What Makes Muscles Grow? (Jeff Siegel with TED-Ed)

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What Makes Muscles Grow?

Muscles: we have over 600 of them, they make up between 1/3 and ½ of our bodyweight. Along with connective tissue they bind us together, hold us up, and help us move.

Whether or not bodybuilding is your hobby, muscles need your constant attention. The way you treat them on a daily basis determines whether they will wither or grow.

What Makes Muscle Grow: Exposure to Load

Say you’re standing in front of a door, ready to pull it open: your brain and muscles are perfectly poised to help you achieve this goal.

First, you need to establish a base of support. This means getting grounded so you are stable else you’ll throw yourself off balance. Thankfully, your inner ear and vestibular system do this for you without you noticing.

Your brain has already made a prediction about how much force it will need to open the door, and it has prepped your body accordingly.

Next, your brain sends a signal to motor neurons inside your arm. When they receive this message, they fire, causing muscles to contract and relax, which pull on the bones in your arm and generate the needed movement.

The heavier the door, the bigger the brain’s signal grows, and the more motor units it rallies to help you achieve your task.

But what if the door is made of solid iron?

At this point your arm muscles alone won’t be able to generate enough tension to pull it open, so your brain appeals to other muscles for help. You plant your feet, tighten your belly, and tense your back, generating enough force to yank it open.

Your nervous system has just leveraged the resources you already have—other muscles—to meet the demand.

How Do Muscles Grow? An Inflammatory Response & Repair

While all this is happening, your muscle fibers undergo another kind of cellular change: as you expose them to stress, they experience microscopic damage, which, in this context, is a good thing.

In response, the injured cells release inflammatory molecules called cytokines that activate the immune system to repair the injury.

This is when the muscle-building magic happens: the greater the damage to the muscle tissue, the more your body will need to repair itself. This is how muscles grow. The resulting cycle of damage and repair eventually make muscles bigger and stronger, as they adapt to progressively greater demands.

Since our bodies have already adapted to most everyday activities, those generally don’t produce enough stress to stimulate new muscle growth. So, to build new muscle, a process called hypertrophy, our cells need to be exposed to higher workloads than they are used to.

In fact, if you don’t continuously expose your muscles to some resistance, they will shrink, a process known as muscular atrophy.

In contrast, exposing the muscle to a high a degree of tension especially while the muscle is lengthening (also called an eccentric contraction) generates effective conditions for new growth.

What Makes Muscles Grow: Hormones & Nutrition

However, muscles rely on more than just activity to grow. Without proper nutrition, hormones, and rest, your body would never be able to repair damaged muscle fibers.

Protein in your diet preserves muscle mass by providing the building blocks for new tissue (in the form of amino acids). Adequate protein intake, along with naturally occurring hormones like insulin-like growth factor and testosterone, help shift the body into a state where tissue is repaired and grown. 

This vital repair process mainly occurs when we’re resting, especially at night while sleeping.

What Makes Muscles Grow: Age, Gender & Genetics

Gender and age affect how muscles grow and repair themselves, which is why young men with more testosterone have a leg up in the muscle-building game.

Genetic factors also play a role in one’s ability to grow muscle. Some people have more robust immune reactions to muscle damage and are better able to repair and replace damaged muscle fibers, increasing their muscle-building potential.

How Do Muscles Grow? Intentional Challenge & Smart Recovery

The body responds to the demands you place on it. If you tear your muscles up, eat right, rest, and repeat, you’ll create the conditions to make your muscles as big and strong as possible.

It is with muscles as it is with life: meaningful growth requires challenge and stress.

Jeffrey Siegel

About TED-Ed Originals

TED-Ed Original lessons feature the words and ideas of educators brought to life by professional animators.

Meet The Creators

  • Educator, Jeffrey Siegel
  • Animator, Brett Underhill
  • Script Editor, Emma Bryce
  • Narrator, Addison Anderson

I’m Jeff Siegel, a wellness coach and mindfulness teacher, helping people upgrade their habits and improve their health.

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If you’d like to explore working together, you can schedule a private 20-min consultation call with me

Jeff Siegel
Jeff Siegel, M.Ed, is a health and wellness coach, Harvard University mindfulness instructor, and personal trainer.

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