Muscle soreness comes from fine tears in the fibres, not from lactic acid. That is why neither stretching beforehand nor cooling down afterwards prevents it.
Training is fine, hard training is not
Light movement improves blood flow and shortens the discomfort. A heavy session for the same muscle group extends it and raises the injury risk, because force production is measurably reduced. Other muscle groups can be loaded as normal.
When it is no longer soreness
- The pain came during the exercise, not the day after.
- It sits in a joint rather than in the muscle belly.
- It lasts longer than five days.
- Urine turns dark -- then see a doctor immediately.
That last point is rare but serious. It follows extreme, unaccustomed load and is not something to wait out.
When in doubt, speak to one of us on the floor. We will look at your form -- that is usually where the cause sits.