There's some paranoia around lower back pain that pervades the fitness world. To be fair, back injuries stink—to put it mildly. Hurting your back can keep you out of the gym for months. It can even lead to lifelong problems. So it makes sense that fit people want to baby their backs, and much of the information you hear about keeping proper form always seems to end with some variation of "You shouldn't be feeling this in your lower back. If you are, you're doing it wrong."
So the other day when I moved into Savasana after a particularly tough yoga class, I was worried when my lower back started pinging. After a few minutes, the muscles relaxed and I forgot all about the sensation—until the next day, when my back was sore, in addition to my core, shoulders, and glutes (it was a tough class!!). It felt like normal soreness, but I was concerned. Should my back ever hurt?
"It's completely normal to feel lower back soreness after doing back or core exercises," reassures Denis Patterson, a doctor of osteopathic medicine who practices at Nevada Advanced Pain Specialists. After all, your core doesn't just include your ab muscles; it includes your back too, and strengthening those muscles is a great way to prevent back pain and trim your waist. (See also: How to Engage Your Core, Plus 7 Great Ab Exercises)
Normal lower back soreness the same as DOMS anywhere else, Patterson says. "It occurs due to your body's natural inflammatory reaction to exercise causing microtrauma to the muscles and surrounding connective tissue."
That said, you should still know the difference between normal back soreness and potential back injury. Back DOMS tends to develop gradually after exercise. You'll start to notice it six to eight hours after your workout, with it peaking 24 to 48 hours later and disappearing 72 hours post-workout. (Have you tried the sexy back workout?)
"Acute back pain that occurs abruptly while or immediately after exercising is not normal, and is usually a sign of an acute lower back injury," cautions Patterson. "Acute back pain that's debilitating and doesn't diminish 72 to 96 hours after exercising could also indicate back injury." Other go-to-the-doctor-now red flags include joint or nerve symptoms like numbness, tingling, weakness, or pain radiating to the lower extremities. (Related: Yoga Poses for Back Pain Relief)
On the whole, though, a little achiness in your back after a workout is nothing to be nervous about. In fact, it could be a sign that you're targeting some hard-to-reach—but very important—muscles. Still, if you've hurt your back in the past, it's a good idea to give your instructor a heads up so he or she can clue you in to any form modifications that can help you avoid aggravating the injury.