Struggling to Build Muscle? Here’s Why Your Muscle Fiber Type is Important

How To Gain Muscle Mass Quickly

Genetics Play an Important Role in How Much Muscle You Build

Have you ever struggled with building muscle no matter how hard you try? If that’s you, then don’t be too hard on yourself. It’s because of your genetics, specifically, the muscle fiber type you were innately equipped with. 

You see, your muscles are made up of muscle fibers, which consist of fast-twitch and slow-twitch fibers. Depending on which one you have more of, will determine how much muscle and strength you build and your capability to excel at certain physical activities. 

Read the rest of this article to learn about the different muscle fiber types, and what they’re best for. 

How to Build Muscle

To build muscle, you need to place your muscles under enough stress that causes them to grow. You do this via resistance training, a caloric surplus, and the progressive overload principle. Of course, you need to do resistance training via lifting weights to build muscle. It’s also important to eat enough calories and protein and follow the progressive overload principle, increasing the weight you’re lifting as often as possible. 

These are the key principles to practice to build muscle, however, they’ll only get you so far, even if you’re doing everything right. That’s because a big culprit to the amount of muscle you’re able to build is based on your genes (1).

That’s right! If you build muscle fairly easily, then it’s likely because you won the genetic lottery when it comes to weight training. On the contrary, if it’s hard for you to build muscle no matter how hard you try, it’s likely because you were dealt a bad hand in the gene pool. 

In fact, a great portion of your strength and muscle mass potential has to do with your genetics.  

Body Types

Before we dive into the specifics of the different muscle fiber types, let’s first take a look at the different body types, somatotypes. Your somatotype is more or less your body’s natural build, and there are three types: ectomorph, mesomorph, and endomorph. 

Below, there’s a list of the difference between each one. 


  • Ectomorph: naturally skinny, hard to build muscle, but also hard to gain fat e.g., skateboarder.
  • Mesomorph: not too skinny, not too big, fairly easy to build muscle and lose weight if on a proper regimen e.g., NFL wide receiver.
  • Endomorph: big, build muscle easily, hard to lose fat e.g., NFL offensive lineman. 

You may have heard of these before, and they play a role in your body’s natural frame and your ability to lift weights. For example, an endomorph will likely be stronger than an ectomorph since they are more massive, however, ectomorphs can cheat more on their diet since it’s harder for them to gain weight. 

Although your body type is important when it comes to your body frame, your muscle fiber type is a good determiner of how much muscle and strength you’ll be able to pack on your frame. 

What Is Muscle Fiber? 

Your skeletal muscle is made of muscle fibers, individual contractile cells within a muscle that is responsible for allowing the muscle to produce force and motion. Depending on the amount of each muscle fiber that makes up most of your muscle, will determine how much force and the power output they’re able to produce (2). 

Muscle Fiber Types

There are two basic types of muscle fibers, slow-twitch, and fast-twitch. Depending on which one you have more of will determine your innate ability to lift weights and how much strength and muscle you build.

Of course, following a structured program and doing everything right to maximize your muscle growth is a huge part of building muscle, however, everyone’s got a genetic potential, and unfortunately (or fortunately, depending on where you fall in the genetic pool and what your goal is), your genetics play an enormous role in how much strength and muscle mass you do gain. 

Fortunately, even if you don’t fall on the end of having the right fiber type that’s churned to build you the most muscle and strength possible, you’ll have other benefits that the genetic freaks lack. Let me explain. 

Fast-twitch muscle fibers are meant for short, explosive bursts e.g., sprinting, powerlifting, whereas slow-twitch muscle fibers types are for endurance activities e.g., marathon runners, long-distance swimmers. More specifically, there are three types of muscle fibers: slow oxidative, fast oxidative (fast-twitch IIA), and fast glycolytic (fast-twitch IIB).  

Fast-twitch Muscle Fibers

fast twitch muscle fiber

Fast oxidative fibers use your anaerobic energy system to produce ATP and produce stronger contractions than slow oxidative fibers. 

Fast-twitch Activities

  • Powerlifting
  • Sprinting
  • Jumping 
  • Strength and agility training

These are activities important for many sports, including football, basketball, and UFC. 

Specifically, fast-twitch type IIA fibers are best for the sports and movements that produce the most force e.g., powerlifting, 40 yard-dash, 100m sprint. Fast-twitch type IIB fibers, aka intermediate muscle fibers, are primed for activities that are explosive but not quite as explosive as fast oxidative fibers (think the 400m sprint, 200m sprint, or lifting medium-heavy weight within a medium rep range). 

Researchers at Ball State University sampling muscle tissues of the world-record holder for the 60-meter hurdles in the 80s found his quad muscles to contain 71% fast-twitch fibers (3). 

Slow-twitch Muscle Fibers

slow twitch muscle fiber

Slow oxidative fibers rely on your aerobic energy system to contract over long periods and are slow to fatigue.

Slow-twitch Activities

  • Cycling
  • Running
  • Rowing

Although elite sprinters have more fast-twitch fibers, elite endurance athletes, such as cross-country skiers have mainly slow-twitch muscle fibers — 60-80% of their muscle fibers (4). 

Everyone’s born with a certain amount of both fiber types. Although you can train yourself to utilize more of one fiber type than the other, a good portion of your potential to maximize one is what you were innately born with (5). With that being said, both fiber types do have their drawbacks.

Pros and Cons of Different Muscle Fiber Types

If you’re a bodybuilder, powerlifter, weightlifter, or average gym-goer looking to build muscle, then you’ll benefit the most from having a lot of fast-twitch muscle fibers. If you don’t naturally have a lof fast-twitch muscle fibers though, it’s okay.

You can train your body to maximize the most it’s capable of producing by training smartly. Also, you’ll have a lot more endurance since you’ll have more slow-twitch muscle fibers, which comes in handy for many tasks as well, including a pickup game of basketball, running, and other endurance sports. 

If you’re an endurance athlete, then you’ll benefit the most from having more slow-twitch muscle fibers than fast-twitch ones. The con to this is obviously that you won’t be able to build as much strength or pack on as much muscle. 

Genetic’s Role

Now that you know the differences in different muscle fiber types and their role in physical activities, you probably have a general idea of which one your body has more of. If you’ve generally struggled with building muscle mass and building strength, then likely you have more slow-twitch muscle fibers. So next time you see a freak of nature in the gym, just remember that it’s likely because they were blessed with more fast-twitch muscle fibers. 

Although for most people, you’ll have a split of 50/50 fast-twitch muscle fibers and slow-twitch muscle fibers (6). 


There’s a lot you can do to maximize your strength and muscle mass, however, it’s to a certain extent. Unfortunately, your genetics and the muscle fiber type you have more of naturally play a big role in how much strength and muscle you’re capable of producing. 

You have both fast-twitch and slow-twitch muscle fibers. Fast-twitch muscle fibers are better for building muscle and explosive activities, including powerlifting and sprinting, while slow-twitch muscle fibers are better for endurance activities, such as rowing and swimming. 

Not being able to build as much strength and muscle mass due to having fewer fast-twitch fiber types isn’t necessarily bad though. It means you’ll dominate endurance activities. 

Which muscle fiber type do you have more of? Let us know and follow us on Facebook, Instagram, and Twitter


1 – Verbrugge, S., Schönfelder, M., Becker, L., Yaghoob Nezhad, F., Hrabě de Angelis, M., & Wackerhage, H. (2018). Genes Whose Gain or Loss-Of-Function Increases Skeletal Muscle Mass in Mice: A Systematic Literature Review. Frontiers in physiology, 9, 553.

2 –Wayne Scott, Jennifer Stevens, Stuart A Binder–Macleod, Human Skeletal Muscle Fiber Type Classifications, Physical Therapy, Volume 81, Issue 11, 1 November 2001, Pages 1810–1816,

3 – Trappe, S., Luden, N., Minchev, K., Raue, U., Jemiolo, B., & Trappe, T. A. (2015). Skeletal muscle signature of a champion sprint runner. Journal of applied physiology (Bethesda, Md. : 1985), 118(12), 1460–1466.

4 – Holmberg, H.‐C. (2015), The elite cross‐country skier provides unique insights into human exercise physiology. Scand J Med Sci Sports, 25: 100-109.

5 – Jansson, E., Esbjörnsson, M., Holm, I., & Jacobs, I. (1990). Increase in the proportion of fast-twitch muscle fibres by sprint training in males. Acta physiologica Scandinavica, 140(3), 359–363.

6 – Fry, A. C., Webber, J. M., Weiss, L. W., Harber, M. P., Vaczi, M., & Pattison, N. A. (2003). Muscle fiber characteristics of competitive power lifters. Journal of strength and conditioning research, 17(2), 402–410.<0402:mfcocp>;2

Terry Ramos
As a personal trainer and writer, Terry loves changing lives through coaching and the written word. Terry has a B.S. in Kinesiology and is an ACSM Certified Personal Trainer and ISSA Certified Strength and Conditioning Specialist. He enjoys playing music, reading, and watching films when he's not writing or training.