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.”