A flare-up is a temporary rise in pain after activity.
The short version
- Most settle on their own within a day or two.
- The usual plan is to keep moving but do less for a few days — shorter, easier sessions rather than complete rest — then build back up.
- Flare-ups tend to get smaller as you keep training over the weeks.
Why it happens
Sore tissues can be more sensitive than usual, so a session that would normally feel fine can leave you sore afterwards. That extra soreness is often about how sensitive the area is right now, not fresh damage.
Flare-ups are common and expected in the early weeks. In one study, people with knee and hip pain doing a supervised exercise programme did get pain spikes after sessions — but the spikes got smaller as the weeks went on, and overall pain came down over the eight weeks. So a flare-up early on isn't a sign the programme is failing.
The other thing worth knowing is that pain after exercise can make people afraid to exercise again, and that fear is what usually leads to doing less and less. Researchers who study exercise for long-standing pain specifically recommend planning for flare-ups rather than being surprised by them — because the aim is to keep the habit going, not to avoid ever being sore.
⚠️ When to see a doctor in person
Get emergency care straight away — go to A&E or call 999 if the flare-up comes with numbness around your groin, genitals, or the area you'd sit on (saddle numbness), a change in genital sensation, sudden loss of bladder or bowel control, or leg weakness that's getting worse. These can be signs of a rare problem where nerves at the base of the spine are being squeezed, and it needs same-day emergency treatment.
- Get medical help the same day if a joint is hot, red, swollen, or unusually painful to touch (a fever may or may not be there — it doesn't need to be present for this to be serious).
- Book an appointment if each flare-up is bigger than the last, the pain hasn't settled after about two weeks of taking it easy, or it's stopping you sleeping or doing normal daily things.
Most flare-ups are a sensitive area reacting to load, not a new injury.
What to do for a few days
- Do less, not nothing. Cut the load roughly in half — fewer reps, less weight, a shorter walk. Full rest tends to make the return harder.
- Keep the rest of your body moving. If squats flare your knee, you can usually still walk or do upper-body work.
- Use whatever settles it. Heat, cold, gentle movement — there's no single right answer, so use what has helped you before.
- Give it a couple of days. Most flare-ups quieten down in that time.
- Then build back gradually. Start at the easier level and work back up over a week or so.
Progression note
Once the flare has settled, go back in at the easier level for a session or two before you return to where you were. If the same exercise flares you every single time, it's a sign the jump in load is too big — make the steps smaller rather than dropping the exercise altogether.
Bottom line
A flare-up after exercise is common and usually short-lived. Doing less for a few days and then building back up is generally more useful than stopping completely. If flare-ups keep getting worse instead of smaller, that's worth a conversation with a physio.