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New Year's resolutions often include a goal to exercise more, so we've put together a few of our favorite fitness-themed episodes into one place for our lovely viewers!

Hosted by: Hank Green
Check out SciShow's podcast SciShow Tangents at
Is Running Really Bad for Your Knees?:

What Causes Runner's High?:

Can Pickles and Bananas Really Help Athletes?:

What's Causing That Stitch in Your Side?:

Are Sore Muscles Growing?:
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Stefan: Exercise is one of those things that most of us know we should get more of, but that we almost all find excuses to avoid, but have you ever wondered if those excuses are valid?  Running is one of the first things people think about when they consider starting a workout routine, but it comes with its share of concerns, especially when it comes to your knees, but is running actually bad for your knees?  Here's some insight.

If you like to run, someone has probably warned you that you're wrecking your knees.

The idea is that when you run, the flexible, tough tissue that cushions your knee joints, called cartilage, takes a pounding. And over time, that cartilage supposedly wears down, which makes your bones rub together, and leads to the pain, stiffness and swelling of Osteoarthritis.

But is this actually true? Past studies that compared runners to non-runners, sometimes collecting data over a couple decades, have found mixed results. A few found that there's more risk of Knee Osteoarthritis in specific groups of runners, like in men younger than 50 who run more than 32 kilometers per week.

But lots of papers comparing long-term runners to swimmers, non-runners, or sedentary people 𝐝𝐢𝐝𝐧'𝐭 turn up evidence that running will doom you to arthritic misery. In some cases, runners even seemed to be 𝐥𝐞𝐬𝐬 likely to get arthritis. So could running actually protect your joints somehow?

To find out, a study published in 2016 looked at what's happening inside your knees when you run. Scientists gathered a group of male and female runners younger than 30, with no history with knee problems, and brought them in for two experiment sessions over two days. They started each session collecting a blood sample, and a bit of synovial fluid, the natural lubricant inside the knee joint.

Then they had the participants spend 30 minutes running on a treadmill one day, and thirty minutes sitting another day. Afterwards, they took more samples. Getting a useful amount of fluid from healthy knees turned out to be tricky.

And the researchers only ended up with complete data from 6 people. But the results are still interesting. The scientists were looking for molecules related to inflammation, because extra inflammation has been linked to the development of arthritis.

They were also a compound called Cartilage Oligomeric Matrix Protein, or COMP, which can be a marker of arthritis if there's a bunch of it in your synovial fluid. After running, the subjects had less of these molecules in their knees, and more in their blood's, spread out in their body. Sitting, on the other hand, slightly increased concentrations of COMP, and some other inflammatory molecules in their knees.

It's hard to tell exactly what this means, but it seems like running might squeeze inflammatory compounds out of your knees, which could reduce cartilage damage and arthritis. But this study 𝐝𝐢𝐝 have a pretty tiny sample size, so... Jury's still out.

But if you like running and it makes you feel healthy, the next time someone says, "But you'll ruin your knees!" Take it with a grain of salt.

So running probably isn't as bad as some people think, and it can actually feel downright good.  Some people even talk about getting a literal high from running, and it turns out there's some science to back that up.  Here's Michael to tell you all about the chemistry behind what some people call 'runner's high'.

Michael: If you’re someone who enjoys distance running, you might’ve felt a sensation of happiness, calm, or reduced pain that sets in after a workout—commonly known as runner’s high.

Since the 1980s, runner’s high was thought to be caused by chemicals called endorphins. Remember the line from Legally Blonde? “Exercise gives you endorphins, and endorphins make you happy!” But now, scientists aren’t convinced that endorphins should get all the credit.

The real culprit might be endocannabinoids, a group of chemicals in your body that act like the compounds in marijuana that cause a high. Endorphins bind to what are known as opioid receptors on neurons all over your body, affecting other chemical signaling that your brain interprets as pain. Because endorphins can cause pain relief in your muscles, they were thought to cause all the feel-good parts of runner’s high.

But endorphins, it turns out, are too big to pass through the blood-brain barrier, a highly selective membrane that protects the brain from potentially harmful stuff in the bloodstream. So they probably don’t cause the general feelings of happiness that come with runner’s high, because they’re not interacting with brain cells. Instead, scientists think it might be the endocannabinoids.

Endocannabinoids interact with the same systems in your brain as THC in marijuana does, but your body naturally makes them. They’re involved in things like soothing anxiety and reducing pain sensitivity. In 2015, a group of researchers showed that mice produced more of an endocannabinoid called anandamide after running on a wheel for around 6 kilometers.

To test for the pain-relief that comes with runner’s high, mice who ran and mice who didn’t were placed on a hot plate to cause pain. And the mice who ran had more anandamide in their blood and took longer to get noticeably agitated on the hot plate. Then, the researchers ran the experiment again.

This time, they gave some of the running mice a drug to block endorphins from having any effect, but the mice still acted calmer when they were plopped on a hot plate. The researchers gave other running mice a drug that blocks anandamide from binding to receptors. These mice were more anxious and sensitive to the hot plate—just like the non-runners.

So, according to this study, it seems like anandamide contributes to the pain and anxiety relief of runner’s high, but endorphins not so much. At least in mice. But what about humans?

In 2012, a study had a few humans, dogs, and ferrets run on treadmills for half an hour, and found that endocannabinoid levels in blood went up in humans and dogs—which are both better adapted to running. And a 2011 study involving 11 healthy male cyclists suggested that anandamide production from exercise might increase neurotrophin levels, a kind of protein that can adjust connections between neurons, and have some antidepressant effects.

Because of small sample sizes, though, it’s hard to say if these studies mean something for all of us. So there’s a good chance that endocannabinoids could be involved in runner’s high, but there’s a lot we still don’t understand about our bodies!

Stefan: So endorphins might not be the feel-good gift from exercise we thought we were, but it seems like there are definitely chemicals elevating our moods after a good workout, and while running or lifting weights can make you feel on top of the world, it also takes a lot of energy.  Some athletes swear by certain foods to get them through their workouts, and some of these foods are a little bit strange, but do they work?

Hank: Athletes do some weird stuff, or at least it looks kind of strange from the outside when you see a marathon runner chugging dill pickle juice before a race or downing a 10-banana smoothie for breakfast. Sporty types have sworn by these foods for decades claiming they prevent the dreaded charlie horse and other muscle cramping during intense workouts. But why even bother eating normal food when you got scientists engineering fancy sports drinks and energy gels in the lab, they probably taste at least a little bit better than ice-cold glass of briny cucumber juice.

Well lots of athletes do use those energy gels and sports drinks but pickle juice and bananas are useful too. Scientists don't actually agree on the exact mechanism that makes muscles cramp after intense exercise but one of the more popular theories is that as we sweat, our muscles dehydrate losing electrolytes like sodium and potassium. Those electrolytes are important because they transmit nerve signals inside our muscle tissue telling it when to relax or contract. That's why sports drinks and energy gels are full of them, plus carbohydrates for extra fuel.

A single pickle spear contains twice as much sodium as an 8-ounce cup of Gatorade and just one serving of banana has 10 times the potassium, so these foods are popular with athletes hoping to pack their muscles with electrolytes before a hard workout.

But recent studies suggest that these foods might actually be good for athletes for other unexpected reasons. In a 2010 study, 10 athletes from Brigham Young University were made to exercise until they were dehydrated which is kind of sad, and then the researchers induced muscle cramping using electricity and gave some of the subjects pickle juice, and some water, and others no drink at all. The athletes who drank pickle juice got rid of their cramps a whopping 45% faster than those who didn't drink anything and 37% faster than their buddies who drank plain water.

Oddly enough, it took only about 85 seconds for the pickle juice to take effect, not even enough time for it to be absorbed by the stomach, much less make its way to the muscles. One possibility is that the juice hitting the back of the throat somehow triggers nerves to instruct the cramping muscles to relax. With only 10 subjects though, it's hard to draw too many conclusions so more studies will have to be done before we know for sure.

Now bananas are pretty awesome too. One study in 2012 gave 14 cyclists either a sports drink or half a banana every 15 minutes over the course of a 75 kilometer bike ride. Both groups performed similarly but the group who ate bananas ended the trail with more elevated dopamine and antioxidant levels so it seems like we have plenty more to learn from bananas as well.

Stefan: It turns out that certain foods really can improve performance, and I guess even pickle juice can help you out with muscles cramps, but what about that specific pain that can emerge on the side of your abdomen.  Is that cramp or something else?  

Hank: You’re running. You’re in the last quarter of the marathon. You’re a beast! A champion! You’re incredible! But then, suddenly -- you feel a sharp pain in your side. It seems to get worse and worse with every breath. It’s an attack of the side stitch! And now you have to stop running for a while so it goes away. But don’t worry! Because you’re not alone.

A 2000 study found that 69 percent of runners had experienced a stitch, also known as exercise related transient abdominal pain, in the past year. And people who exercised in other ways, like swimming or bike riding, also reported having gotten a stitch. But even though they’re so common, researchers aren’t really sure where the pain comes from.

Remarkable that we don’t know these things, even about our own bodies. One possibility is that it’s all about your diaphragm, the dome-shaped muscle between your chest and abdomen. When your diaphragm contracts, your lungs expand and fill with air. When your diaphragm expands, your lungs get smaller and the air is forced back out. But when you exercise, you might be overextending that muscle. When you’re running, for example, your foot consistently hits the pavement at the same time as you exhale -- when your diaphragm is at it’s highest and tightest -- it might strain the diaphragm, causing it to spasm. It could also be that forceful movement, like the up-and-down motion of running, bounces your internal organs around, straining the ligaments trying to keep them from sliding around inside you. Which is a really gross thought.

But the most likely explanation seems to point to the peritoneum, a two-layered membrane that lines your abdominal wall and helps support your organs. Now, normally, there’s fluid in between the layers to make sure they don’t scrape together too much. Because when they do, you end up with that sharp pain. When you eat a large meal, your stomach pushes out on the inner layer, and when you’re dehydrated -- like if you’ve been sweating a lot from exercising -- there’s less fluid between the layers. The way your body’s moving around might bump the two layers against each other, too.

So how do you make the pain go away? First, just stop exercising for a bit. The pain will eventually pass -- unless it doesn’t. In which case, go see your doctor. Because I am not a doctor. And if you want to make sure it doesn’t happen again, it’s probably worth listening to your mom’s advice: wait a while after a big meal before jumping in the pool or going out for a run. Working to strengthen your core might help, too. That should reduce the movement in your abdomen while you’re exercising, meaning less strain on your ligaments and membranes. This way, your internal organs and the stuff inside of your abdominal cavity can stay right where they’re supposed to be, without causing stitches that might slow you down.

Stefan: So once you've taken that break after eating, you might be ready for a good workout, but what if you're still sore from that lifting session a few days ago?  Did you overwork your biceps or are they just still growing from the last time you hit the gym?  Hank gives this question a good workout.

Hank: Alright. Let's face it. We've all mostly given up on our New Year's Resolutions by now. But if you are one of the few people who recently started hitting the gym on a regular basis, you've probably experienced the muscle pain and stiffness that shows up over the first few days after you work out. It's known as "delayed onset muscle soreness," and it probably has to do with tiny rips in your muscles. But while muscle growth does seem to be related to damaged cells repairing themselves, more pain doesn't necessarily mean more gain.
See, when you work your muscles harder than usual, it can cause micro-trauma, or microscopic tears. Now, that's more likely to happen after particular kinds of exercise, like when you're doing what's called eccentric contractions: when your muscles are contracting and lengthening at the same time. For example, bringing a dumbbell down during a bicep curl? Your bicep is sort of slowly relaxing, but it's still contracting to hold the weight up. These kind of motions put extra tension on the proteins within the muscles, and cause tiny tears that trigger inflammation. That swelling, along with the tears themselves, activates nociceptors, the neurons responsible for sensing pain; as time passes, your muscles heal, and the pain goes away.
But: are your muscles stronger now? Well, maybe.
We know that building muscle does involve minor damage and healing. But it isn't totally clear if muscle soreness and growth use the same mechanism, especially because researchers have found that feeling extra-sore doesn't necessarily mean you're growing more muscle. Either way, though, the next time you work out should be easier, thanks to the so-called "repeated bout effect." Since muscle soreness mainly comes from exercises your muscles aren't used to, after a while, they'll start to adapt, and you won't be in as much pain. It's almost like your body wants you to get back to the gym.

Stefan: So if you're feeling really sore, you might not be getting more swole, but your muscles should hurt less the next time you do that tricep dip.  Once you've rested for a day or two, go ahead and hit the gym again.  Your body will thank you for it.  

Thanks for watching this episode of SciShow.  If you're still craving some mental exercise, you can check out our video to find out if thinking harder burns more calories.