Think Muscle - Bodybuilding and Fitness

Fiber Types, Training, and Hypertrophy

From the Hypertrophy-Specific Training Series

By Bryan Haycock, Editor-in-Chief

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The issue of fiber type that continues to resurface in the discussion of different training methods. Some people insist that in order grow to get maximum growth, muscles must be trained according to fiber "types". A look at what determines a fiber's "type" should help clear up the issue and help you make a decision as to its relevance to training specifically for muscle growth.

"Fiber types" and how they are classified.

Muscle fibers were first classified according to their "function". Fast-twitch and Slow-twitch are the two basic types. It was later discovered that the twitch (twitch = contraction) characteristics were the result of different kinds of contractile proteins. Some proteins were good at contracting quickly, and were also dependant on "fast oxidative" pathways (ATP, and fast glycolytic pathways). The other type, slow-twitch, has contractile proteins that were different than those in fast-twitch fibers, and were dependant on "slow oxidative" pathways (beta-oxidation, fatty acid oxidation).

The two distinct metabolic profiles of fast- and slow-twitch fibers, give them distinct fatigue profiles. Fast-twitch fibers fatigue rapidly because their fuel source, ATP, is depleted rapidly. I use the term "depleted" loosely. Slow-twitch fibers fatigue slowly because their fuel source (fatty acids) take a long time to deplete.

There is another factor in the fatigability of fast- and slow-twitch fibers. The amount of power they are able to generate. Because fast-twitch fibers contract quickly, they are able to produce more "power" than slow-twitch. So fast twitch fibers use their available more quickly because the "motor-units" are larger. A motor-unit is a group of fibers connected to a single motor neuron. Keep in mind that power is a function of work over time.

The purpose of each different type of fiber?

Fast-twitch fibers are used to move your body mass quickly. This is important for running, jumping, and reflex movements (e.g. pulling your hand away from a hot stove). This requires short burst of relatively high force (but with low precision). Slow-twitch fibers are used to support the body posturally. This requires long/sustained contractions of relatively low force (but with high precision).

You will find a high proportion of slow-twitch muscle in the calves, and trunk (spine) and in the forearm predominantly. This makes sense when you think about it. Your calves, which contain your toe and foot muscles, are constantly working to balance your body while standing and walking. They are contracting constantly when you are standing. Your trunk muscles hold you upright when you are standing or sitting unsupported. Your forearms house your finger and hand muscles. These are used to hold things. Holding requires constant contraction of your finger muscles.

Isolating fiber types in training.

Forget about the notion of isolating fiber types while training for hypertrophy. You can't isolate fiber types per se when lifting a weight sufficiently heavy to cause muscle growth. Let me explain. Your brain activates muscle fibers in a specific sequence and manner based on the kind of movement it desires. This progressive activation of muscle fibers is called recruitment. Small "motor units" (motor neuron-muscle fiber unit with a low threshold of activation) are activated first to produce precise movements. These small motor units use slow-twitch fibers.

If activation of the inductive small motor units is insufficient to produce the desired movement, the brain activates progressively larger and higher threshold motor units. These larger motor units involve fast twitch fibers.

So, slow-twitch fibers are recruited first, followed by fast twitch fibers, based on the needed amount of strength (force or power). Because of this recruitment pattern, you could theoretically isolate small slow-twitch fibers, but you couldn't isolate fast twitch fibers because your brain activates slow-twitch first during any contraction. The greater the force of contraction, the greater the number of fast twitch fibers will be activated, but only after all slow twitch-fibers are activated.

So picture in your mind a dial that goes from 0 - 11. The numbers indicate how much force you want the muscle to generate, 0 being none and 11 being maximum intensity contraction. ON the dial, going from 1-5 the body will activate an increasing number of small motor units (slow twitch fibers) until it has activated them all. From 5-11, the small motor units will remain activated, but the body will add to them, large motor units (fast twitch fibers) until the desired muscular force is achieved. You progressively fine motor control as the amount of force goes up. This is a manifestation of the recruitment pattern just described.

Fiber type and muscle hypertrophy

Both slow twitch and fast twitch fiber are able to hypertrophy when exposed to overload. In a study by Hortobagyi, muscle fiber size of the quadriceps were compared after 36 sessions (12 weeks) of maximal isokinetic concentric or eccentric leg extensions. Type I fiber areas did not change significantly, but type II fiber area increased approximately 10 times more in the eccentric than in the concentric group.

There is a tendency for fast twitch fibers to experience more damage from training, thus fast twitch fibers tend to hypertrophy "more readily" to heavy resistance exercise. Nevertheless, both fast and slow twitch fibers hypertrophy. If you look at a bodybuilder's cross section of muscle fibers, you will find both fiber types hypertrophied, this being due to the inclusion of both concentric and eccentric contractions under load.

In conclusion fibers are classified into two different types, fast and slow. The distinction between the two types of fibers is based on both their contractile properties, as well as their metabolic properties. Slow twitch fibers, associated with small motor units, are activated first when a effort is applied against an object. Once all small motor units have been activated large motor units, involving primarily fast twitch fibers are activated.

All exercises performed by a person trying to build muscle are, of necessity, performed using sufficient weight to activate all slow twitch fibers and most fast twitch fibers. Both slow and fast twitch fibers will then hypertrophy. Fast twitch fibers will hypertrophy first, and to a greater extent, due to their susceptibility to cellular micro-trauma during the eccentric portion of every rep.

When trying to grow muscle, it is worthless to try to adjust the program to "stimulate" or "isolate" any specific type of fiber. Recruitment patterns involved in lifting weights heavy enough to cause hypertrophy activate all fibers, both fast and slow.


Cope, T. C, and M. J. Pinter. The size principle: still working after all these years. News Physiol. Sci. 10: 280-286, 1995

Hortobagyi T, Hill JP, Houmard JA, Fraser DD, & colleagues. Adaptive responses to muscle lengthening and shortening in humans. J. Appl. Physiol. 80(3): 765-772, 1996

About the Author

Bryan Haycock M.Sc. is an exercise physiologist and NPC judge. Bryan has been bodybuilding for over 20 years and holds certifications with the NSCA, ACE, and is a member of the American College of Sports Medicine. Bryan is currently the Editor in Chief of and is the founder and CEO of Bryan is a highly sought after authority on the physiology of muscle growth and fat loss. Bryan also specializes in the management of type-II diabetes through diet and exercise.