Why your muscles hurt so much the day after you work out

Ever woken up the day after a workout and wondered what you did to deserve such pain?

I’m talking about soreness. That pain can stem from several sources, and understanding what's behind yours will help you best remedy it—and find ways to potentially prevent it in the future.

“There’s muscle soreness that could be due to, say weight training, which can cause what we call delayed onset muscle soreness, which is kind of a diffuse soreness in the muscle,” says Thomas Brickner, head team physician for a number of sports at the University of North Carolina. “It usually starts a day or two after a new workout, or a workout that you’re not typically accustomed to.”

Delayed Onset Muscle Soreness (DOMS) is the kind that happens the day after you dive into your first barre class, first run in a few months, or first time trying out weights. And though it can feel like you can barely move, when worst comes to worst you can straighten your arms if need be.

We experience DOMS because of diffuse microscopic injuries to the muscles themselves and the inflammation that results from it. (It's a common myth that it results from the build up of lactic acid. Lactic acid does cause that intense burning feeling during your last rep or right when your muscles are about to give in. But your body is able to eliminate it from your blood in a few minutes.)

“Usually the delayed onset muscle soreness is just kind of a discomfort in the muscles themselves that is somewhat diffuse, but the pain is usually just kind of mild and [the muscles] won’t typically lose much in the way of motion,” says Brickner. You don't typically have much in the way of swelling in the area either, he says. For example, if you did some bicep curls a day earlier, your biceps might feel sore, but you'd still be able to straighten your elbows.

For DOMS, certain types of exercise may make you more sore than others, especially workouts that include what are called eccentric contractions—ones that cause the muscles to tighten and lengthen at the same time. A good way to visualize this would be to picture doing a squat: the quadricep muscles in your thighs are starting to lengthen as you lower your body, but they are also tightening so you don’t go down too fast. Running downhill can cause this too, Brickner says.

Usually, this type of muscle soreness goes away on its own in a couple of days. But when you are feeling the brunt of it, there are steps you can take to make yourself feel better while it runs its course, and potentially allow the muscles to heal faster, too. Brickner says staying hydrated is extremely important. Your muscle cells need water to properly repair damaged tissue through protein synthesis. If you are bold, he says, and have access to multiple bath-sized bodies of water, a contrast bath—going from a warm bath to a cold one—could be helpful as well. Contrast baths work by opening and closing blood vessels, which creates a "pumping action" that decreases pain and inflammation in the area. You also can't go wrong with gentle massage of the muscles in pain, which research has shown to switch on genes that decrease inflammation as well as activate mitochondria-producing genes.

DOMS is painful, and can really wreck havoc on your daily activities. But you should still be able to do things, albeit a little more slowly. However, if you literally can’t straighten your arm a few days after a round of bicep curls, it's probably time to call the doctor. Brickner says that this is a sign of rhabdomyolysis, a severe injury to the muscles from an excessive workout. Extreme exercise can actually cause cell death of the muscles themselves. When the cells die, they release toxins into the bloodstream, which can cause immobility of the muscles in question, stiffness, swelling, and a release of myoglobin into the kidneys, which can make your pee look bloody. Not so good.

“A lot of people go and they work out hard and they think, ‘Oh, the muscle soreness is normal, it’s normal soreness from working out,’ but it might not be,” says Bricker. “It might be rhabdomyolysis, which is an abnormal type of muscle soreness. If you have that awareness of what rhabdo is, that is very important thing.” Rhabdo is rare, but if you have intense muscle stiffness, pain and swelling paired with red or brown urine, it's best to see your doc or head to the emergency room.

Anyone can fall victim to muscle soreness, whether you are a national championship-earning basketball player or someone who gets wiped out just walking up the stairs (I fall into the latter category). It all comes down to not pushing yourself too hard, especially if you are just now getting back into the swing of things when it comes to working out.

“I see that in our athletes every so often," says Brickner. Oftentimes they are in great shape, division one athletes, but maybe they just got back from winter or summer break and do a ton of pull ups, more than they had done in weeks or months. Even though they are in great shape, he says, they just overdo it. And the result is muscle soreness.

How do you know not to overdo it and spend the next few days feeling stiff as a board? For the first few times doing a new workout, you should feel like you could keep pushing a little bit harder if you wanted to. For lifting, Brickner recommends picking a weight that doesn’t fatigue you. The weight that you lift should make you think ‘hey, I could go further’ the first few times. As for beginning runners, Brickner says, if you’ve never jogged before, start on an every-other-day basis, and maybe trying something like jogging for a mile and then walking for a mile to gradually build up your distance.

“Almost all of these things occur because somebody goes into a program too quickly without training the body for it,” he says.

Beyond not wearing yourself out on round one, hydrating and having good nutrition for the workout you’re taking on might help (carbs for aerobic exercise like running, or protein for weight lifting). Warming up and cooling down is also essential to protecting yourself from injury, Brickner says.

“Muscle soreness can happen in the best trained athlete, it can happen in the least trained athlete,” he says. “I think that it can happen to anyone, but the prevention, no matter who you are, is to start off small with any activity that you’re not used to.”