[00:06] Muscles.
[00:08] We have over 600 of them.
[00:10] They make up between 
1/3 and 1/2 of our body weight,
[00:14] and along with connective tissue,
[00:16] they bind us together, hold us up,
and help us move.
[00:20] And whether or not body building
is your hobby,
[00:22] muscles need your constant attention
[00:25] because the way you treat 
them on a daily basis
[00:27] determines whether 
they will wither or grow.
[00:31] Say you're standing in front of a door,
ready to pull it open.
[00:34] Your brain and muscles are perfectly
poised to help you achieve this goal.
[00:39] First, your brain sends a signal
to motor neurons inside your arm.
[00:43] When they receive this message,
they fire,
[00:45] causing muscles to contract and relax,
[00:49] which pull on the bones in your arm
and generate the needed movement.
[00:53] The bigger the challenge becomes,
the bigger the brain's signal grows,
[00:57] and the more motor units it rallies
to help you achieve your task.
[01:01] But what if the door 
is made of solid iron?
[01:04] At this point, your arm muscles alone
[01:06] won't be able to generate 
enough tension to pull it open,
[01:10] so your brain appeals 
to other muscles for help.
[01:13] You plant your feet, tighten your belly,
and tense your back,
[01:17] generating enough force to yank it open.
[01:20] Your nervous system has just leveraged
the resources you already have,
[01:24] other muscles,
[01:26] to meet the demand.
[01:27] While all this is happening,
[01:28] your muscle fibers undergo 
another kind of cellular change.
[01:32] As you expose them to stress,
they experience microscopic damage,
[01:37] which, in this context, is a good thing.
[01:40] In response, the injured cells release
inflammatory molecules called cytokines
[01:45] that activate the immune system
to repair the injury.
[01:50] This is when the muscle-building 
magic happens.
[01:53] The greater the damage 
to the muscle tissue,
[01:55] the more your body 
will need to repair itself.
[01:59] The resulting cycle of damage and repair
[02:01] eventually makes muscles 
bigger and stronger
[02:04] as they adapt to progressively 
greater demands.
[02:08] Since our bodies have already adapted
to most everyday activities,
[02:11] those generally don't produce 
enough stress
[02:14] to stimulate new muscle growth.
[02:17] So, to build new muscle,
a process called hypertrophy,
[02:20] our cells need to be exposed to higher
workloads than they are used to.
[02:25] In fact, if you don't continuously expose
your muscles to some resistance,
[02:29] they will shrink,
[02:31] a process known as muscular atrophy.
[02:34] In contrast, exposing the muscle
to a high-degree of tension,
[02:37] especially while 
the muscle is lengthening,
[02:40] also called an eccentric contraction,
[02:42] generates effective conditions
for new growth.
[02:46] However, muscles rely on more than
just activity to grow.
[02:50] Without proper nutrition, 
hormones, and rest,
[02:53] your body would never be able
to repair damaged muscle fibers.
[02:56] Protein in our diet preserves muscle mass
[02:59] by providing the building 
blocks for new tissue
[03:02] in the form of amino acids.
[03:04] Adequate protein intake,
along with naturally occurring hormones,
[03:08] like insulin-like growth factor
and testosterone,
[03:11] help shift the body into a state
where tissue is repaired and grown.
[03:16] This vital repair process mainly occurs
when we're resting,
[03:19] especially at night while sleeping.
[03:22] Gender and age 
affect this repair mechanism,
[03:25] which is why young men 
with more testosterone
[03:27] have a leg up in the muscle building game.
[03:30] Genetic factors also play a role
in one's ability to grow muscle.
[03:34] Some people have more robust 
immune reactions to muscle damage,
[03:38] and are better able to repair
and replace damaged muscle fibers,
[03:42] increasing their 
muscle-building potential.
[03:44] The body responds to the demands
you place on it.
[03:47] If you tear your muscles up,
eat right, rest and repeat,
[03:52] you'll create the conditions to make your
muscles as big and strong as possible.
[03:57] It is with muscles as it is with life:
[03:59] Meaningful growth requires challenge
and stress.