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Muscles.

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We have over 600 of them.

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

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and along with connective tissue,

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

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And whether or not body building
is your hobby,

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muscles need your constant attention

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because the way you treat 
them on a daily basis

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determines whether 
they will wither or grow.

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Say you're standing in front of a door,
ready to pull it open.

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Your brain and muscles are perfectly
poised to help you achieve this goal.

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First, your brain sends a signal
to motor neurons inside your arm.

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When they receive this message,
they fire,

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causing muscles to contract and relax,

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which pull on the bones in your arm
and generate the needed movement.

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The bigger the challenge becomes,
the bigger the brain's signal grows,

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and the more motor units it rallies
to help you achieve your task.

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But what if the door 
is made of solid iron?

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At this point, your arm muscles alone

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won't be able to generate 
enough tension to pull it open,

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so your brain appeals 
to other muscles for help.

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You plant your feet, tighten your belly,
and tense your back,

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generating enough force to yank it open.

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Your nervous system has just leveraged
the resources you already have,

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other muscles,

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to meet the demand.

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While all this is happening,

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your muscle fibers undergo 
another kind of cellular change.

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As you expose them to stress,
they experience microscopic damage,

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which, in this context, is a good thing.

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In response, the injured cells release
inflammatory molecules called cytokines

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that activate the immune system
to repair the injury.

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This is when the muscle-building 
magic happens.

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The greater the damage 
to the muscle tissue,

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the more your body 
will need to repair itself.

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The resulting cycle of damage and repair

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eventually makes muscles 
bigger and stronger

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as they adapt to progressively 
greater demands.

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Since our bodies have already adapted
to most everyday activities,

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those generally don't produce 
enough stress

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to stimulate new muscle growth.

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So, to build new muscle,
a process called hypertrophy,

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our cells need to be exposed to higher
workloads than they are used to.

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In fact, if you don't continuously expose
your muscles to some resistance,

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they will shrink,

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a process known as muscular atrophy.

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In contrast, exposing the muscle
to a high-degree of tension,

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especially while 
the muscle is lengthening,

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also called an eccentric contraction,

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generates effective conditions
for new growth.

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However, muscles rely on more than
just activity to grow.

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Without proper nutrition, 
hormones, and rest,

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your body would never be able
to repair damaged muscle fibers.

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Protein in our diet preserves muscle mass

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by providing the building 
blocks for new tissue

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in the form of amino acids.

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Adequate protein intake,
along with naturally occurring hormones,

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like insulin-like growth factor
and testosterone,

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help shift the body into a state
where tissue is repaired and grown.

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This vital repair process mainly occurs
when we're resting,

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especially at night while sleeping.

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Gender and age 
affect this repair mechanism,

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which is why young men 
with more testosterone

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have a leg up in the muscle building game.

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Genetic factors also play a role
in one's ability to grow muscle.

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Some people have more robust 
immune reactions to muscle damage,

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and are better able to repair
and replace damaged muscle fibers,

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increasing their 
muscle-building potential.

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The body responds to the demands
you place on it.

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If you tear your muscles up,
eat right, rest and repeat,

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you'll create the conditions to make your
muscles as big and strong as possible.

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It is with muscles as it is with life:

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Meaningful growth requires challenge
and stress.