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Type IIa Muscle Fibers: Training for Explosiveness

Type IIa Muscle Fibers: Training for Explosiveness

Reading Time: 7 minutes 44 seconds


DATE: 2022-04-01

Type IIa Muscle Fibers – Training for Speed, Strength, and Power

For your more serious, athletic clients, those who want to go beyond basic fitness and hit more difficult power goals, it may be time to consider training to type.

Type of what? Muscle fiber, of course.

As a trainer you know about different muscle fibers, type I, type IIa, slow twitch, fast twitch, and so on. But your clients may not know that training can get more specialized, that you can design programs to specifically target the muscle fibers that will help them meet their goals and optimize performance.

Those who are more interested in getting stronger, more powerful, and faster than being able to pound out a marathon or an Ironman may benefit from training for type IIa muscle fibers. Here’s what they—and you—need to know.

Muscle Fiber Types – Slow Twitch, Fast Twitch

Muscles are made up of multiple motor units. Each one contains a bundle of nerves and fibers. Muscle fibers can be any combination of three different types: type I, type IIa, and type IIx. And these are categorized as slow twitch or fast twitch, depending on how quickly they produce tension in a muscle.

The proportion of each type depends on several factors, such as what you’re born with, the individual muscle, your age, and your type and level of physical fitness and training.

Understanding the differences between these fibers, what they do, how they are engaged during activities, and their relative proportions can help you and your clients develop and reach more specific training goals.

Slow Twitch vs. Fast Twitch

Slow twitch muscle fibers are known as type I fibers, and the name describes the fact that they contract slowly and steadily. These fibers are recruited for endurance and lower-intensity activities. They can keep contracting and working without fatiguing for long periods of time, creating energy through aerobic processes.

Fast twitch muscle fibers on the other hand contract quickly and more powerfully. They are needed for high-intensity but short-duration activities, like sprinting or lifting a really heavy weight. They contract quickly and fatigue quickly. This is because they largely use anaerobic metabolism for energy, which results in lactic acid production and soreness that causes the muscle to fatigue.

Type I Muscle Fibers

These are the slow twitch muscle fibers. They are smaller than type II fibers, are slower to produce tension, and they produce less force and power. They do, however, have the advantage of being slow to fatigue. Type I fibers rely on oxygen for energy and can keep going for long periods of time.

They are used for endurance activities, and endurance athletes tend to have a higher proportion of these muscle fibers. Long-distance runners, triathletes, and distance swimmers and cross country skiers really rely on type I fibers.

Type IIa Muscle Fibers

Type IIa muscle fibers are fast twitch, meaning they fire more quickly. They are also more powerful than type I fibers and are recruited for activities that require more intensity: sprinting, lifting heavy weights.

These fibers provide major strength, but they also fatigue more easily than type I fibers. They rely on anaerobic processes and produce lactic acid, so they can’t keep going as long as type I. Strength and power athletes, like sprinters and weightlifters, have higher proportions of type IIa fibers in their muscles.

Type IIx Muscle Fibers

There is a second type of fast twitch muscle fiber called type IIx. These are even faster and more powerful than type IIa. They are also even more inefficient, fatiguing very quickly. Type IIx fibers are used for activities of very short duration that require significant power and strength.

The type IIx fibers are sometimes referred to as “couch potato” muscle fibers. Even people who are inactive need to be able to run quickly or lift something in an emergency. This is where the couch potato fibers come in handy. They are metabolically efficient at rest but still allow you to react to a stimulus if necessary.

If you work out, even just a little, those IIx fibers will quickly convert to the more useful, longer lasting IIa fibers. During a period of inactivity, they revert back to IIx.

The Size Principle of Motor Unit Recruitment, or How to Target Type IIa Muscle Fibers

In order to increase the size and number of any muscle fiber type, including type IIa muscle fibers to improve strength and power, you need to understand how different fiber types are recruited during muscle contraction.

The process and order in which muscles rely on fiber types to contract is described by the Size Principle of Motor Unit Recruitment, which tells us that:

  • Motor units are used during muscle contraction in order of increasing recruitment threshold and firing rate.

  • Those fibers with a low threshold and slower firing rate will be used first. So, slow twitch, type I fibers go first.

  • Motor units with muscle fibers that have a higher threshold and faster firing rate are recruited and used next.

  • Only after type I fibers are recruited do type II fibers begin to fire, first type IIa and then type IIx.

  • When engaging in an activity, such as running fast, type I fibers may be enough. If they are not, only then will the body engage type II fibers.

In other words, when your body is trying to do some type of activity, such as lifting a weight, it will try to do it with type I muscle fibers first. If these don’t provide enough force, your brain sends a signal to get the type II muscle fibers to finish off the job.

What this should tell you about training for power and strength is that you need to get past the type I recruitment in order to access and exercise type II muscle fibers.

If you only ever do low-intensity, endurance activities your muscles will never get to the type II fibers and they won’t grow or increase in number. Focusing on high-intensity strength training and explosive movements produces improvements in force and power by recruiting and using type II fibers.

Why Train for Type IIa Muscle Fibers

While evidence that training to improve a specific muscle fiber type are mixed, it can be worthwhile. Here are some important reasons you or your client may want to focus on type IIa muscle fibers when training:

  • These muscle fibers act fast, so you get a performance advantage when you have more and larger type II fibers.

  • Type II fibers are very responsive to training. Fast twitch muscle fibers tend to grow II5 to 7 percent more in response to training versus slow twitch.

  • The peak power of the type IIa fibers is also greater, so when you have more of them you can lift heavier weights and run faster.

  • Power athletes generally have more type IIa muscle fibers, which indicates that to be able to perform weight lifting, sprinting, and similar sports, you need to develop these fibers.

  • As we age, there is a decline in lean muscle mass, including type I and type II fibers. Fast twitch, or type IIa and type IIx are larger fibers and contribute to metabolic efficiency. If we don’t keep these muscle fibers active, we lose them over time. This contributes to age-related metabolic dysfunctions, increased risk of injuries like falls, and less healthy body composition changes.

Can Training Really Target and Improve Muscle Fibers by Type?

The short answer is maybe.

The longer answer is that evidence from research is mixed. There are some studies that show that more intense training, such as with heavier weights, does increase growth in type II fibers. They also demonstrate that lower intensity training increases the amount of type I fibers in muscles.1

Other studies, though, have not gotten the same results, and in fact showed that both types of training increase type I fiber growth.2

So is it worth trying to train specifically for one type of muscle fiber? Probably. It certainly can’t hurt as long as you ensure your client gets some variation in training. Let them focus mostly on power and strength for increasing type II muscle fibers, but also insist on one or two endurance sessions per week.

Knowing Your Muscle Fiber Type

Some people naturally have more type I or type II fibers because of factors like age or genetics. There is easy way to reliably determine any individual’s fiber type makeup, unfortunately. A muscle biopsy can tell you your percentage of each type, but this is pretty invasive for the average individual or even athlete.

There also may be differences in muscle fiber type percentages from one muscle in your body to the next. The type of training you do can also be a clue as to which type of fiber dominates your muscles:

  • If you do more endurance activities, like distance running you probably have more type I, slow twitch fibers, up to 70 to 80 percent.

  • On the other hand, if you are more of a sprinter or bodybuilder, you likely have more type II fibers.

You really can’t know for sure how your muscle fibers are split, and neither can your clients. And even if you did, it shouldn’t limit you from other types of activities. For instance, if you found out you tend to naturally have more slow twitch fibers, you wouldn’t let that stop you from trying power and speed exercises.

If your clients want to know where they stand on muscle fiber type, try to impress upon them that it isn’t that important. But those who do want to improve strength, power, and speed, can focus on training session that target and build type IIa fibers.

So How Do we Hit Type IIa Fibers in Training?

The concept is pretty simple. If you perform more exercises that require fast twitch muscle fibers, you’ll develop these fibers, increase their size, and essentially train your brain to access type II fibers.

On the other hand, endurance workouts will train you to recruit type I fibers.

So, if you are training for power, to maximize type IIa muscle fibers, you need to do more power, high-intensity, and strength workouts and fewer endurance workouts. Think intensity, speed, and explosive force, not long and slow.

Also important to consider in type II muscle fiber training are high and low impact movements. Lower impact, plyometric movements focus on power in speed dominant sports. The resistance you need to overcome is typically lower.  

High impact plyometric exercises focus on the opposite and require more resistance. In the list below, lower impact training includes Olympic lifts and sprints. Higher impact training includes drop jumps and jump squats.

Here are some examples of exercises you can use with your client to train for type IIa fibers. Sets with 10 to 15 repetitions are ideal for recruiting and maximizing fast twitch fibers.

  • Weighted plyometric exercises, such as jump squats.

  • Speed squats.

  • Speed benches.

  • Olympic lifts.

  • Drop and catch moves.

  • Drop jumps.

  • Sprints.

Most people working out with a trainer benefit from a combination of endurance and power workouts. But, if you have that one client who really wants to focus in on the latter, help him learn how to do it. Train to type and you can really push him to his goals of greater power, strength, and speed.

The ISSA offers a certificate in strength and fitness that delves deeper into the area of focused training. You can sign up here and learn how to better help your clients with their fitness goals.

Type IIa Muscle Fibers: Training for Explosiveness

Click HERE to download this handout and share it with your clients!


  1. Netreba, A.I., Popov, D.V. Liubaeva, E.V., Bravyi, I.R., Prostova, A.B., Lemesheva, I.S. Vinogradova, O.L. (2007) Physiological Effects of Using the Low Intensity Strength Training without Relaxation in Single-Joint and Multi-Joint Movements. Ross Fiziol Zh Im I M Sechenova. 93 (1) 27-38

  2. Campos, G.E., Luecke, T.J., Wendeln, H.K., Toma, K., Hagerman, F.C., Murray T.F., Ragg, K.E., Ratamess, N.A., Kraemer, W.J., Staron, R.S. (2002). Muscular Adaptations in Response to Three Different Resistance-Training Regimens: Specificity of Repetition Maximum Training Zones. European Journal of Applied Physiology, 88 (1-2) 50-6

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