How do Muscles Grow?

How do muscles grow? What happens inside your muscles after your workout? And how do you trigger muscle growth? In this article, we’ll dive into the mechanisms of muscles hypertrophy, and what the most important factors are to build muscle mass.

How Muscles Grow

Whenever people are talking about building muscle mass, you hear the term “muscle hypertrophy” quite often. And this term, hypertrophy, actually tells us how muscles grow. In the context of physiology, hypertrophy means that your existing cells, or in our case existing muscles fibres, get bigger. So you don’t grow more muscle fibres, but rather the ones you already have are getting bigger in size.

So how do your existing muscle fibres become bigger? There are two mechanisms leading to this, which both play into each other.

The first mechanisms involves satellite cells. These cells are kind of a muscle stem cell. And after an appropriate stimulus, these satellite cells multiply and some of them fuse with your existing muscle fibres. What that does is add volume to the muscle fibres (because two cells fuse to become one), but it also adds another nucleus to the muscle fibres.

The nucleus contains your DNA, which is the blueprint for making proteins. Therefore, the more nuclei (add text: nucleus singular, nuclei plural) a muscle fibre has, the more proteins it can produce.

And this is actually the second mechanism of muscle hypertrophy: Muscle protein synthesis. Meaning that when your muscle fibres get the correct stimulus, they will start producing more muscle proteins. And since these proteins all take up space, more muscle proteins means larger muscle fibres means larger muscles overall.

Mechanisms of Muscle Hypertrophy

So what are these stimuli that trigger satellite cell fusion and muscle protein synthesis? Well, in the current literature, three mechanisms are being discussed:

  • Mechanical tension

  • Metabolic stress

  • Muscle damage.

And these three mechanisms are pretty much self-explanatory: Muscle damage means that there is damage to your muscle fibres. However, it’s not quite clear yet if and how much muscle damage contributes to muscle growth, with some studies suggesting that muscle damage is actually not promoting muscle growth at all. So, it’s probably the least important factor of the three.

Slightly more important is metabolic stress, which means there’s a build-up of metabolic by-products, such as lactic acid, within your muscle tissue.

But by far the most important factor for muscle hypertrophy is mechanical tension. Basically, this means that your muscles are being stretched under load, which, in turn, activates satellite cells and increases MPS.

And these three factors are heavily influenced by certain training variables, such as progressive overload, rep ranges, training intensity, and rest between sets. But I'll cover all these variables and their influence on muscle growth in separate articles; you can find them here.

Nina is a virologist, aspiring bodybuilder and science communicator.

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