(Describer) A man points at a waveform.

So squeeze your butt for me. Oh! OK, squeezing. Oh!

(Describer) It changes.

There you go. And then relax. Relaxing. Nice. Wow! This is really cool. This is really cool!

(Describer) In a studio...

I know that you're watching Physics Girl, and now maybe you know too. Hi, I'm Dianna, and this is not a fitness channel. But Blogilates is, and it just so happens to be the most popular Pilates channel on YouTube. And Cassey from Blogilates got me to do a butt workout.

(Describer) In that video...

I love that you're able to talk doing this, Cassey, because I'm starting to die! Good! That was way outside of my comfort zone, but I took her afterwards to a lab and experimented on her. So, ha!

(Describer) In the lab...

MAN: So we will place these electrodes right on the skin, right above certain muscles that we want to see activate. I'm going to put this electrode on your gluteus medius. We'll hook them up to these cables and then to these wireless transmitter boxes.

(Describer) Dianna wears them.

And so, we can get the electrical activity of the muscle sent to the computer wirelessly, and can look at when the muscles are on and how much they're on, that sort of thing. I was really curious to see which of your butt muscles light up, and which of your muscles are activated, when you do different exercises, like squatting versus running. And then, in those different exercises, which of your glute muscles are actually working. Like, I know you can feel it when you're doing exercises, but it's like it's symphony of muscles working together, and you can't always hear the individual butt instruments. That whole metaphor was just so that I could say "butt instruments." But we also had a secret motive. That's cool. This is really cool. Now, I need to find the exercise that gives me the most. Maybe. I don't know. It depends on what your goals are, really? I think everyone wants a bigger butt. Cassey wanted to know how to get a big butt. Look, this is not everybody's goal in life. It's not necessarily mine. But as far as trends in body shape go, this one seems to be at least intriguing at the moment. And we learned about how much exercise can actually affect the shape and strength of your body. I'm going to say right now that there are a ton of factors that go into the strength and shape of your body, like diet, exercise, and genetics. That one, genetics, that one's out of our control completely. But if you're interested in the effect of exercise on the shape and strength of your body, let's charge onward. First of all do we even know what a butt is? Well, what do you mean by butt, right?

(Describer) K. Michael Rowley:

It's sort of whoever you're talking to. There's that saying that if you have a hammer, everything looks like a nail, right? So if you're talking to a biomechanist, when you asked me about a butt, I thought, oh, she means the hip. And she means the hips rolling, running. But if you talk to a nutritionist, they'll probably have an entirely different interpretation. Michael's point was that there's a lot going on around your butt area. There's muscles, joints, there's fatty tissues, which is mostly what contributes to the average person's butt looking bigger. There's also bones around their butt. So if someone says they broke their butt, they're probably talking about the coccyx. They broke their tailbone. People say that? Some people do. I broke my butt.

[LAUGHTER]

We get it, the butt can refer to a lot of things, but since we're thinking about the effective exercise on the size of your butt, then we're going to focus on the muscles. There are three main muscles that most people would consider comprising the butt or the glute muscles-- the gluteus maximus, the gluteus medius,

(Describer) ..on the hip bones...

and the gluteus minimus. But we focused on the gluteus maximus, the gluteus medius, the lower back muscles, and the thigh muscles. There's a reason that we ignored the gluteus minimus.

(Describer) Michael:

The gluteus minimus is the third gluteal. Where's that one? That's actually right below the medius. So they do almost identical things. Like right here. No, actually below, as in deeper.

CASSEY: Ah! Oh! Beneath? Since the gluteus minimus is covered by the gluteus medius, we wouldn't be able to use the electrode technology to get the signal from that, because it works by sensing electrical activity of the muscles. But since the gluteus medius is closer to the skin, than you would probably, unfortunately, get the signal from that muscle instead of the minimus. Anyways, Michael placed the electrodes on our muscles, and then when we activated a muscle, the electrode picked up the electrical signal from that muscle, transmitted it to the computer, and we see the signal as this yellow graph readout. How is it measured?

(Describer) ...Cassey.

Yeah, so it's measured in millivolts. So an electrical measure. So it's how much the muscle is being recruited. It's not the same-- recruited by the neural system. So how much your body is saying, OK, use this muscle a little bit, use this muscle a lot. It's not the same as muscle force. So we're measuring the electrical activation of those muscle fibers, essentially. It's literally like your brain is picking the muscle. I choose you! On this configuration, we're seeing the gluteus maximus,

(Describer) ...on graphs.

the gluteus medius, the paraspinals, or the lower back, and the quadriceps. The quads don't show up at every shot though, because the electrode fell off. I'm so curious. Do you think squats are more effective than doing, like, a butterfly bridge-- I'm just, like, curious. So we have to do a small series of things at a time. So what do you want to do first?

[LAUGHTER]

DIANNA: Cassey did four bridge exercises here-- a rolling bridge, one pushing forward, single-legged, and butterfly. DR. ROWLEY: Then we would look at the amplitude.

DIANNA: Of these four exercises, the amplitude of the gluteal muscles was highest for the single-legged bridge. That makes sense. DR. ROWLEY: We'll look at the duration.

DIANNA: It looks like here the rolling bridge lasted longer than the pushing bridge. Then we can compare glutes to back. This exercise this version had a higher ratio of backs to glutes.

DIANNA: That was the pushing bridge. I guess you put your back into it.

(Describer) Cassey, subtitled: "Look at how effective the one we did this morning was. You can't even compare that because it's single leg, but look at that. That's like our version of the normal one." "Yeah, the butterfly?" It's shown in Cassey's video.

CASSEY: Look at how effective the one we did this morning is. This one. You can't even compare that to the single leg. Look--that's our version of the normal one.

DIANNA: Yeah, the butterfly? This burns!

DIANNA: Cassey then tried comparing squats, bridges, and butt pulses, and we found something intriguing about the move that had the most glute activation in that set. This is single leg. Oh! So the butt pulse.

(Describer) They compare graphs.

That's the butt pulse. Oh, that's the butt pulse.

(Describer) She lifts one leg back and pulses.

That was the tiniest move. Whoa. One factor to consider is that you were at end range. So when you do the butt pulse, you're already at full hip extension. Right. So you're taking the muscle-- a muscle can't produce as much force when it's short. So you've shortened the muscle, and then you tried to create force. So you had to use a lot of the muscle. That looked hard. Oh, yeah. Aha.

(Describer) Dianna tries it.

Oh, yeah! Aha! I make my students pulse all the time. Yeah. They hate me for it. Now I know why!

DIANNA: We tested so many other things, like, does Cassey get tired after 20 butterfly bridges? No. Does using a pointed versus a flexed foot make a difference for butt muscle activation? Not that we could tell when Cassey tried. Unfortunately, Cassey had to leave after that, but I was still super curious to test a more common activity, like running, against a targeted glute exercise. What is, like, "the" glute exercise? DR. ROWLEY: Probably squats. Squats? A fitness person would probably tell you squats. So squats versus running would be a good comparison. I would guess we're going to see more activation in running than in squatting. Interesting. I was skeptical. But then I squatted for science, then lunged for science, and then ran for science. Squat, lunge, run. Circuit training. So here we had five squats.

(Describer) He points out a small waveform on the graph.

There might have been a break here where we had the lunges across the floor,

(Describer) ...slightly bigger.

and then here we have them running all the way back. So some of these big spikes are because of the wire bouncing with each run, but you can still see in between the spikes all that activity, the dense activity there, and how much higher that is than squatting and lunging. It's almost like I didn't do anything anything. Muscles are generally more active when they're slowing down a motion, rather than when they're speeding something up. Yeah. And so since on every foot strike, when you are running, your glute is activating to prevent you from falling down. Yeah. So it's sort of absorbing the impact of each step. So, yeah, we noticed that running recruited my glutes more than squatting. But that doesn't necessarily mean that running gives you the bigger butt. It depends on how fast you run, for how long, how many squats do you, and many, many other factors. Also, you run on one leg at a time, so you have to control all your weight with one leg. Whereas with a squat, the weight is distributed between your two legs. That could explain why we saw a higher activation with running. And I'm also a sample size of one. That's not statistically significant. Statistically signif-- statis-- that term cannot be said. Statistically significant. And actually, one study found that long-distance running or swimming might give you a leaner butt. The study found that female athletes training for these endurance sports had lean butts that were no different from less active people. But athletes in sports that involve sprinting and stopping, like soccer or squash, or jumping, like volleyball, have bigger gluteus maximus muscles than average. One example is figure skaters. Figure skaters often have well-developed butts, right? And part of the reason is because of their boots. So I did my undergraduate research thesis looking at the loading of your joints when you land flat-footed, versus when you land with a pointed foot. And when you lay with a pointed foot, your ankles take up a good portion of the load. Yeah. But when you're forced to land flat-footed, like you are in rigid figure-skating boots, the demand gets shifted to the hip. So figure skaters having to land flat-footed all the time are really absorbing a lot of that landing impact with their hip muscles instead. Interesting. And so that's contributing to their large butts. So the sports you do can contribute to butt size, but there's also this other thing to remember. I also want to add that the size of someone's butt is a combination both of the size of the gluteus maximus muscle, but also the subcutaneous fat that's there. And also remember, whether working out actually changes your body depends on a ton of environmental factors, your gender, and other factors, like your genes. There is an interesting meta-analysis on twins that showed high heritability of things like waist circumference. But then another meta-analysis showed low heritability of things like cardiorespiratory fitness. And yet another meta-analysis showed that as you get older, the contribution that food and exercise make increases, and the genetic contribution decreases. There is a lot to it. I will link to all of these studies in the description. So if you want to try to contribute to the strength and mass of your butt muscles, well, take up volleyball. But also we saw the effectiveness of the activation of your glutes with Cassey's butterfly bridges. So go try out her workout. It just went out today. It is the very first video of her 30-day 100-glute challenge. I was day number one! So there you have it. All of your butt questions answered. Never thought I'd say that. Thank you so much for watching, and--

BOTH: Happy physics-ing!

(Describer) Titles: You know what really makes me smile? Facial muscles. Haha. Accessibility provided by the US Department of Education.

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