Early this morning, I went to the new Precision Run studio in New York—AKA the shiny, new stand-alone studio that evolved from Equinox’s Precision Running class—ready to log some miles. It was totally dark in the room, except for the yellow LED light strips that set the mood (which, BTW, adjust to blue during recovery periods) and it was lined wall to wall with the most high-tech treadmills I’ve ever seen.

I stepped onto the machine without having stretched, and David Siik—founder and creator of Precision Running—started to lead the class into a warm-up. “Get into a light jog,” he told us. I cranked up my tread and started sauntering about, warming up my muscles. But to do this, Siik didn’t just have everyone slowly take our speeds up a notch. First we did kick-backs, where you kick your foot behind you towards your butt, which stretches your hamstrings. Then: “Now do some high-knees!” Whenever I hear those two words together, I literally cringe. I loathe high-knees. They’re just hard. But Siik had us doing them for the sake of our hip flexors.

Hip flexors are actually one of the tightest muscles in your body, since they’re used in basically every type of movement on top of the fact that they’re closed off and under lots of stress when you’re sitting for most of the day (thanks, desk job). “Doing high knees is a great dynamic warm-up to help activate the hip flexors,” Siik tells me. “They hydrate [the hip flexors] and get them ready to run. This can really help prevent the risk of an injury.”

It’s actually really easy to get injured if you don’t stretch your hip flexors, BTW—if they’re really tight, it makes it hard for blood to flow properly in the area, you can get a misaligned pelvis, the list goes on. So, even though high knees can be a pretty annoying, strenuous move, you should really get those knees up whether you’re on the tread, running outside, or basically doing any type of intense workout. The hips don’t lie, after all.

Another tight part of your body is the IT band—here’s what you need to know. And here’s a 5-minute bedtime stretch that’ll stretch your muscles and help you get right to sleep.