Lyle McDonald

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August 18, 2016

Hyperplasia vs. Hypertrophy in Skeletal Muscle

I received the following question in the mailbag and, for a fairly short question I’m going to give a fairly long answer since it gives me something to write about today.

Question: Does the number of fast/slow twitch muscle fiber types in your body actually change in response to strength or endurance stimulus? Or just the volume, and you’re stuck with what your genetics dictate?

The short answer is yes-ish.  Here’s the long answer.

Let me make one clarification here.  Well, two.  The first is that I am talking about skeletal muscle.  Cardiac muscle acts a little bit differently in how it grows with stress and we don’t lift weights for a bigger heart (perhaps if we did there would be more love in the world).

Also, I’m talking about training induced growth.  You can cause some goofy stuff to occur when you ablate a muscle (i.e. cut a muscle in a larger group and you see the other muscles grow like crazy) or with other distinctly non-physiological types of research methods.  Here we’re talking about moving iron (the original question asked about endurance training but there’s no reason to begin to suspect that hyperplasia occurs from that type of training in my mind).

Hyperplasia vs. Hypertrophy: Definitions

First, let’s define the terms hypertrophy and hyperplasia.  Hypertrophy means an increase in cell size.  Fat cell hypertrophy occurs when the fat cell increases in volume (by storing fatty acids as triglyeceride) and skeletal muscle hypertrophy occurs when skeletal muscle increases in volume.

Hyperplasia means an increase in cell number.  Fat cell hyperplasia (which does occur in adults, contrary to old belief) is an increase in fat cell number.  Skeletal muscle hyperplasia would be an increase in muscle cell, or in this case, fiber number.

Read the rest of Hyperplasia vs. Hypertrophy in Skeletal Muscle

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