Around every 10 years the idea that we should be training agreeing to muscle-fiber type reemerges. Most lately, a paper published in the Strength and Conditioning Journal can bring up the matter once again. Before we talk about the evidence one way or another, let’s temporarily define what we mean by fiber types.
Muscle fibers were first described according to their “function.”
Slow-twitch and fast-twitch are that two basic types and are usually referred to as type I and type II, respectively. It was recently discovered that the twitch or contraction characteristics were the result of different types of contractile proteins. Some proteins let 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 let slower fiber contraction and are dependent on “slow oxidative” pathways (ATP and fast glycolytic pathways)
There are two distinct metabolic types of fast- and slow-twitch fibers give them separate fatigue profiles. Fast-twitch fibers fatigue quickly because their fuel source, ATP, is depleted promptly. I use the term “depleted” lightly since the motor units are bigger.
Researchers 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 have 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.
Take this advice wherever you go. Seriously, write it down. You’ll be thanking us for sure.