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How Muscles grow: a new hypothesis.

How Muscles Grow: A New Hypothesis.

Scientists have been studying muscle growth since at least 1897.

But more than a century later, there’s no consensus on how muscles actually get bigger.

Let’s be clear: We’re not talking about why muscles grow. It’s well-established this primarily happens as an adaptation to mechanical loading.

Read: You pick up a challenging weight, put it down, and repeat. Do that frequently enough, and over time, your muscles undergo structural changes that make the same weight easier to lift.

Okay, we all know that.

What we’re referring to here is how those structural changes occur. Like, at a microscopic level.

Yeah, it’s a bit nerdy. So if you dig that kind of thing, stick with us.

The anatomy of a muscle

Just so you have a visual, here are the basic components of a muscle.

For our purposes, let’s zoom in on the myofiber, or muscle fiber. Eighty percent of a muscle fiber’s size is made up of myofibrils. 

These house the actual contractile filaments of a muscle fiber and are what allow you to produce force.

It’s speculated that myofibrils adapt to mechanical loading by adding new filaments. That seems to make sense: More contractile filaments would allow you to lift heavier weights.

What’s more, these new filaments increase the size of the myofibril, which makes individual muscle fibers bigger, and ultimately, increases the overall size of a muscle.

This is known as myofibril hypertrophy, and it’s what most exercise science students have been taught. (Another type of growth is sarcoplasmic hypertrophy—which we won’t discuss here but is thought to represent about 20 percent of a muscle fiber’s growth.)

But there’s also some research that suggests muscle fibers expand due to the creation of new myofibrils. This is called myofibril hyperplasia. There’s less evidence for this, but it hasn’t been disproven in humans. (It’s a thing in cats.)

Which brings us to a new hypothesis—one that integrates both these concepts.

It’s called the myofibril expansion cycle, and it’s detailed in a new paper from Kent Jorgenson and Troy Hornberger, PhD, of the University of Wisconsin, and Stuart Phillips, PhD, of McMaster University.

It goes like this: With mechanical loading, myofibrils add new filaments and expand in size. Once the myofibrils reach a certain size, they split and form two smaller daughter myofibrils. This is how new myofibrils are created.