You know that on-top-of-the-world feeling you have when you leave the gym after slaying a workout? You’ve earned a well-deserved rest day and so you schedule another burner for 48 hours later. Well, nothing kills that buzz like waking up on that second day, ready to tackle the gym again—only to have your plans ruined at the hands of delayed onset muscle soreness. You’re not sure you can get out of bed…much less swing a kettlebell. It begs the question: WTF?

Turns out, this is an all-too-common phenomenon in the fitness world even has a nickname: DOMS (for delayed onset muscle soreness). “It affects everyone differently. Some people feel mild soreness and some people have pain so bad it restricts their range of motion,” says Adam Rosante, strength and nutrition coach and founder of The People’s Bootcamp. “The truth is, no one’s really isolated a single cause for DOMS, but it’s most likely a combination of damage to the muscle tissue and inflammation.”

And, that intense second-day soreness or DOMS shouldn’t be brushed off as a lack of stretching or foam rolling. It’s actually a lot more complicated than that. Keep reading for the 411 on DOMS and how to know if intense soreness could be something more.

So, what exactly is delayed onset muscle soreness?

“DOMS is a feeling coming from deep muscle fiber tears. This generated from things like HIIT training or heavy eccentric lifting in certain types of workout programs. Usually, those who begin working out for the first time or those doing an unfamiliar activity will experience DOMS.” says Kevin Mejia, trainer at the Dogpound Gym in New York City.

While it most commonly affects those who are new to working out (or those who’ve taken a few weeks off, let’s say), it can also strike new muscle groups that aren’t worked out commonly. That means if you hit a new class or give a new lifting series a whirl, you might be more prone to experiencing it. “Let’s say, for instance, that your preferred workout is going to a spinning class three days per week. Then one day you try a lower body strength training session and can’t walk for a week,” explains Rosante. That’s DOMS.

It occurs because when you’re spinning, your muscles are contracting concentrically, and when you work them eccentrically (the lowering phase of lunges or squats, for example), your legs aren’t used to the type of movement, which creates tiny tears in the muscle fibers. “It’s also gonna be worse for untrained or lesser trained muscle groups, which is why it can hit you hard at the start of a new exercise program,” says Rosante. “It can totally throw you for a loop, but don’t panic: DOMS is normal. Know that your body is repairing so that it can handle a greater workload moving forward.”

What’s the best way to get rid of delayed onset muscle soreness?

You might not want to hear this but the best thing you can do to ease the soreness is exercise. Seems a bit counterintuitive, but it works. According to one study, where various methods were tested to help soothe DOMS-related pain (including massage therapy, cryotherapy, and stretching, to name a few) additional exercise was the most effective remedy.

“What will probably help you most is nutrient-rich blood flow to those sore muscles. Just lighten up the intensity and know that, in time, it’ll pass,” Rosante says. And you don’t have to go hard again, just get moving. The study on DOMS also notes that it’s a good idea to lower the intensity of your workouts for one to two days after you experience DOMS. You may also want to focus on the upper body on those days if your lower body is sore (or vice versa). “Find ways to move your body around if that’s yoga or 100 air squats at your desk every hour or even something like foam rolling will help the sensation significantly,” adds Mejia.

When should you be worried about delayed onset muscle soreness?

“If the sensation, or even potential pain, becomes intolerable this could be a bad sign. Normal everyday functions shouldn’t be painful,” says Mejia. Knowing when to stop versus when to push yourself through the soreness is important too. “If the pain comes on suddenly during your workout, like a pop or a snap or like something isn’t quite right, that’s a sign of an injury and you should stop your workout immediately,” advises Rosante.

Another tell-tale sign that your pain is something other than normal muscle soreness? Rosante says if you feel pain anywhere other than in your muscles, like in your joints for example. Stop working out and seek medical advice, because that’s not normal. “Just remember that severity differs for everyone and can range from mild soreness to pain so bad it limits your range of motion,” adds Rosante. But it should only come in the muscles and after a few days of feeling it, you should be ready to go hard again.

Need more recovery tips? Here, top trainers weigh in on their top tips for beating post-workout soreness. And check out this NYC studio dedicated entirely to recovery.