Not All Muscle Growth Is Equal

picture of a muscle fiber

Types of Muscle Growth

Generally speaking the two accepted types of muscle growth are myofibrillar and sarcoplasmic hypertrophy. Hypertrophy just means an increase of an organ or tissue. For our sake it’s tissue as we are in the gym to build muscle.

Which type of hypertrophy do we want? I will get to that shortly but first a general overview of muscle fibers.

diagram showing muscle fiber breakdown

There are three major types of muscle fibers:

  • Type I
  • Type IIa
  • Type IIx

Type I fibers are considered “slow twitch.” Think of them more as the endurance fibers; they do not tire quickly, but they are not very powerful in terms of strength. Type IIa and IIx are considered “fast twitch.” Think of explosive movements, powerful and high-output in terms of strength generating (i.e. lifting heavy weights).

The ratio of muscle fibers is determined by our activities, and adapts accordingly.

If you did a lot of endurance training, e.g., long-distance running, you would develop more Type I fibers in your legs than Type II. If you did a lot of heavy squatting, you would develop more Type II fibers than Type I. People are generally more dominant in one fiber type over another, but the body does adapt to what you do. Your genetic disposition is why you will hear people say “I’m naturally a good runner” or “I’m better at strength training”. But as I mentioned, your body can and will adapt properly.

Types of Hypertrophy:

  • Myofibrillar hypertrophy refers to an actual increase in size of the muscle fibers (myo means “muscle” and a fibril is a threadlike cellular structure). Think of dense muscle.
  • Sarcoplasmic hypertrophy refers to an increase in the volume of the fluid, non-contractile components of the muscle (glycogen, water, minerals, etc.). Sarco means “flesh” and plasmic refers to plasma, which is a gel-like material in a cell containing various important particles for life. Think of puffy soft muscle.

diagram showing difference between sarcoplasm and myofibrillar hypertrophy

If we look at rep ranges for lifting weights from very heavy weight for 1 rep to very light weights for 30+ reps, the following generally holds true:

Heavier weightlifting tends to increases power and induce myofibrillar hypertrophy, and lighter weightlifting preferentially increases endurance and induces sarcoplasmic hypertrophy. Heavy lifting does result in some sarcoplasmic hypertrophy and vice versa. But heavy lifting places the emphasis on myofibrillar hypertrophy.

rep range venn diagram

If you’re familiar with my work, you’ll recall that I have stated several times that as natural guys, (no steroids) our goal is to get stronger over time. This is what will build muscle. And to get stronger we need to lift heavy weights and stimulate the type IIa and IIx fibers which induce myofribillar hypertrophy. Having optimum testosterone levels helps a lot. And don’t waste money on fad supplements, stick to the safe and proven supplement, creatine.

4-8 reps is the best range for myofribillar hypertrophy – this should be your main focus and will build a strong dense base of muscle that will last for years to come. Sarcoplasmic hypertrophy has its place, but is for more advanced lifters and for those who are nearing their genetic potential in terms of muscle growth.

The general public is told to train in the 8-12 rep range which is more conducive to sarcoplasmic hypertrophy. The problem with that is it is hard to get stronger over time in that rep range. Unless you are on steroids, good luck progressing long term in regards to strength in that rep range. Plus, the muscle you do build is more fluid based and will disappear the minute you stop lifting. Myofibrillar hypertrophy is more permanent as it is the growth of actual muscle fibers.

The bottom line is the correct focus of a natural weightlifting routine is myofibrillar hypertrophy, and that means focusing on heavy 4-8 rep weightlifting.

Go lift some heavy-ass weights.

Be Fit. Dress Great!


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