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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 typesWe'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 exhaustingThe 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 layersthe 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 muscleDuring

  • development, you have cells in the mesoderm layer called myoblaststhese things are

  • cells that are dedicated to becoming muscle fibersMyoblasts 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 cellsNow, one thing that makes skeletal

  • muscle fibers unique compared to cardiac or smooth muscle is how it has multiple nuclei

  • in each fiberThat'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 bodyIf 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 sizeSo 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 muscleit

  • seems like our muscles grow mostly because our existing fibers get biggerWeightlifting

  • 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 yearBy 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 changesUnfortunately, 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 fibersPart 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 othersand skeletal muscle has a lot for its sizeThey 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 mitochondriaAnd 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 exerciseOne 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

  • fallsStudies 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 tooGoing 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.

Hey everyone! Just wanted to explain the change of scenery. Usually, we film these videos

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科学によると、筋肉を構築する方法 (How to Build Muscle, According to Science)

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    Summer に公開 2021 年 01 月 14 日
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