If you watch sports on TV, at some point you may hear a commentator talk about an athlete having explosive or powerful muscles. For example, professional football players like JJ Watt receive a lot of attention for off-season conditioning programs that include exercises like flipping a large truck tire. You may have noticed that when discussed, the sportscaster will note that an athlete’s particular training technique is done to develop 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 type of muscle fiber?

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 have been identified including type I, type IC, type IIC, type IIAC, type IIA, type IIA and type IIX they are generally classified as being either slow twitch or fast twitch.

Here are 6 things to know about slow twitch, type I, muscle fibers:

  1. Slow twitch fibers contain mitochondria, the organelles that use oxygen to help create adenosine triphosphate (ATP), 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 creating 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. The slow twitch fibers have a low activation threshold, meaning they are the first recruited when a muscle contracts. If they can not 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.

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 include:

  • 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 which can help improve their ability to utilize oxygen to produce energy. Using an ActivMotion Bar can provide excellent feedback for developing slow twitch fibers – moving the bar as slow as possible to prevent the ball bearing from shifting can keep muscles under resistance longer, helping to develop the aerobic capacity of the slow twitch fibers and forcing your core to activate and stabilize as the bar shifts.
  • Resistance training exercises using lighter weights with slower movement tempos for higher numbers of repetitions, more than fifteen, can engage the slow twitch fibers to use aerobic metabolism to fuel the activity.
  • Circuit training, alternating from one exercise to the next with little-to-no rest, using lighter weights can be an effective way to challenge slow twitch fibers. Here is an example of a lower body exercise circuit using the ActivMotion Bar to help develop strength and aerobic capacity. Try adding a Core Hammer or set of Core Flytes to the circuit workout as well to increase the training benefits and fun factor!
  • When working with bodyweight-only or lighter amounts of resistance, allow for shorter rest intervals of approximately thirty seconds between sets to challenge the slow twitch fibers to use aerobic metabolism to fuel the workout.

Here are things to know about fast twitch, type II, muscle fibers:

  1. Fast twitch fibers can be further classified into fast twitch IIa – fast oxidative glycolytic because they use oxygen to help convert glycogen to ATP and 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 provide.
  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.

The characteristics of fast twitch fibers are more suited for explosive, strength-and-power-based sports like football. If you want to engage more fast twitch fibers to help you increase strength levels or become more explosive her are a few specific techniques that work:

  • 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 will be recruited. Using the heavier ActivMotion Bars can help recruit more fast twitch fibers, especially when a lift is performed until a muscle achieves momentary fatigue.
  • Performing explosive, power-based movements; whether it is with a barbell, kettlebell, medicine ball, ActivMotion Bar or simply your own bodyweight, fast, explosive muscle contractions 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, from two-to-six, at a time for maximum effectiveness.
  • Because they deplete energy quickly, fast twitch fibers require longer rest periods to allow motor units to recover and replace spent ATP. So, allow at least sixty-to-ninety 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, and identifying whether you are fast or slow twitch dominant would require an invasive muscle biopsy. However, if you find that you tend to enjoy more endurance-based activities and they are relatively easy for you, then 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, then 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 workouts.

What muscle fiber types are you more interested in training, and why?

Thanks for reading!

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