype testing has been able to identify which individuals will respond better to heavier loads and lower numbers of repetitions, and which will respond better to lighter loads and higher numbers of repetitions (Jones et al. 2016). 

Personally, I believe you can blame 10% of your muscle preposition on genetics. But as long as your environmental factors are in place (sleep, nutrition, ect) you will not look significantly different than other people.

Applying Muscular Fiber Knowlege to Strength Training and Muscular Hypertrophy

Muscle Hypertrophy is a term used for an increase of muscle cells, making it grow.

Here is a chart depicting what fibers are trained in repetitions:

Overview Growth In Muscle Fibers Below
Repetition Range Type I Type IIa Type IIb Strength Increases
1-2 repetitions Minimal Minimal Minimal Excellent
3-5 repetitions Minimal Minimal Good Excellent
6-8 repetitions Minimal Good Great Good
9-12 repetitions Low Excellent Very Good Specifically Within Rep Ranges
13-15 repetitions Good Very Good Good Endurance
16-25 repetitions Very Good Diminishing Minimal Endurance
25-50 repetitions Excellent Low Minimal Endurance

There has been a recent, carefully controlled study that there are to no differences in the hypertrophic effects of training with heavy and light loads (Morton et al. 2016). 

The takeaway from this recent study here is to prove that as long as you are continuing strength training, you will achieve muscle hypertrophy. But the key is to train with all rep ranges since everyone has a different genetic deposition and everyone will respond to different training regimens.

Stay Strong,


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Jabir Ahmed


Jabir is a natural 89kg class athlete & a NASM certified personal trainer. With the achievement of elite strength standards along with a vast amount of knowledge within nutrition, dieting, bodybuilding, and strength training, he helps his clients achieve the body they have always dreamed of.