Muscle Fiber Types: Fast-Twitch vs. Slow-Twitch

If you watch sports on TV, at some point you’ve probably heard a commentator talk about an athlete having explosive or powerful muscles. For example, professional football player JJ Watt has received a lot of attention for his off-season conditioning program, which includes flipping a large truck tire. A sportscaster was recently discussing Watt’s training techniques and mentioned that Watt was working on his fast-twitch muscle fibers in an effort to become more explosive. At first, this sounds kind of hokey—fast-twitch muscle fibers? Is that really a thing, and is it possible to do certain exercises that focus on one muscle fiber type?

The answers, in short, are yes and yes. 

Yes, there are different types of muscle fibers in the body, which are classified based on how they produce energy. Yes, the different muscle fibers can be trained using specific exercises designed to focus on how they create energy or generate force. While a variety of types of muscle fiber has been identified, including type I, type IC, type IIC, type IIAC, type IIA, and type IIX, they are generally classified as being either slow-twitch or fast-twitch (see table). 

6 Things to Know About Slow-twitch, or Type I, Muscle Fibers:

1. Slow-twitch fibers contain mitochondria, the organelles that use oxygen to help create adenosine triphosphate (ATP), which is the chemical that actually fuels muscle contractions, and are considered aerobic.
2. Slow-twitch fibers are also called red fibers because they contain more blood-carrying myoglobin, which creates a darker appearance.
3. Because they can provide their own source of energy, slow-twitch fibers can sustain force for an extended period of time, but they are not able to generate a significant amount of force.
4. Slow-twitch fibers have a low activation threshold, meaning they are the first recruited when a muscle contracts. If they can’t generate the amount of force necessary for the specific activity, the fast-twitch muscle fibers are engaged.
5. The tonic muscles responsible for maintaining posture have a higher density of slow-twitch fibers.
6. Steady-state endurance training can help increase mitochondrial density, which improves the efficiency of how the body uses oxygen to produce ATP.

As you can see, slow-twitch fibers have specific characteristics for how they function, which means they can be trained to be more aerobically efficient with the proper exercise program. 

Techniques for Training Slow-twitch Fibers:

• Exercises that feature sustained isometric contractions with little-to-no joint movement keep the slow-twitch muscle fibers under contraction for an extended period of time. This can help improve their ability to utilize oxygen to produce energy. Examples include the front plank, the side plank and the single-leg balance.
• Resistance-training exercises using lighter weights with slower movement tempos for higher numbers of repetitions (i.e., more than 15) can engage the slow-twitch fibers to use aerobic metabolism to fuel the activity.
• Circuit training, which involves alternating from one exercise to the next with little-to-no rest while using lighter weights, can be an effective way to challenge slow-twitch fibers.
• Body-weight exercises for higher numbers of repetitions can be an effective way to challenge aerobic metabolism, which helps improve the efficiency of slow-twitch fibers.
• When working with body-weight only or lighter amounts of resistance, use shorter rest intervals of approximately 30 seconds between sets to challenge the slow-twitch fibers to use aerobic metabolism to fuel the workout. 

8 Things to Know about Fast-twitch, or Type II, Muscle Fibers:

1. Fast-twitch fibers can be further classified into (1) fast-twitch IIa - fast oxidative glycolytic, because they use oxygen to help convert glycogen to ATP, and (2) fast-twitch type IIb - fast glycolytic, which rely on ATP stored in the muscle cell to generate energy.
2. Fast-twitch fibers have a high threshold and will be recruited or activated only when the force demands are greater than the slow-twitch fibers can meet.
3. The larger fast-twitch fibers take a shorter time to reach peak force and can generate higher amounts of force than slow-twitch fibers.
4. Fast-twitch fibers can generate more force, but are quicker to fatigue when compared to slow-twitch fibers.
5. The phasic muscles responsible for generating movement in the body contain a higher density of fast-twitch fibers.
6. Strength and power training can increase the number of fast-twitch muscle fibers recruited for a specific movement.
7. Fast-twitch fibers are responsible for the size and definition of a particular muscle.
8. Fast-twitch fibers are called “white fibers” because do not contain much blood, which gives them a lighter appearance than slow-twitch fibers. 

As you can see, the characteristics of fast-twitch fibers are more suited for explosive, strength- and power-based sports like football. Therefore, when an announcer talks about how a training program benefits a specific type of muscle fiber, they are being accurate with the science. 

If you want to engage more fast-twitch fibers to help you increase strength levels or become more explosive, here are a few specific techniques that work.

Techniques for Engaging Fast-twitch Fibers:

• Resistance training with heavy weight stimulates muscle motor units to activate more muscle fibers. The heavier the weight, the greater the number of fast-twitch fibers that will be recruited.
• Performing explosive, power-based movements, whether it is with a barbell, kettlebell, medicine ball or simply your own body weight, will recruit greater levels of fast-twitch fibers.
• Fast-twitch fibers will fatigue quickly, so focus on using heavy weight or explosive movements for only a limited number of repetitions (e.g., two to six) for maximum effectiveness.
• Because they deplete energy quickly, fast-twitch fibers require longer rest periods to allow motor units to recover and to replace spent ATP. Therefore, allow at least 60 to 90 seconds of rest after each explosive or strength exercise. 

Understanding how the physiology of the body adapts to exercise can help you develop more effective exercise programs for your specific needs. Genetics determines how much of each muscle-fiber type you possess; however, identifying whether you are fast- or slow-twitch dominant would require an invasive muscle biopsy. Therefore, if you find that you tend to enjoy more endurance-based activities and that they are relatively easy for you, you probably have a greater number of slow-twitch fibers. Conversely, if you really dislike going for long runs, but enjoy playing sports that rely on short bursts of explosive movements, or if you like weight training because it is relatively easy, you are probably fast-twitch fiber dominant. An exercise program that applies the right training strategies for your muscle fibers can help you to maximize the efficiency and enjoyment of your workout time.