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.