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.

And that's it.

Thank you so much for watching and happy physicsing.