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About every 10 years the idea that we should be training according to muscle-fiber type reemerges. Most recently a paper published in the Strength and Conditioning Journal brings up the matter anew. Before we discuss the evidence one way or another, let’s briefy define what we mean by fiber types. 

Muscle fibers were first classifed according to their “function.” Slow-twitch and fast-twitch are the two basic types and are often referred to as type I and type II, respectively. It was later discovered that the twitch (twitch = contraction) characteristics were the result of different kinds of contractile proteins. Some proteins allowed the fiber to contract quickly and are dependent on “fast oxidative” pathways (ATP and fast glycolytic pathways). The other type, slow-twitch, has contractile proteins that allow slower fiber contraction and are dependent on “slow oxidative” pathways (oxygen requiring beta-oxidation and fatty-acid oxidation). 

The two distinct metabolic profles of fast- and slow-twitch fibers give them distinct fatigue profles. 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) would take a very 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’re able to produce more “power” than slow-twitch. So fast-twitch fibers use their available fuel more quickly because the motor units are larger. 

Campos conducted an eight week study comparing three different rep ranges on fiber-type hypertrophy. So what happened?  Did the slow-twitch fibers increase most in the high-rep group? Did only the fast-twitch fibers hypertrophy in the low-rep group? If you believe you must do high reps for slow-twitch fibers to grow and low reps for fast-twitch fibers to grow, then that’s exactly what should’ve happened!

On the other hand, if hypertrophy is a matter of load, and all fibers hypertrophy in response to increasing load, then hypertrophy should go up as load goes up. In other words, the group who lifted the heaviest relative weight should’ve experienced the greatest amount of hypertrophy in all fiber types irrespective of the number of reps (within reason). And that is exactly what happened. The high-rep group saw an increase in slow-twitch fiber size of 10.3%. The low-rep group saw an increase of 12.4% in  slow-twitch fibers. Keep in mind that all groups trained to failure. As for fast-twitch fibers, they also increased more as weight loads increased from ~11% in the high-rep group and ~24% in the low-rep group.

As to this most recent paper published on the topic, it’s argued that slow-twitch fibers will not hypertrophy to their fullest extent unless high reps are used that increase the level of fatigue. They cite a paper published in 2012 in which two training protocols were compared; three sets of leg extensions using 80% 1RM taken to failure versus three sets using 30% 1RM taken to failure. This was carried on for 10 weeks. Similar to previous studies comparing high- and low-rep training, they found  that there was no difference in growth produced by the two protocols. However, unlike previous studies, they found that slow-twitch fibers grew more in the high-rep group (+23%) than they did in the low-rep group (+16%), though the difference was not statistically significant.

I don’t believe that training according to fiber type is necessary or an accurate way of thinking to maximize growth, and I take as my evidence the aforementioned study by Campos and others that concur with his findings. That being said, I do agree with the authors of this recent paper when they recommend that a program designed to elicit maximum gains in size should include both high and low reps, as evidence by the method I espouse: hypertrophy-specific training (HST). – FLEX