We’ve all experienced delayed onset muscle soreness, AKA DOMS, coming on a day or two after a hard workout. Sometimes it’s because we haven’t done that type of exercise in a while. Or maybe we’ve pushed harder than usual. It could be that we added in a bunch of downhill running, uphill running, sprint work, or something else that modified the load on our muscles.

No matter how it starts, the soreness comes on in a day or two and leaves a few days after that, giving us some discomfort but not messing up our lives too much. At least, not usually.

While DOMS is an annoyance, is it actually something bad happening to our muscles? Is it a sign of damage or just a body signal that we’ve done too much? Let’s do a deep dive on DOMS to see what’s happening at the molecular level, why it’s actually good for us, and exactly what our body is doing other than just hurting.

The Science of DOMS

There’s a lot going on with delayed onset muscle soreness, and it all starts with muscle damage, just like all workouts do. A hard workout damages your muscles—that’s just a fact. But that’s okay. The damage is what signals your body to repair stronger and fitter. We have to hurt ourselves a little bit to get the gains that we want. But what kind of damage are we doing exactly? For that, we need to zoom in to the muscle cells.

Structural Damage: Z-Discs and Sarcomeres

Here’s a drawing of a sarcomere. It’s the basic unit of a muscle. Muscles have millions of these, your body has trillions, all organized in the same way. There are anchoring lines at the ends called Z lines that provide structure and contractile fibers in the middle that pull toward each other, like climbing a rope hand over hand.

When the fibers pull toward one another, they shorten, pulling the muscle tighter and moving the bones they’re attached to. Boom, contraction.

All these fibers are anchored to the Z lines. If they weren’t held tightly in place, they’d just pull on each other and the muscle wouldn’t contract in an organized way—it’d go all weird and squishy, balled up and disorganized. That wouldn’t be a useful way for a muscle to work. So the Z lines anchor the contractile fibers and give the muscle its structure. They’re also something that can get damaged with lots of exercise.

The constant pulling of the fibers on these Z lines leads to what we call Z disc streaming. The neat little anchors that align your muscle fibers start to smear and misalign after hard exercise. Because they’re being pulled hard in each direction, they can go out of whack and not be as straight as they’re supposed to be. They end up all wiggly and disorganized.

This Z disc streaming is actually a good thing, since it’s one of those effects of exercise that triggers the body to remake that area stronger. After exercise, your body gets going on remodeling, building back the Z lines thicker, anchoring them down with even more proteins. The body doesn’t like to “waste” effort. It wants you to prove that the extra anchoring is actually necessary. So it waits until the Z lines are damaged, then rebuilds them stronger.

Protein Breakdown and Microscopic Gaps

That’s one element, but there’s even more going on at the fiber level. The key proteins that cause muscles to contract and keep them organized are actinin, titin, and nebulin, among others. These are molecules that hook together, providing the muscle structure and scaffolding required for the muscle to do its job. Just like with the Z lines, if these aren’t properly anchoring the muscle, it’s not going to work right.

One of the types of damage that happens after a hard workout is directly on these proteins that are holding things together. They get banged up and tweaked, like the twisted beams of a building after an earthquake, just at a much smaller level.

When these proteins are damaged, the body flushes them out, leading to actual, microscopic holes being left behind in the muscle tissue. These gaps in the muscle fibers make the muscles weaker, but only in the short term.

In the longer term, your body is going to fill these gaps in with even more proteins, like reinforcing a structure by adding extra support beams. The healing process will increase the total number of proteins in the area, making the scaffold stronger. But in the short term, having a bunch of microscopic holes in a muscle is going to make it hurt.

Mitochondrial Stress and Swelling

A new element of DOMS that we’ve only discovered recently is how it could be caused by your mitochondria swelling. We all know that mitochondria are the powerhouses of the cells, and muscles need a lot of power to do their work. Muscle cells can have dozens of mitochondria in them, if not more, and these work hard when muscles are working, providing the energy they need to contract and relax.

When mitochondria are called upon to work hard, they get stressed. Researchers took biopsies of people who had recently exercised harder than normal and were experiencing DOMS, placed those samples under microscopes, and were able to see exactly what was going on. The mitochondria in the sore tissue was swollen and the proteins that anchored them to their spots in the cells were disorganized. The powerhouse of the cell feels the strain too.

Immune Response and Muscle Swelling

The last mechanism that causes DOMS (and can make you feel weird after hard exercise) is muscle fiber swelling. In the days following a difficult workout, muscles call immune cells into themselves, causing them to swell up as the cells enter and bring water with them. This is a totally normal reaction to hard exercise.

Immune cells are there to clear out the damaged tissue so that the muscles have room to rebuild. If we don’t clean out the damage, we don’t have space to add new stuff. In order for an immune reaction to work well, tissues have to swell.

This increases the gaps between the muscle cells, letting immune cells leave your bloodstream and slide into place, cleaning up the damage. The swelling is like an access road to the damaged area, letting the immune cell workers get into place and do their jobs.

The Bottom Line: DOMS Is Your Body Rebuilding

We used to think that DOMS was caused by muscle tears or necrosis (tissue death), but that’s not what is going on. Instead, there are several small factors that lead to the pain from DOMS: Z-disc streaming, protein damage, mitochondrial swelling, and immune cell infiltration.

These processes happening together are actually beneficial—they allow the body to recognize and repair the damage that a workout has done, growing back stronger in the future. DOMS isn’t a sign that you’ve hurt yourself; it’s a sign that your body is actively upgrading your muscles.

Managing DOMS: Patience Is Key

How do you get rid of DOMS? Unfortunately, there aren’t really any good methods to speed up the process. The best thing you can do is stay hydrated and take some electrolytes (minerals like sodium and potassium that help with muscle function)—your body needs these to rebuild effectively.

Things like massage, foam rolling, ice, heat, and various supplements haven’t been demonstrated to significantly reduce DOMS duration. The one thing that does provide temporary relief is gentle to moderate exercise of the sore muscles. This has a pain-relieving effect, but it doesn’t last very long and doesn’t actually speed healing.

Otherwise, you’ve just got to wait it out. But don’t worry—it’ll just be a day or two and is a sign that your body is actively healing and rebuilding. Rather than dreading DOMS, try to see it as evidence that you’ve provided your muscles with the stimulus they need to grow stronger. Embrace the soreness as part of the adaptation process that makes you fitter and more resilient.