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Muscles.

We have over 600 of them.

They make up between
1/3 and 1/2 of our body weight,

and along with connective tissue,

they bind us together, hold us up,
and help us move.

And whether or not body building
is your hobby,

muscles need your constant attention

because the way you treat
them on a daily basis

determines whether
they will wither or grow.

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, 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 bigger the challenge becomes,
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.

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.

The resulting cycle of damage and repair

eventually makes 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-degree of tension,

especially while
the muscle is lengthening,

also called an eccentric contraction,

generates effective conditions
for new growth.

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 our 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.

Gender and age
affect this repair mechanism,

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.

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.