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You know the drill: Cross your arm over your chest, count to twenty, then cross your other arm over. And then bend down and touch your toes.

It’s the same old stretching routine that gym teachers have been using for decades, because it supposedly helps with flexibility, which helps with exercise. Unfortunately, those few minutes of stretching before running won’t exactly keep you from getting injured or drop your mile time. There’s more to it than that.

But the good news is: if you want to be more flexible, stretching will eventually pay off. Flexibility is just the ability to move a joint through a range of motion without getting hurt. And the kind of stretching where you lengthen a muscle and hold it for a few seconds — called static stretching — will lead to bendy, flexible muscles if you do it right and you do it enough.

At the smallest level, muscles are made up of sarcomeres, overlapping bands made of proteins called actin and myosin that slide back and forth when the muscle moves. When you flex your biceps, those bands slide together, shortening the muscle. And when you stretch, those bands lengthen, but only to a point.

Any farther, and they can snap… which isn’t what you want. So your body has a built-in defense for that: muscle spindles. Muscle spindles are sensory nerves that detect muscle stretch.

And when they feel a stretch, they send a signal that makes the muscle tense up and protect itself from tearing. That’s why you’re supposed to hold a stretch for a little while. The muscle spindle needs to get used to the stretch, and turn down that signal.

After all of your muscle fibers have been pulled taut, a lot of the increase in flexibility comes from the connective tissue surrounding them. Stretching can cause chemical changes and straighten out jumbled bunches of connective tissue, kind of like how you can brush tangled hair. And that can lead to a bendier body!

Now, this all makes it seem like your muscles either contract and shorten or stretch and lengthen. But when you run, or jump, or pick up a heavy box, there are a lot of different forces at play. And your muscles go through eccentric contractions — where they’re tensed up, but the sarcomeres are still lengthening because they’re being pulled by other forces.

These types of contractions aren’t inherently bad. Without them, we wouldn’t be able to control our bodies. Like, you had to eccentrically contract your leg muscles if you sat down to watch this video.

But if some kind of force overpowers these contractions during exercise, that’s when muscle strains happen. They’re actually a super common injury. The research is pretty mixed about whether a few minutes of static stretching before exercise will reduce the risk of muscle strain.

On the one hand, stretching may help if you’re overly tight and being pushed towards extreme ranges of motion, like huge lunges during a tennis match. On the other hand, muscle strains usually happen within a normal range of motion, like if you’re just jogging. And static stretches won’t help with that.

Also, if you’ve had a hard workout, you’re probably familiar with the ache of delayed onset muscle soreness. Scientists think that lactic acid buildup from burning energy, muscle and connective tissue damage, or inflammation might be at play here, but they’re not totally sure. So far, the only effective way to get rid of soreness is waiting.

Stretching doesn’t seem to help. So your gym teacher’s stretching routine probably won’t keep you from getting hurt. But can it make you a better athlete?

Well, it depends on the type of stretching. Many studies have shown that static stretching right before your exercise actually decreases your performance for at least part of that workout. And like the muscle soreness mystery, we’re not totally sure why.

But researchers have got some guesses. The first idea has to do with your muscle spindles being less sensitive after a bout of stretching, like we mentioned earlier. This means that muscle fibers won’t be recruited to work as quickly.

And if you’re lifting heavy weights, you want as many muscle fibers to fire as often and as quickly as possible. The other idea is that the area where muscle meets its connective tendon — called the musculotendinous junction — has a bit more slack in it right after stretching. But that bit of slack means that less energy is transferred from your muscle into your skeleton, which makes for a slower run or a shorter jump.

So you might want to ditch the static stretching in exchange for a dynamic warm-up — basically anything that increases muscle temperature before you exercise. There have been some studies on a handful of people, from general volunteers to professional soccer players, that compared dynamic to static stretching. And participants who used dynamic warm-ups jumped higher, ran faster, and produced more muscular force than the ones who did just static stretching.

The big benefit seems to come down to how warm the muscle is. An increase in muscle temperature allows for faster nerve signaling, faster uptake of oxygen, and more recruitment of all muscle fibers — including fast twitch muscle fibers, which are used in bursts of movement. And all this means more power.

So, basically, if your goal is to be more flexible, static stretching can help you do that. But if you want to up your gains at the gym, you might want to rely on some other strategies. Thanks for watching this episode of SciShow, which is produced by Complexly.

We produce over a dozen channels, including The Financial Diet where Chelsea and Erin dive into personal finance topics with no fear. From the emotional side to technical financial terms, you'll learn tricks to improve your life by getting a better handle on money. Check out new videos every Tuesday and Thursday! [ ♪ Outro ].