If you’ve ever had to stop mid-run because of a calf cramp, felt your hamstrings seize up during downward dog, or woke up in the middle of the night with a charley horse, you know how painful it can be to suffer through a cramping muscle. Muscle cramps are not only zero fun, they can be so debilitating that you have to pause your workout until the cramp subsides (a major inconvenience if you’re in the middle of a fitness class or running a race).

Whether or not you experience muscle cramps yourself, you’ll be fascinated by a video of a leg cramp in action posted on Facebook last week. The video, posted by Facebook user Angel Bermudez and reported by Mashable, shows his calf cramping up after his workout. The calf muscle doesn’t just flex, as you might expect—you can actually see the muscle moving involuntarily, as if spiders were crawling underneath his skin. “I’m not doing anything,” Bermudez says in the video in between grunts of pain.

Hard to watch, right? If you’ve never actually seen a leg cramp in action, know that this isn’t entirely uncommon. For example, here’s another video showing a man’s calf cramping up while he’s in the gym:

Some people’s cramps are visible, while others aren’t—it depends on a few different factors.

You’ve probably had a cramp or two that felt incredibly painful but wasn’t visible or moving like the ones in the videos above. So why did Bermudez’s cramp look like that? “Being able to see a cramp is a result of the amount of muscles cramping at once, how deep the cramping muscles are, and whether there is fat over them,” Nadya Swedan, M.D., a physical medicine and rehabilitation specialist for sports injuries based in New York City, tells SELF. “Because cramping is an involuntary, irregular muscle spasm it does not recruit an entire muscle in a normal way,” she adds.

“Some calves will ball-up; others will twitch and tick,” says Michele Olson, Ph.D., a fellow of the American College of Sports Medicine and adjunct professor of sports science at Huntingdon College in Montgomery, Alabama. “It changes from person to person,” she tells SELF.

Muscle cramps aren’t fully understood, but they’re most likely caused by an imbalance of the chemicals that make your muscles fire and relax.

There are many factors that may be the culprit for your muscle cramps. “Cramping is not fully understood in the medical field, but an imbalance in nutrients including potassium, calcium, and magnesium is associated with cramping,” explains Dr. Swedan. These chemicals are responsible for causing the muscle cells to contract and release.

Excessive sweating, most often due to hot weather, can make muscle cramps more likley. That’s because the nutrients required for muscle contraction float in the blood’s plasma, and sweating can affect the proper concentrations of plasma and nutrients. “If you sweat too much due to high heat and humidity, you will draw too much fluid from your plasma,” Olson explains. “As you lose more and more plasma to create sweat, you also lose sodium, chloride, and calcium, which have to be in the proper balance to both contract and create the relaxation of the muscle fibers.”

Extreme heat and cold can also lead to dehydration, which predispose cramping. Other risk factors include pregnancy, growth spurts, or hormonal changes, explains Swedan. “Overtraining and fatigue along with insufficient stretching can also lead to cramping,” she adds. Fortunately, there are some ways to prevent muscle cramps that you can try.

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