What stretching actually does to your body ft. Sofie Dossi

[MUSIC PLAYING] ANNOUNCER: Thanks to Curiosity
Stream for supporting PBS Digital Studios. So stay like this, and
then lift up like this. Use this momentum to kick
your legs up even if you don’t go all the way yet. And like bend your legs,
and see if you can go to just the top of your head. Just kidding. Hey. I’m Diana. You’re watching “Physics Girl.” And last week, I got to
hang out with contortionist and YouTuber, Sophie Dossi. We did some science
experiments for her channel. And while we were hanging out,
I realized that, as a physicist, I know absolutely
nothing about what stretching does to your
body and whether there is a scientifically-supported
way of becoming more flexible. There is. Stay tuned. I didn’t know, so I
headed over to USC to meet with a bio mechanist
to find out the latest science. I’m Michael Rowley. I am a PhD candidate
in the division of biokinesiology
and physical therapy. My first question
was, is there something different about Sophie? I’m just going to
get into a back bend, and like lift up my feet. Oh my gosh. Yes, yes. Very different. But I mean, like
biomechanically, does she have a superpower? There’s evidence that people
who are hyper flexible– Quick pause. Hyper flexible or hypermobile
is actually a scientific term. And there is a test used to
determine whether someone is hyper flexible. It’s called the
Beighton score, and I’ll link to it in the description
if you want to test yourself. OK. So people who are hypermobile– Those people have a
little bit more relatively of the more stretchy collagen
than the type I collagen. Uh-huh. I need a quick refresher on
collagen. Take it away Michael. So a bone to bone
connection is a ligament. And then a muscle to bone
connection is the tendon. Tendons and
ligaments are made up of mostly collagen, which
is a type of protein. And there’s different
types of collagen. So tendons and ligaments have
primarily type I collagen, which is a really strong type,
and then also some type III collagen, which will
be more stretchy, and then may also include
some elastin, which is another type of
connective tissue that’s very elastic
and stretchy. So the ratios of those
things can be different. Got it. So Sophie might actually have
a different material makeup of the tissues in
her body, like she might have more of that
type III collagen, which would allow her to do things
that the average person can’t do. Fact was, I almost can’t
overstretch anymore because I’m just completely in half. So it’s like I have
pushed the limit on that, like there’s just– I mean you can’t–
you’re not a ghost. You’re not a ghost. But how flexible is Sophie? Yeah. Before I started
contortion, I was always more flexible than
that average gymnast. Oh, than even the
average gymnast. Yes. Not even the average person. Yeah. When I was little,
I use to watch TV with my feet over my head. Yeah. That’s flexible. OK. But how do you get
flexible biomechanically? I guess my first question
is, when you stretch, what parts of your body
are stretching out? Yeah. So when you stretch, you’re
going to increase flexibility and excursion in all those
tissues– muscles, tendons, ligaments. The most short-term, the
fastest, most acute effects will be in the muscle. So your tendons
and your ligaments are stretching out too? How does that work? They seem so sturdy. This is getting to the heart
of what I really, really want to know, which is, what
is physically happening to all of these tissues? The actual pieces
of physiology that change with stretching,
the body will change the water content in the material. That’s probably the fastest way. And then you can
also induce some stem cells that still live in
the tendon material themselves. And those can differentiate
into more cells. They can produce more
or slightly different types of febrile or
proteins like that collagen that I talked about. Humph. So your body can change
the material properties of your tissues by stretching? Cool. Another way is that you
actually modify sensation. So you might experience some
mild pain the first time you’re going to stretch,
but you’ll actually desensitize yourself to
that pain over repeated stretching multiple days
a week for multiple weeks. Hold up. Really? Yeah. One way of getting
more flexible is just becoming
more numb to pain? It actually really feels good. It feels like a
nice, good stretch. It doesn’t feel like
a hurt stretch at all. Crazy. I have no chance. Another change that
happens physiologically with stretching, our muscles have
an alpha-gamma reflex loop. So when you stretch
your muscle, there’s this automatic signal that
gets sent to the spinal cord. It doesn’t even go up
to the brain and back. The spinal cord then tells
the muscle to contract. That coactivation would be
bad for stretching, right? If you’re stretching the
muscle, and then the muscle’s trying to pull back on you. So over time, you can actually
down regulate that reflex loop, and that’s been shown to
happen when you stretch for long periods of time. Lastly, you actually add
length to the muscle fibers. And you do that by
adding sarcomeres, which are sort of
the contractile unit producing the active
contraction of muscles. Wow. Bodies are so complicated. So let’s recap. Stretching can change the
water content in your muscles, stem cell differentiation,
how much you feel pain, that alpha-gamma reflex
loop from your muscles to your spinal cord, and the
length of your muscle fibers. That’s cool. Now I know what’s going on
when you get more flexible. Biomechanics is cool, almost
as awesome as physics– maybe more useful though– which
brings me to my last question. What does science have to say
about how to get more flexible? Well, Michael pointed me
to this really interesting meta-analysis, which is like
a big study that actually synthesizes all the
findings from other studies, in this case, 23
other studies, which looked at how to get the best
benefits on improved range of motion, or flexibility. And they found that
static stretching, that is when you
ease into a stretch and then hold it for 30
to 60 seconds, that’s the best for improved range
of motion, or flexibility. And as far as how
much to stretch, they found that
five minutes total per week of one muscle group
was sort of the lower limit. But you didn’t see much more
of a benefit over 10 minutes a week of stretching. And six days out of the week was
the plateau of seeing returns. So the more you
stretch per day doesn’t keep increasing the
benefits, but that frequency, six days a week and
then the five minutes total for that week,
is what’s important. So commence operation
stretching muscle group for six days a week,
five minutes total. Now, of course, different
types of stretching have different
benefits, so I will link to some of the other
literature in the description. But more importantly
in the description is the video that I did over
with Sophie on her channel. I channeled my inner YouTuber,
and we did a challenge where all the science
experiments I was doing, Sophie had to do with her feet. She’s insane. It was amazing. I had so much fun filming with
her, so check out that video and subscribe to
Sophie’s channel. And thank you to
Michael at USC who blew my mind with biomechanic facts. Physicists don’t get to learn
a whole lot of biomechanics. And that’s it. Subscribe to Physics
Girl if you want to learn more stuff
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