If you're just starting out, you'll get to know it. You wake up the next day, or the one after, and going down the stairs hurts. It's delayed onset muscle soreness (DOMS), and although it's annoying, it's normal.
What it is
It's the muscle pain that shows up 24 to 72 hours after training. It's caused by small amounts of damage in the muscle and the inflammation that comes with it, especially when you do a new exercise or more load than usual. The lowering part of each rep (the eccentric) is what causes it most.
It isn't lactic acid
It's a very common myth. Lactate clears from the muscle shortly after training. It has nothing to do with the pain you feel two days later.
Does soreness mean you trained well?
No. It means you did something your body wasn't used to. Once you've been doing the same exercises for a few weeks, it shows up much less even though you're still making progress. Don't chase it as a sign of a good workout.
Can I train when I'm sore?
- If it's mild: yes. Warm up well and you'll notice it fades as you move.
- If it's severe: train a different part of your body or do a lighter session.
- Moving helps: walking, easy cycling or mobility work usually ease it more than staying still.
How to reduce it when you start
- Start with low volume: 2 or 3 sets per exercise.
- Add load gradually.
- Repeat the same exercises for a few weeks before changing them.
- Sleep well and eat enough protein.
When it isn't just soreness
See a doctor if you notice very intense muscle pain, a lot of swelling, marked weakness or very dark urine after training. That isn't normal and needs to be checked.
Sharp pain in a joint, or pain that gets worse every time you train, isn't soreness either. In that case, have a physiotherapist look at it.
In short
Soreness is the price of getting started. It goes away in a few days, shows up less and less over time and says nothing about how well you trained. If you want to avoid the typical mistakes of the first few months, read the common mistakes when you start training.