---
title: 'Science of Muscle Hypertrophy'
source: 'https://youtube.com/watch?v=Uz4ZrvFY6b4'
video_id: 'Uz4ZrvFY6b4'
date: 2026-06-28
duration_sec: 1066
---

# Science of Muscle Hypertrophy

> Source: [Science of Muscle Hypertrophy](https://youtube.com/watch?v=Uz4ZrvFY6b4)

## Summary

This video explains the mechanisms behind skeletal muscle hypertrophy, starting with the anatomy of muscle fibers and sarcomeres. It then details three types of hypertrophy (myofibrillar, connective tissue, and sarcoplasmic) and the three primary stimuli (mechanical tension, metabolic stress, and muscle damage) that drive them. The video also discusses how different rep ranges and training methods like vascular occlusion influence muscle growth.

### Key Points

- **Muscle Structure** [0:39] — A whole muscle is surrounded by fascia, contains fascicles (bundles of fibers) wrapped in perimysium, and each muscle fiber is encased in endomysium. The cell membrane is called the sarcolemma.
- **Satellite Cells** [1:40] — Satellite cells are muscle stem cells located between the sarcolemma and basal lamina. They donate nuclei to muscle fibers to support growth.
- **Sarcomere and Contraction** [2:20] — Myofibrils contain contractile proteins actin (thin) and myosin (thick), arranged in sarcomeres. Contraction occurs when myosin heads walk along actin, shortening the sarcomere.
- **Three Types of Hypertrophy** [5:56] — Myofibrillar hypertrophy: increase in contractile proteins and sarcomeres. Connective tissue hypertrophy: growth of fascia and other connective tissues. Sarcoplasmic hypertrophy: increase in organelles, fluid, and energy stores.
- **Three Stimuli for Hypertrophy** [8:43] — Mechanical tension is the primary driver. Metabolic stress (e.g., lactate buildup) and muscle damage (e.g., from eccentric contractions) are additive stimuli.
- **Rep Ranges and Volume** [13:20] — Moderate rep ranges (6-12 reps) are often most beneficial due to a combination of mechanical tension and metabolic stress. High volume (more sets and reps) also promotes growth.
- **Hypoxia and Vascular Occlusion** [15:46] — Hypoxia (low oxygen) from vascular occlusion training increases lactate and nitric oxide, leading to cell swelling and enhanced growth signals.

## Transcript

hi everybody dr. Mike here in this video
we're gonna take a look at skeletal
muscle hypertrophy now we all know that
going into the gym lifting weights
undergoing resistance training increases
our muscle mass muscle volume muscle
size and muscle cross-sectional area all
of which we term skeletal muscle
hypertrophy but the question is how does
this occur so in this video I'm going to
go through the mechanisms that promote
skeletal muscle hypertrophy but the
first thing we need to go through is the
skeletal muscle itself so let's just say
we have a whole muscle like a bicep for
example now we know that this whole
muscle is surrounded by connective
tissue that we term fashio and if you
have a look inside of that whole muscle
you'll find that there's these discrete
areas called fascicles they themselves
are surrounded by connective tissue
which we call perimysium now you can see
within each fascicle are gonna we're
gonna have skeletal muscle cells known
as muscle fibers and you can see these
fibers now if I would have take one of
these fascicles and pull out one of
these muscle fibers which we call a
muscle cell you're gonna see a couple of
things firstly it's surrounded by
connective tissue as well so the whole
muscle connective tissue right the
fascicle connective tissue and then the
muscle fiber connective tissue now this
is called the endomysium that connective
tissue around the muscle fiber and
underneath that you've got the cell
membrane cuz all cells have a cell
membrane and we called this four muscle
cells the Sarco lemma alright now you
can see that I've drawn up a couple
things first of which these little blue
things here are called satellite cells
they're going to be very important when
it comes to hypertrophy sunlight cells
are sitting between the sarcolemma so
the cell membrane and a connective
tissue the basal lamina okay and they're
sitting there in the quiescent which
means they're asleep and they're waiting
to be stimulated and do something very
specific in order first skeletal muscle
hypertrophy to occur the other thing you
can see is the nuclei of skeletal muscle
they have more than one and again this
is important when it comes to sunlight
cells remember the nucleus is where DNA
is transcribed we make more DNA
now remember proteins come from amino
acids which come from DNA that's
important you can also see that if we
were to take out one muscle fiber that
inside they're filled they're really
filled up with these things could
myofibrillar
tubes now these myofibrils are made up
of proteins two major types of proteins
there's more but two major types you
need to know called actin and myosin now
what you're going to find is that these
proteins inside the myofibrils their
contractile proteins because not all
proteins are contractile right but these
are contractile proteins now you'll see
that dispersed between the myofibrillar
have some organelles some sub cellular
components things like mitochondria
right mitochondria is important for
producing ATP energy which we need for
skeletal muscle contraction we also have
things like sarcoplasmic reticulum
now the sarcoplasmic reticulum is the
endoplasmic reticulum but for skeletal
muscle and their important role is that
they are the storage unit for calcium we
also need calcium for muscle contraction
actual fact calcium and ATP are two of
the major things that we need for muscle
contraction there's other organelles as
well but let's just focus on those two
all right now let's take a myofibrillar
out have a look at this right we've got
the Maya febrile sitting within the
muscle fiber sitting within the fascicle
sitting within the whole muscle each
component is surrounded by connective
tissue we take a my fibrillate and we
looked at look at the actin and myosin
contractile proteins within and you'll
see they have a particular arrangement
what I've drawn up here is something
called a sarcomere now you keep hearing
this word Sarco as a prefix when we talk
about scalar muscle because it means
flesh all right so you can see the two
major types actin which we call the thin
filament and myosin that we call the
thick filament now in order for muscle
to contract muscle its job is to
contract so that force can be generated
usually things can move and you've got
cardiac muscle to pump blood smooth
muscle to push things through the hollow
insides of those of that lumen and we've
got skeletal muscle attached to the
skeleton so things can move right so
what we need to do is that this thick
myosin filament needs to bind to the
thin actin filament and these little
myosin heads need to walk their way
along the actin filament and it pulls it
in what this does is it shortens the
skeletal muscle in actual fact it
shortens this thing called the sarcomere
now you're gonna have sarcomeres lined
up in series like this or or I should
say and lined up in parallel okay and
this is going to be important when it
comes to skeletal muscle hypertrophy as
well now remember the myosin can't bind
to the actin unless calcium is present
calcium frees the actin so it can bind
and calcium sits within the sarcoplasmic
reticulum and we need ATP the thing with
ATP is we don't actually have a lot of
it just freely floating around you think
we would but we don't we need to
generate ATP and so skeletal muscle
needs to generate ATP fast and we'll
talk about that with skeletal muscle
hypertrophy soon all right
couple things now when we talk about
hypertrophy I told you increasing the
size increasing the volume increase in
the mass increasing the cross-sectional
area what do you think is increasing
here is it the whole muscle is that the
fascicle is it the muscle fiber or is it
the sarcomere or is it the protein
subunits of the sarcomere well it's not
just these individual things so for
example a lot of people rely on scooter
muscle hypertrophy has just been an
abundance of the proteins that
contractile proteins and their
arrangement within the sarcomere people
think that scooter muscle hypertrophy is
only more sarcomeres lined up in series
or lineup in parallel but that's only
one type of skeletal muscle hypertrophy
right called myofibrillar hypertrophy I
think we should write this down right so
you can have myofibrillar hypertrophy
and like I said what that is is an
increase in the protein subunits for
contraction and an increase in the
amount of sarcomeres we have either in
series or in parallel but we know that
up to 20% of the whole muscle is
connective tissue like I stated before
right so you've got that fashion you've
got the Paramecium you got the
endomysium you have the epimysium on the
outside here so up to 20% is connective
tissue so in connective tissue is
dynamic it doesn't
sit there connective tissue can grow and
it can contribute to strength so you can
have connective tissue hypertrophy now
the last or third type of hypertrophy
that we need to talk about is
hypertrophy of non contractile subunits
and non connective tissue which has to
do with the stuff sitting between the
cells and in the cells for example which
includes things like the organelles and
the fluid and also the metabolic energy
sources that are stored inside of these
muscle cells so for example now we call
this let's just write this down
we call this sarcoplasmic hypertrophy so
what we're referring to for sarcoplasmic
hypertrophy is an increase in the size
or abundance or volume of the organelle
so it could be the mitochondria it could
be the sarcoplasmic reticulum could be
the T tubules that skeletal muscle needs
to propagate that action potential to
tell it to contract right it could be an
accumulation of fluid inside and it
could be an accumulation of the
metabolic products needed to produce
energy so up to three percent of
skeletal muscle is made up of glycogen
up to five percent is made up of
triglycerides so you can increase the
abundance of these and they're going to
come back later when we talk about
hypertrophy so these are the three major
types and the thing is what causes these
types of hypertrophy so doctor shown
field dr. Brad shown field is the king
of skeletal muscle hypertrophy and all
the research that I've performed most of
it has come from dr. Shawn field and so
what dr. Schoenfield states is the three
major causes or stimuli for hypertrophy
include the following mechanical tension
metabolic stress and muscle damage so
these three stimuli are what can result
in these three types of hypertrophy now
your question may be well which of these
hypertrophy is occurring usually all of
them are going to be occurring to some
degree you're probably going to think
with resistance training probably the
most common type of hypertrophy is Maya
febrile hypertrophy but in saying that
when you look at bodybuilders and
compare them to power lifters their
training regime is different but both of
them exhibit quite significant
musculature quite significant muscle
growth and they train differently right
so for example bodybuilders have a
moderate load with shorter rest periods
and you're going to find that power
lifters have a higher load with longer
rest periods and what you might find is
that some bodybuilders may more so have
a connective tissue based and
sarcoplasmic based hypertrophy because
they tend to have more stores of
glycogen within their cells and they
tend to have more connective tissue
products as well compared to the power
lifters so training type can be specific
to the type of hypertrophy that you get
all right so mechanical tension
metabolic stress and muscle damage of
these mechanical mechanical tension is
king right and you'll find that
metabolic stress and muscle damage are
additive to this process because you
really get skeletal muscle hypertrophy
on metabolic stress alone and muscle
damage alone now let's talk about some
things let's talk about how this can
result in this all right so when you've
got mechanical tension on a muscle so
you lifting a particular load over time
and so we know that we have eccentric
portions of a movement and concentric
portions in the eccentric portions where
you've got lengthening of the muscle but
these sarcomeres are contracting you can
have damage occurring to the
myofibrillar units now damage promote
some form of inflammatory response it
can recruit cytokines and inflammatory
cells and this promotes your repair
that's great what also happens is this
type of damage
wakes up the sleeping satellite cells
the quiescent satellite cells and they
are myogenic cells that means they're
basically muscle stem cells and what
they can do is they fuse since they're
sitting underneath the basal lamina
right but above the sarcolemma they fuse
with the muscle cell they're all muscle
cells remember they fuse with them and
they donate a nuclei that's why I've got
more nuclear here donates a nuclei and
what that means is there's now more
nuclei more DNA more transcriptional
activity more amino acids and proteins
being produced in actual fact when you
look at skeletal muscle the larger the
muscle is correlated with more nuclei
and this has to do with that satellite
cell so the satellite cell is really
important for donating the nuclear but
in addition the satellite cell can call
upon particular transcriptional
activities right since it's got this
additional nuclei transcribe more
pathways associated with anabolic or
growth pathways so for example mTOR map
kinase calcium signaling pathways all of
these can be recruited pathways for
growth and ab ilysm alright so we'll
focus on that now when you look at the e
centric and you start to go into that
pattern of moment eccentric stretching
at the same time that it's contracting
you can find that in some studies animal
studies predominantly that the types of
growth that you get at the sarcomere is
an addition of sarcomeres in series and
when you get more column centric
movements the addition of sarcomeres is
in parallel alright so again the type of
that movement can change what type of
hypertrophy you have at the sarcomere so
talking about the my fibrillar
hypertrophy if we actually take all them
all the proteins within a skeletal
muscle what you're gonna find is up to
70% of them are my fibrillar proteins
right seventy percent twenty percent of
them are sarcoplasmic proteins and
around about 10% our mitochondrial
proteins alright now a couple of other
things is when so if we have a look at
different types of loads so when we look
at load lifting load it's usually going
to be represented as a percentage of our
one-hour
and we usually equate that to how many
reps we can perform other particular
weight all right so you can have a low
rep scheme a moderate rep skein and a
high rep scheme
all right low rep scheme zero to six
reps mutter at six to twelve reps
anything above 12 13 14 15 reps and
above is going to be a high rep scheme
so what they've found is that the low to
moderate rep scheme seems to be more
beneficial to hypertrophy compared to
high rep skeins and the reason is a
little bit complex but has to do with a
couple of things has to do with
mechanical tension and has to do with
metabolic stress and muscle damage but
let's look at metabolic stress now so
when you lift a particular load we need
ATP and I told you that ATP is needed
for muscle contraction and I told you we
don't have many reserves of ATP so we
need to produce it now when we need to
produce it immediately we use something
called the Fausto creatine pathway
creatine likes to hold onto phosphate
called phosphor creatine and it donates
that phosphate to an adp to basically
turn it into ATP we have energy but that
only lasts for like one two three
seconds not very long at all so the low
rep scheme tends to favor the
phosphocreatine pathway when we go into
the moderate rep scheme right six to
twelve reps we go past that and we start
to go into glycolysis we start to use
glucose for energy now if we're doing
this quick and fast and remember when
you have glycolysis and it jumps into
the mitochondria a user's oxygen to
produce heaps of ATP if we need more ATP
than we have available oxygen has to go
through another pathway which is
producing lactate right and ATP as well
another thing is lactate production is
associated with hypertrophy in this
sense lactate can pull water has a
strong osmotic gradient right it pulls
water into the cell and causes the
muscle cell to swell this swelling of
the muscle cell puts pressure on the
sarcolemma sort of cell membrane and
this actually stimulates a couple of
things it stimulates amino acid
transport and it stimulates anabolism or
protein synthesis brilliant so this is
something that lactate can do lactate
production is also associated with
testosterone levels and growth hormone
levels as well alright so it seems to be
this is
why the moderate reps game seems to be
represented as potentially the most
beneficial when it comes to hypertrophy
but let's not forget volume all right so
that's load volume is adding up all your
reps and sets in the single session and
it seems to be the higher the volume the
better it is for hypertrophy as well now
let's talk about something that involves
both metabolic stress and muscle damage
which is hypoxia
so hypoxia is a reduction in oxygen
going to that tissue and feeding that
tissue so often we call this vascular
occlusion and the some sort there's some
types of training where you occlude the
vasculature with a cuff for example and
it seems to be that this type of hypoxic
or or banded vascular occlusion training
can increase muscle hypertrophy if done
in accordance with exercise and it
doesn't even need to be under high load
so what the found is that hypoxia
training or vascular occlusion training
without any exercise just the vascular
occlusion can maintain muscle size and
strength for individuals who are
bedridden if you mix it in with exercise
it seems to be an additive effects with
hypoxia and exercise to promote
hypertrophy as well so that's really
interesting why why does this hypoxic
event promote hypertrophy couple of
theories one of which is that of lactate
so when you've got no oxygen we're
forced to use glucose to produce ATP and
lactate and like I said lactate pulls
water into the cell it swells promotes
amino acids and protein synthesis and
also promotes
growth hormone and testosterone coming
in the other thing is when you have a
hypoxic environment low oxygen you get
the production of reactive oxygen
species one of which is nitric oxide
nitric oxide tells blood vessels to
dilate so after that training when you
release the cuff or the band you get
increased in nitric oxide tells blood
vessels to dilate you get hyperemia
which is more blood to that area
carrying all the metabolites that you
require for growth so maybe that is also
another factor coming into play so what
I've gone through here is the anatomy of
the skeletal muscle and gone through the
different types of hypertrophy and also
the different causes of these particular
high
a hypertrophic events hopefully that
helps them make sense