Muscles have different types of fibers, and each type has a different way of using energy and producing force.  

Because of the different energy systems, they have different contractile speeds and different fatigue rates.  While this may seem like high-level information that doesn’t directly impact your training or nutrition, having an understanding of the muscle fiber types and how they work will help you understand why we recommend training the way we do, and how you can eat to fuel your training performance and hypertrophy. 

After reading this blog, you’ll understand:  

→ The different types of muscle fibers 

→ How each type works 

→ Implications for training and nutrition

The Types of Muscle Fibers

You may have heard of “fast-twitch” and “slow-twitch” muscle fibers, and think of sprinters vs. marathon runners, respectively.  But there is a lot more to muscle fiber types than those two black and white categories.

There are actually 4 types of muscle fibers:  

1.  Type 1 
2.  Type 2 a 
3.  Type 2 x 
4.  Type 2 b

Before diving into the differences, I need to define some terms that I’ll use to describe and categorize the 4 types.

Aerobic: Producing energy with the use of oxygen 

Anaerobic: Producing energy without the use of oxygen Motor

Neurons: These supply muscle fibers with nerves, and trigger a contraction within the muscle. 

Mitochondrial Density: Mitochondria in the muscle, when increased in size and number, can more efficiently convert energy to ATP, meaning more energy is available to working muscles.  Greater mitochondrial density = training harder and longer. 

Capillary Density: Capillaries are small blood vessels that transport blood in and out of the muscles.  When more oxygen is needed in the muscles, more blood flows there.  When there is more capillary density, more oxygen can get to the working muscle. 

Oxidative Capacity: How efficiently the muscle can use fat for energy. 

Glycolytic Capacity: How effectively the muscle can use carbs (glycogen) for energy. 

ATP: Adenosine Triphosphate, provides energy to working muscle cells.  Muscles store a small amount of ATP (enough for a few seconds of work).

Now that we know these terms I can explain the different types of muscle fibers and how they work.

TYPE 1 

Type 1 muscle fibers are also called slow-twitch.  

These are the muscle fibers that respond to aerobic activities like long-distance running, walking, or riding a bike.  

Their reaction time is slow, and resistance to fatigue is high.  Although these muscle fibers can contract for hours, the power they can produce is low, and they have the smallest motor neurons.

Other characteristics of type 1 fibers: 

- Mitochondrial Density: High 

- Capillary Density: High 

- Oxidative Capacity: High 

- Glycolytic Capacity: Low 

- Storage Fuel: Triglycerides

These characteristics mean you can perform long-duration exercise without relying on shorter-term fuel like glycogen.  However, you can’t produce much power, so it will have to be low-intensity.

These fibers have less potential for growth than type 2, faster twitch fibers, which is part of the reason you don’t see long-distance runners with jacked quads, even though they’re working them hard.

TYPE 2 

Type 2 muscle fibers are called fast-twitch.  

There are 3 sub-types of type 2 fibers: a, x, and b.

Type 2 a fibers have more endurance than type 2 x fibers, but less than type 1 fibers. They’re fairly resistant to fatigue, and are used for anaerobic activities that last between 5 and 30 minutes.

Other characteristics of type 2 a fibers:

- Mitochondrial Density: High 

- Capillary Density: Intermediate 

- Oxidative Capacity: High 

- Glycolytic Capacity: High 

- Storage Fuel: Creatine Phosphate, Glycogen

These are the fibers that would be used for activities like a timed mile run, a 5K race, or a 10-minute row for distance. 

 Type 2 x fibers are fast-contracting fibers that fuel anaerobic activities less than 5 minutes.  They are less resistant to fatigue but produce more power.

Other characteristics of type 2 x fibers: 

- Mitochondrial Density: Medium 

- Capillary Density: Low 

- Oxidative Capacity: Intermediate 

- Glycolytic Capacity: High 

- Storage Fuel: Creatine Phosphate, Glycogen

These fibers are used for activities like a quarter-mile sprint, a high rep set of squats, or a 2-minute assault bike ride. 

Type 2 b fibers are the least resistant to fatigue and contract the fastest, and have the largest motor neurons.  They’re used for anaerobic activities that last less than a minute.

Other characteristics of type 2 b fibers: 

- Mitochondrial Density: Low 

- Capillary Density: Low 

- Oxidative Capacity: Low 

- Glycolytic Capacity: High 

- Storage Fuel: Creatine Phosphate, Glycogen 

These fibers are used for activities like a 100m sprint, a 3 rep max lift, or a box jump.

TRAINING TO SHIFT MUSCLE FIBER TYPE

It used to be said in the fitness industry that you could change the types of muscle fibers you had by training in different ways.  

For example, if you trained for endurance you’d shift to more slow-twitch, and if you sprinted or trained with Olympic lifting you’d shift to more fast-twitch. 

The truth is you don’t shift your muscle fiber types with training.  In fact, a study by Fry et. Al in 2003 found that powerlifters and the general population had really similar proportions of muscle fiber types.

When you train for hypertrophy and your muscles grow, what happens is your type 2 muscle fibers grow in size, not in proportion. Type 2 fibers grow about 25-75% more than type 1 fibers in response to training.  They hypertrophy, but don’t multiply.  

So this means you probably shouldn’t worry about training to shift your muscle fiber types, just train for the adaptation you want.  If you want bigger muscles train for hypertrophy, if you want to improve endurance train for endurance. 

According to Henneman’s Size Principle, your nervous system starts by activating type 1 muscle fibers, and activates more and more of them until it needs to activate type 2, then activates more and more until you can’t produce any more force.  At that point, you have to end your set or sprint. 

Since your type 2 fibers are the ones with the most potential for growth, if you want bigger muscles you need to activate them.  

Taking Henneman’s Size Principle into account, that means you need to take your sets pretty close to failure regardless of the number of reps you’re doing.  

This is why most of the hypertrophy training we design for our clients takes most sets to 1-3 RIR.

This is where you want to consider what your rate limiter for a given exercise is.   

You want the thing that makes you stop the exercise to be the target muscle, not your lungs, grip, core, or any other muscle group. 

How to make sure the target muscle is your rate limiter:

→ Starting with a weight that allows for 6-15 reps means you’ll probably effectively activate your type 2 muscle fibers because you’ll be taking the muscle close to failure before your lungs give out. 

→ Using an exercise that doesn’t cause your grip to give out ensures you are taking the target muscle close to failure. Wrist wraps are a useful tool for preventing grip giving out. 

→ Selecting exercises like hack squat over front squat takes the core and upper back out of the equation. 

→ Use an exercise where stability isn’t a limiting factor.  For example, if you want to do a unilateral hamstring exercise, a b-stance Romanian deadlift may be better than a single-leg Romanian deadlift where you’re trying to balance.

This doesn’t mean the exercises that don’t have a target muscle as the rate limiter are bad, it just means for hypertrophy they may not be optimal. 

[For more on choosing the best exercises for hypertrophy beyond rate limiters, read the Exercise Selection 101 blog post.]

Type 1 fibers have a high capacity for oxidating fats, a low capacity for using glycogen as fuel, and store triglycerides as fuel. 

Type 2 fibers have a low capacity for oxidating fats, a high capacity for using glycogen as fuel and store creatine phosphate and glycogen as fuel.

This means if you are training for endurance and using more type 1 muscle fibers you want to have triglycerides available (You always have triglycerides available. Even lean people can run for days on stored body fat.) 

If you’re training in shorter-duration and using more type 2 muscle fibers you want to maximize the glycogen and creatine phosphate available as fuel. 

Since most of you reading this are training for strength, performance, or hypertrophy, this means you will primarily rely on carbohydrates for fuel while training.  

Regardless of your rep range or goal in weight training, you’ll be using your type 2 muscle fibers that need the glycogen.

If you’ve ever been in a fat-loss phase and ended up with relatively low carbohydrate intake, you know the feeling of trying to lift weights without enough glycogen stored.   

This is why as a general guideline we suggest:  

→ Setting your protein around 0.8-1.2 grams per pound of bodyweight  

→ Setting fat at the minimum effective dose of 0.3-0.4 grams per pound of bodyweight 

→From there, filling the remainder in with carbs

This ensures that as much as possible of your fuel comes from carbs, while still getting the health benefits you need from fat, and getting enough protein to build and preserve muscle mass.

So that explains the stored glycogen but what about adenosine triphosphate (ATP)? 

The first 10 seconds of an exercise are fueled by the phosphagen system.  This system replenishes stores of ATP, which provides energy to working cells.

ATP is broken down by removing a phosphate, which means it’s left with two phosphates and is now called adenosine diphosphate (ADP).  

To make more ATP, the ADP needs to find another phosphate.  Enter creatine phosphate.  Creatine phosphate will give one of its phosphates to ADP to create ATP to use for more fuel.  In short, more creatine = more potential ATP. 

Having more ATP to fuel anaerobic activities means you may be able to lift a little heavier, and you may get an extra rep or two here and there.  Over time, this adds up to more volume, hypertrophy, and more strength. 

Creatine is found naturally in red meat, but most take it in supplement form as creatine monohydrate.  

As an aside, beyond creatine’s effect on strength and power by generating more ATP, it can improve muscle protein synthesis, improve glycogen storage in the muscle, and can even have neuroprotective benefits. 

Because type 1 muscle fiber types don’t use creatine phosphate as a fuel source, taking exogenous creatine won’t improve your endurance.  However, because of its other benefits, I recommend everyone take it anyway unless contraindicated by your doctor.

Even though you can’t shift your muscle fiber types by the way you train, knowing the different types and the order in which they’re recruited is useful information in the pursuit of more muscle hypertrophy, and knowing how to properly fuel your activity to get the adaptation you’re training for.  

You may not need to know the capillary density of the muscle fiber types in order to actually build muscle, but knowing why your muscles work the way they do means you’re not just blindly following a program with certain rep ranges or RIR because you saw it somewhere, but you know why you’re doing it. 

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We'll take care of all of the nuances like muscle fiber types, and build the perfect protocol to get you to your goal physique - all you have to do is execute.