How to Build Your Fast & Slow-Twitch Muscle Fibers

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Twitch Muscle Fibers

Digging Deeper Into Muscle Physiology

If you haven’t read our previous article about fast and slow-twitch muscle fibers, be sure to give it a once-over, as it’s a great primer about the differences between the two and how that might inform your training protocols.

What it didn’t get into were the specifics around how you would go about targeting certain fibers for certain training goals. It is important for trainers and athletes to understand the variables at play. A failure to integrate this information into your programming represents a serious competitive disadvantage compared to your more knowledgeable competitors.

How To Target Specific Muscle Fiber Types

The two categories of muscle fiber—slow-twitch and fast-twitch—can be further divided into three groups: Type I, Type IIa, and Type IIx. Type I is slow-twitch, Type IIa is fast-twitch, and Type IIx is a special variety of fast-twitch that is even more powerful and fatigues even more quickly than regular Type IIa.

A great model for understanding this topic starts with the question: “What type of fuel is being used for this activity?” If you can identify the fuel being used, you can accurately determine which muscle fiber type you are stimulating.

Twitch Muscle Fibers- Chart

Take a look at this chart. If fatty acids are the primary fuel being used, then the slow-twitch fibers (Type I) are performing most of the work. Activities in this category are low-intensity, things like walking, biking, swimming, or other similar movements. This type of fuel use can be prolonged for extended periods of time (hours). A clue that you’re using fatty acids for fuel is that you do not feel any “burning” sensation in the muscle.

If glycogen—the storage form of glucose in the muscle—is being used, then the fast-twitch fibers (Type IIa) are performing most of the work. Activities in this category involve intense muscle contractions, things like high-intensity intervals, lifting heavy weights, running, or plyometrics. This type of fuel can only last for limited amounts of time (minutes). A clue that you’re using glycogen for fuel is that you do feel a “burning” sensation in the muscle.

If creatine phosphate is being used, then the specialized fast-twitch fibers (Type IIx) are performing most of the work. Activities in this category include all-out efforts, things like sprinting, maximum-effort weight lifting attempts, explosive jumping, or anything you’re doing “as hard as you can.” This type of fuel can only sustain you very briefly (seconds).

Before we continue on and get into more specific applications of this information, recognize that the active ingredient here is force demand. These three types of muscle fibers are activated in response to the force that is being demanded of them.

Low force demand = only the slow-twitch fibers (Type I) will be called into action.

High force demand = the slow-twitch fibers (Type I) and the fast-twitch fibers (Type IIa) will be called into action.

Maximum force demand = the slow-twitch fibers (Type I), the fast-twitch fibers (Type IIa), and the specialized fast-twitch fibers (Type IIx) will be called into action. In other words, “all hands on deck!”

The Best Way to Build Your Slow-Twitch (Type I) Muscle Fibers

So let’s put some theory into practice.

As discussed above, the slow-twitch fibers (Type I) are the most commonly used in any skeletal muscle, since they handle low-force workloads for long periods of time. Getting out of bed in the morning, walking around the house, going about your day, and even a light jog will all be accomplished by the slow-twitch fibers.

These fibers produce low levels of force, but they can endure for a long time and recover quickly. Therefore, the best way to train these fibers is to perform low-force activities for extended periods of time on a high-frequency basis (daily or close to it).

Great methods for this are:

  • 30-60 minutes of Zone 2 training
  • A fast-paced hike lasting one to two hours
  • 30-60 minutes of slow swimming
  • A bicycle ride lasting one or two hours

The name of the game here is 1) low force, 2) extended duration, and 3) frequent performance.

The Best Way to Build Your Fast-Twitch (Type IIa) Muscle Fibers

Force demand is the lever we can adjust to incorporate higher-order muscle fibers. So, to include the Type IIa muscle fibers in our exercise stimulus we must increase the force demand. As a consequence of this increase in intensity of effort and production of force, we will only be able to continue this activity for a limited amount of time, and we will require more rest in between performances.

So the formula here is high-force, shorter duration, less frequent. What are our best options?

  • Weight lifting with 60% of your one-rep max or above
  • Running, biking, rowing, or otherwise moving at a pace that produces a burning sensation in the muscle
  • High-intensity interval training

As long as it’s high force demand, cannot be sustained for more than 30-45 minutes, and produces a burning sensation in the muscle, it is recruiting and stimulating both the Type I and Type IIa fibers. But what about the elusive Type IIx fibers?

The Best Way to Build Your Specialized Fast-Twitch (Type IIx) Muscle Fibers

We look once again to force demand to gain access to these highest-threshold muscle fibers. To provoke the Type IIx muscle fibers, we have to increase the force demand to the absolute maximum of which we are capable.

As a consequence of this maximal increase in intensity of effort and production of force, we will only be able to continue this activity for a very short period of time, and we will require even more rest in between performances.

So, we want the highest force demand possible, that cannot be sustained for very long at all, and that is performed infrequently to allow for recovery. Things like:

  • Sprinting
  • Maximum-effort lifts
  • Jump training
  • Plyometrics

Or You Can Just Use ARX

Let’s walk through a set of Leg Press on an ARX machine for an example of optimal muscle fiber recruitment.

In the ARX software, let’s say you set the timer for two minutes. Then let’s say you program the travel time to five seconds in the concentric and five seconds in the eccentric, with no pause in between repetitions. You math whizzes out there will have already calculated that twelve repetitions will be performed in these two minutes.

During the first fifteen seconds of maximal effort, all available muscle fibers will be called into action. The Type I, Type IIa, and Type IIx fibers are all working at maximum against the resistance of the machine, which is changing moment by moment to accommodate your changing levels of force through the range of motion. This is most potent in the eccentric, as the foot plate is moving towards you and you are producing as much force as possible to try to resist the foot plate.

After this fifteen seconds, your muscles’ supply of creatine phosphate has run out, and the quick-fatiguing Type IIx fibers drop out. You’re now down to the Type I and Type IIa fibers, burning primarily glycogen.

As you move past the one-minute mark, and further past ninety seconds, you experience the orderly drop-out of Type IIa fibers as they fatigue in series, one-by-one. You can watch this fatigue play out on the computer screen as you lose the contributions of these bigger, stronger fibers.

By the end of the two minutes you are left with just the Type I fibers, the slow-twitch fibers that can endure for longer, but cannot produce that much force. The screen will register the lowest force readings of the entire set, oftentimes sixty percent or more below your maximum force readings from the beginning.

At the end, the muscle cells will be full of lactic acid and unprocessed pyruvate, which is produced as a result of glycolysis (burning glycogen). This pyruvate is then processed aerobically in the mitochondria for the next several hours. So yes, there is a potent aerobic stimulus here too.

So instead of having to schedule all those different types of activities to stimulate the different types of muscle fibers, you need only perform a set on an ARX machine. Yes, if you want to improve your performance at a specific skill you have to practice that skill, whether it’s running, biking, swimming, or juggling bowling pins.

But if you don’t much care which activity you perform and just want the stimulation for your muscle fibers, ARX is your one-stop shop and will improve your performance in anything that uses those muscles.

And That’s How You Build Slow and Fast-Twitch Muscle Fibers!

You can go hard to hit the fast-twitch fibers, but you can’t go for long.

You can go for longer to hit the slow-twitch fibers, but then you can’t go that hard.

Or you can use ARX to safely go at your maximum (Type IIx), continue the set as you fatigue (Type IIa), and even accumulate deep metabolic stress as your strength decreases (Type I).


ARX simplifies the most comprehensive full-body workout through perfectly matched, motorized resistance. Short for Adaptive Resistance Exercise, ARX is scientifically proven to deliver quantifiable results in less time. The all-in-one strength training machine dynamically adjusts resistance in real-time to personalize every workout. ARX empowers and challenges individuals to achieve their fitness goals one perfectly calibrated repetition at a time. No dangerous weights to drop and no adjustments to make, just exact resistance. Founded in 2016, ARX is headquartered in Austin, Texas.

For more information, please visit ARXFit.com and join the community on FacebookInstagram and YouTube.