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Hey everyone! Just wanted to explain the change
of scenery. Usually, we film these videos
in a studio, but we’re gonna be filming
from my house from now on. Hair check, hair check.
I’m gonna stress about one thing
today is my hair. Alright! Aging is inescapable,
for now anyway. And aging itself encompasses
a lot of physiology.
Some aspects of aging are hard to
picture and are the topic of ongoing research,
but one that we know quite a bit about is
muscle. But muscle is complex. Not only is
it always adapting to how we use it, but it
changes multiple times over the course of
our lives. Today, we’ll learn about how
our muscles change from the time we’re just
a fetus, to our last mortal moments.
Before we get too deep in the weeds here, we need
to clarify what we mean by muscle. Some of
you may remember back from episode one that
muscle is one of our four distinct types of
tissue along with nervous, epithelial, and
connective tissue. Each type of muscle has
a different function, and there are three
types. We’ve got two types of muscle that
we control involuntarily: the special cardiac
muscle in our hearts and smooth muscle around
our blood vessels and certain organs. We don’t
consciously contract our heart muscles. Thank
goodness, that would be exhausting. The focus
of this episode though is skeletal muscle,
the type of muscle that lets you move your
body. Skeletal muscle is the most massive
group of tissue in your body making up for
a serious chunk of your body weight, so where
does it come from? This tissue is one of the
first ones formed while you’re still an
embryo, so that’s where our journey begins.
In your early days as an embryo, your cells
divide into three distinct layers — the
endoderm, mesoderm, and ectoderm. Literally
the inner, middle, and outer layers. These
layers become the different tissues of your
body, and it’s the mesoderm that we’re
interested in. It becomes the tissues of your
cardiovascular system, reproductive system,
connective tissue, and skeletal muscle. During
development, you have cells in the mesoderm
layer called myoblasts — these things are
cells that are dedicated to becoming muscle
fibers.  Myoblasts grow and replicate until
they encounter a certain chemical that gets
them to start the process of turning into
proper muscle. After they encounter that chemical,
if they bump into another myoblast, they’ll
start linking together into chains of myoblast
cells.  Now, one thing that makes skeletal
muscle fibers unique compared to cardiac or
smooth muscle is how it has multiple nuclei
in each fiber. That’s because during this
next stage as an embryo, myoblasts fuse together
into structures called myotubes then they
ditch their individual cell membranes as they
fuse into one fiber. Each myoblast had a nucleus,
so the myotubes, as well as mature muscle
fibers, have multiple nuclei too. Some of
those myoblasts don’t differentiate though,
and they hang around as satellite cells, a
type of cell that sometimes gets called muscle
stem cells. These things are super useful.
Our mature muscle fibers don’t divide like
other cells do, but satellite cells can divide
and grow if our muscles are injured. Although,
that statement comes with a lot of it depends
attached to it. There’s quite a bit that
can influence how those satellite cells work.
At this point in the journey, you are a human
that’s alive in the world, crawling around
with about six hundred muscles, eating mashed
bananas or Cheerios or whatever they feed
babies these days. I don’t know, I don’t
hang out with any. From the time you’re
a newborn to your younger years, you keep
roughly the same number of muscle fibers,
but each one gets bigger because of those
satellite cells. This muscle cell growth is
called hypertrophy and it explains the vast
majority of muscle growth that happens in
your body. If you’ve lifted weights before,
you might’ve heard that term thrown around
a bit in reference to those hashtag gains.
And it’s true, your muscles can hypertrophy
as a result of a weight lifting routine. But
hypertrophy also means an increase in any
cell size. So your fat cells can hypertrophy
too, that kind of thing. Plenty of research
has investigated the possibility of muscle
hyperplasia, or growing new muscle fibers,
but at this point in our knowledge of muscle, it
seems like our muscles grow mostly because
our existing fibers get bigger. Weightlifting
aside, at this point in your lifecycle, your
muscles are growing. After a few years, puberty
happens and your muscles grow again. And your
muscles keep growing in size and strength
until about your twenties and maybe thirties.
It seems kind of silly to say this, but every
body is different. Your body is still awesome
regardless of when your muscle mass peaks.
I also included that maybe in that last line
because nothing special happens to your body
at the stroke of midnight when you turn thirty.
Although that was when I got my first grey
hair. As life goes on, and you creep into
your thirties, this is about when aging starts
to have its effect on your muscles. Sometime
after your thirties, you’ll lose about one
to two percent muscle mass every year. By
age 70, you’re looking at twenty five to
thirty percent muscle loss from your peak.
This aging-associated muscle loss is called
sarcopenia, which involves a few long term
processes that all happen at the same time.
It’s kind of a weird, in-between term that
sometimes refers to a disease but also just
being a thing that happens as a result of
age-related changes. Unfortunately, because
there’s so much happening at once, it’s
hard to figure out what causes muscle loss,
and therefore how we can fix it. It involves
both changes in function and physical changes
that both influence each other. Some of it
is influenced by your nervous system. As you
age, the nerves that control your skeletal
muscles reorganize themselves. We start to
lose type 2 muscle fibers, which are fast
twitch fibers that let us generate power,
and start to rely on the slow twitch, type
1 fibers. Part of that involves losing connections
between nerves and type 2 muscle fibers. With
less muscle working together for any given
movement, and switching to weaker fibers in
general, your strength decreases and eventually,
so does the size of your muscles. Muscle aging comes
with all kinds of changes in the composition
of muscle. We also see an increase in the
fat and connective tissue between and around
muscle fibers. Plus, we see a decrease in
our muscle’s ability to repair itself. Part
of this is because we don’t make as much new
protein to go into those muscles. We also
see fewer of those important satellite cells,
again, cells that become skeletal muscles,
so we can’t take advantage of their repair
and regeneration abilities like we used to.
And then we start losing mitochondria. If
you remember back to our mitochondria episode,
certain cell types have more mitochondria
than others, and skeletal muscle has a lot
for its size.  They need to make quite a
bit of energy to do so much work, so in order
for muscle to work properly, they depend on
functioning mitochondria. And when muscle
mitochondria don’t work as well, neither
do your muscles. We’re still not totally
sure why mitochondria stop working as well,
but we think it has to do with slight tweaks
in gene regulation, or how your body turns
certain genes on or off. There are a few other
reasons for muscle loss as well like increased
general inflammation, hormonal changes, and
we just tend to be less active as we get older.
So it won’t come as a surprise that the
most effective treatments for preventing muscle
loss due to normal aging include a combination
of nutrition and exercise. I know I just spent
the last two minutes talking about how your
muscle wastes away as you get older, but I’ll
leave you with a bit of hope. Exercise is
one of the most powerful things you can do
to maintain health as you get older, and you
can see gains from exercise long into life. A
handful of studies have shown that adults
between age sixty and eighty can improve their
aerobic fitness by twenty to thirty percent
with exercise.  One study even showed that
seniors in their nineties could get stronger
and see thigh muscle hypertrophy with strength
training. This becomes a huge deal when it
comes to staying independent and avoiding
falls. Studies use different styles or prescriptions
of exercise, but they usually include some
kind of weight lifting component. Those treatments
help reduce muscle loss by improving those
satellite cells, preventing further loss of
nervous connections, and making more mitochondria.
But I doubt you’re really gonna care how many
mitochondria you have when you’re that old.
You’re probably more concerned with stuff
like balance and day to day tasks. Thankfully,
strength training helps with that too. Going
into the future, researchers are working on
identifying the finer details behind sarcopenia
and how to best treat it, maybe with drugs
or other strategies. In the meantime, I’m
strangely motivated to lift all of a sudden.
Thinking about frailty and mortality can make
people feel uneasy, I know, but part of what
makes life so special is the fact that it’s
fragile. Ultimately, it’s what makes our
lives human. Thanks for watching this episode
of Seeker Human, I’m Patrick Kelly.