If you’ve ever exercised intensely after taking some time off—or you just tried a boot camp class at your gym for the first time—you know that you’re likely to feel pretty sore the next day. That pain you feel that leaves you waddling around—and can feel even worse two days after a workout—is thanks to a phenomenon known as delayed onset muscle soreness, or DOMS.
So why exactly do you feel sore the next day, instead of right away? Here’s what you should know about DOMS—and when you should be concerned about that second-day ache.
What Is DOMS?
Unlike acute soreness, which is pain that starts while you’re exercising, DOMS usually begins 12 to 24 hours after your workout, with the greatest pain felt anywhere from one to three days later, explains Carol Wilder, M.D., a family and sports medicine physician at The Sports Medicine Clinic at UW Medicine in Ballard.
Research shows that DOMS is caused by microscopic damage to muscles and the surrounding connective tissues, which leads to inflammation and shifts of fluid and electrolytes. When your body starts to repair the damage, you begin feeling sore. Muscle soreness is especially common after doing a workout you’re not used to, says Wilder. Which is why if it’s been months since you last went hiking and you decide to trek to Mailbox Peak your first day out the gates, you’ll probably pay for it for the next few days.
And unlike what your high school track coach may have told you, lactic acid buildup in your muscles is not what causes DOMS. It turns out that old-school theory has long been debunked, says Wilder. “It’s just one of those buzzwords that people hear about,” she says.
Lactic acid comes into play during anaerobic exercise (think: sprinting, weight training or interval training). It’s responsible for that muscle burn you feel during the final push of a tough workout, but it’s not going to make you sore two to three days later, Wilder explains.
When Should You Worry About Sore Muscles?
While muscle soreness is common after an exercise hiatus, you shouldn’t feel as if you’re overreacting by talking to your doctor about your pain, says Wilder. Feeling tired after trying something new? Totally normal. But visible swelling, joint discomfort or pain felt right away could be signs of injury.
A severe case of DOMS could be the condition rhabdomyolysis, which is a breakdown of muscle tissues that releases the protein myoglobin into the bloodstream and can lead to kidney damage and even, in some cases, total kidney failure. Signs of the condition, often referred to as “rhabdo,” include flu-like symptoms, dark urine and significant muscle pain.
“If you notice any of these symptoms following exercise, you should be seen by a doctor for urine and lab tests,” says Wilder.
No Pain, No Gain? How to Avoid Sore Muscles
Contrary to popular belief, you don’t need to work yourself to the bone and feel super sore the next day to get a good workout, says Wilder.
“A little bit of muscle soreness means you pushed the limit, but it’s a bell curve,” she says. “Being tired, achy and sore is different than being so sore you can’t walk.”
If you do get sore, it can be tempting to wait it out on the couch. After all, who wants to exercise when it hurts to walk up the stairs or get out of your car? But keep moving! You’ll probably want to give your muscles some time off from heavy lifting, but gentle movement can help stimulate blood flow to aid in faster muscle repair, says Wilder. Research also suggests that massage, foam rolling or an ice bath might help alleviate some of that discomfort you’re feeling.
And while everyone is susceptible to soreness, you’ll likely start to experience it less intensely once your body has acclimated to exercising. The best way to avoid DOMS is to not do too much, too fast, says Wilder.
“My major message is to return to activity gradually after a hiatus,” she says. “Being a weekend warrior has its costs.”