MuscleGeek 2: Muscle Fiber Types

For Science Enthusiasts and Gym Rats

Types of Muscle Fibers and Specificity in Training

If you recall from Issue One of Muscle Geek, there are thousands of parallel muscle fibers similar to strands of hair in a ponytail, or spaghetti noodles wrapped together that make up your muscles. In this issue, we are going to discuss (1) the types of muscle fibers there are, (2) how to test which types you have, and (3) how to train for a specific type of fibers, because the type of fibers you have CAN CHANGE!

What Kinds of Muscle Fibers Are There?

There are two types of muscle fibers: Type I, being classified as a slow-twitch fiber, and Type II, fast-twitch fibers. Type II fibers are also further classified in two more groups, type IIa, and type IIx, which differ in contractile and biochemical properties.

Slow-Twitch, Type I

These fibers have a high concentration of mitochondria and myoglobin in cells. Mitochondria are the energy-producing organs in the body. These fibers are also surrounded by more capillaries than the other types of fibers, which supports their capacity for aerobic metabolism, and resistance to fatigue during long bouts of exercise. These fibers produce less force, are slower to contract and create maximal tension compared to other fibers, but can work for longer periods of time, and are important for stabilizing the body as well as posture. Walking on the treadmill for hours, standing at work, and even holding yourself upright uses these types of muscle fibers.

Fast Twitch, Type II

Your fast twitch fibers generally have lower concentrations of mitochondria, myoglobin and surrounding capillaries than slow-twitch fibers, and therefore; tire faster. These fibers are larger and can produce quick and great amounts of force, which are important for power activities, weight lifting, and plyometric-type exercises. These muscle fibers can produce more force than other types, but can not work for as long as the slow-twitch fibers can.

Fast-Twitch, Type IIx

These muscle fibers produce the most force out all the types, but because they run anaerobically (think sprints) they are inefficient, and cannot work for long periods of time. When you do all out, 1-RM activities or other power movements that you can only repeat a few times and with rest, this is the fiber type you are using.

Fast-Twitch, Type IIa

These muscle fibers are also called intermediate fibers, and are a mix of type I and type II muscle fibers. Therefore, they have a mix of contractibility and force generation. Something really cool about these fibers is that they can run aerobically, as well as anaerobically. They fatigue more slowly than type IIx.

So everyone has at least a mix of all the fiber types, but as you can see for varying athletic goals, abilities, and skills, the proportion of these respective types will differ. A sprinter and powerlifter would have trained their type II fibers much more than a dance instructor, dog-walker, or casual weight lifter would.

Picture courtesy of ucl.ac.uk

How to Test Which Types of Fibers You Have?

Now that you know the different types, are you curious as to the types you have? You could get a multiple muscle biopsies (ouch!) and as not al muscles are the same, build a profile of what you are, since you have fast and slow muscle fibers in all of your muscles. But generally speaking, non-trained individuals will have about a 50/50 ratio of fast and slow types. Highly skilled, elite athletes will have significant differences, (for example, an Olympic sprinter could have as much as 75% fast-twitch fibers; marathon runners could have as much as 80% slow-twitch or type 1 fibers).

The other large factor in muscle fiber type is age. As we continue to age, we lose muscle mass and decline in type IIx fibers in particular, and an increase in type I (or slow-twitch fibers). Fast-twitch fibers are also bigger in size, so aging can also lead to metabolic changes (the need for fewer calories to maintain body weight), body composition changes. Weight training as we age can combat falls, as well as the decline in the desirable types of muscle fibers.

How to Train for the Fibers you Want

If you were genetically blessed with lots of fast-twitch muscles but you want to run marathons, half-marathons, or even an Ironman, you can modify your fiber types to some degree.

If you want more Type I fibers, endurance training, lower resistance weight training, lower intensity, but longer duration and repetitions will be key.

If you want more type II fibers, strength training is the only way.

Lifting weights increases the size of both fiber types, but the hypertrophy (muscle development and size) occurs in the type II fibers. Along with hypertrophy in these fibers, strength training also increases the ability for more force production.

Type IIx can be trained to become type IIa by consistent weight training.

If you taper your training program to prepare for a meet, or event (reduce the volume and the intensity) strength and power of type IIa fibers can occur without the decrease in type 1 fiber performance.

Questions? Comments? How to Contact Us?

If you have any questions regarding this issue of Muscle Geek, or would like to share anything with us, please email us at: moralesfitclub@gmail.com. We would love to hear from you.

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References:

1. Clark MA, Sutton BG, and Lucett SC (Editors). (2014). NASM Essentials of Personal Fitness Training (4th edition revised). Burlington, MA: Jones & Bartlett Publishing.

2. Powers SK, and Howley ET. (2012). Exercise Physiology: Theory and Application to Fitness and Performance, (8th Edition). New York, NY: McGraw Hill.

3. Akasaki Y, Ouchi N, Izumiya Y, Bernardo B, LeBrasseur N, and Walsh K. (2013). Glycolytic fast-twitch muscle fiber restoration counters adverse age-related changes in body composition and metabolism. Aging Cell 13:80-91. doi: 10.1111/acel.12153

4. Vanhatalo A, Poole DC, DiMenna FJ, Bailey SJ, and Jones AM. (2011). Muscle fiber recruitment and the slow component of O2 uptake: constant work rate vs. all-out sprint exercise. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 300:3, 700-707.

5. Trappe S, Harber M, Creer A, Gallagher P, Slivka D, Minchev K, and Whitsett D. (2006). Single muscle fiber adaptations with marathon training. Journal of Applied Physiology, 101:3, 721-727. doi: 10.1152/japplphysiol.01595.2005