This article originally appeared in Shape.

Your backside does a lot of work-including the lion's share of propelling you forward when you walk, run, and move. It's a particularly heavy workload for your hamstrings and calves. And if you've ever been woken up by Charley horse cramps, you likely know that it's important to know how to stretch your calves. (Related: How Weak Ankles and Ankle Mobility Affect the Rest of Your Body)

The good news: Tight calves (and even Charley horses themselves) likely aren't caused by your workouts or your runs. "Charley horse cramps are caused by involuntary muscle contractions," says Jason Karp, Ph.D., an exercise physiologist and run coach based in San Diego, CA.

"If you're getting them when you're resting, chances are they're triggered by dehydration or a mineral deficiency [often sodium, potassium, or magnesium]." Some medications, including diuretics and cimetidine (a heartburn reliever), may also be the culprit. "To prevent cramps, drinking plenty of fluids throughout the day, eating high-potassium foods such as bananas and oranges, and stretching your calves before you go to bed," Karp says. If you wake up with a cramp, walk around for a few minutes, massage the area, and then stretch.