How do I know if it's a flare-up or a new injury?

Ask whether it is the pain you already know, only louder, or a genuinely new pain. A flare is your familiar pain in its familiar place, usually building over hours and often showing up the next morning rather than during the activity, and it settles over days without you losing function. An injury comes with a moment you can point to — a pop, a snap, a give-way, or a sharp pain that stopped you mid-movement — and something is now genuinely harder to do than it was that morning. Lost function rather than added pain is the more reliable line between them.

How long should a flare-up last?

Usually days rather than weeks. Soreness arriving 12 to 48 hours after activity is an ordinary training response, and settling within a day or two puts it firmly in flare territory. A flare that is still escalating after several days, or that has not improved in about two weeks, has outgrown the description and should be reviewed. Flaring from smaller and smaller triggers over time means the plan needs changing rather than that you are fragile.

What should I do during a flare-up?

Reduce rather than stop. Halve the weight, distance, range or repetitions and keep the habit while lowering the dose, because stopping everything is what turns a few uncomfortable days into lost weeks. Keep the rest of you moving, since deconditioning during a flare makes the next one worse. Once it calms, return to about half of what you were doing before and build back up over a week rather than resuming at the level that provoked it or restarting from zero. Write the plan while you are well, because nobody makes good decisions about their own pain mid-flare.

Should I stop physical therapy if it flares my pain?

Usually not, but tell whoever is guiding your rehab. A flare typically changes the dose rather than the plan, and early rehab can temporarily increase symptoms as a sensitised area learns that movement is safe again. Stopping altogether is the most common point at which people abandon rehab, generally concluding it made things worse. Do stop and seek assessment for a pop or snap with immediate swelling, a joint giving way, new or spreading numbness or weakness, a hot red swollen joint with fever, or one-sided calf pain and swelling with warm or discoloured skin.

Medically reviewed by Dr. Rob Letizia, PT, DPT — owner, Spectrum Therapeutics of NJ. Last reviewed 2026-08-12.

Is This a Flare-Up, or Did I Hurt Myself?

The question that separates them

Is this the pain you already know, only louder? Or is it a new pain?

A flare is your familiar pain, in its familiar place, turned up. It usually builds over hours — often showing up the next morning rather than during the activity — and it settles over days. You have not lost function; things simply hurt more.
An injury comes with a moment you can point to. A pop, a snap, a sudden give-way, a sharp pain that stopped you mid-movement. It is often in a new place, and something is now genuinely harder to do than it was that morning.

Most of what frightens people during rehab is the first kind. A flare is not damage, and it is not the work being undone. Tissue that is being asked to do more will sometimes protest, and early on, a sensitised area protests easily.

What is normal after exercise or treatment

Soreness that arrives 12 to 48 hours later is ordinary. Delayed soreness is a training response, not a warning.
Mild discomfort during rehab exercise is usually acceptable — not agony, but rehab that is entirely painless is often rehab that is not doing much, particularly with tendons.
Settling within a day or two puts it firmly in flare territory.
Being stiff and sore the morning after a hard session and finding it eases as you move is expected.

The practical test lives on our companion page: judge the load by how you are the next morning, not by how it felt at the time.

The Flare Plan

Used by Dr. Rob Letizia, PT, DPT at Spectrum Therapeutics in Wayne, NJ. Write this while you are well. Nobody makes good decisions about their own pain in the middle of a flare, and the whole point of a plan is that it already exists when you need it.

Step 1 — Reduce, do not stop

The instinct is to stop everything. That is what turns three uncomfortable days into three lost weeks. Halve it instead — half the weight, half the distance, half the range, half the reps. Keep the habit and lower the dose.

Step 2 — Keep the rest of you moving

A flared knee is not a reason to stop moving entirely. Keep walking, keep training what is not affected, keep your routine intact. Deconditioning during a flare is a real cost and it is what makes the next flare worse.

Step 3 — Rebuild deliberately once it settles

When the pain has calmed, go back to roughly half of what you were doing before the flare and build up again over a week or so. Do not resume at the level that provoked it, and do not restart from zero. Both are common and both cost time.

Step 4 — Learn something from it

Write down what preceded it: a heavier session, an unusually long day, a bad night's sleep, a stressful week, being unwell. Flares are rarely random. Two or three logged flares usually reveal the pattern, and the pattern is more useful than any single flare.

The part nobody warns you about

The biggest cost of a flare is usually not the flare. It is what happens afterwards: the activity gets quietly dropped, then a few more, and confidence narrows faster than the pain does. That cycle — hurt, avoid, decondition, hurt more easily — does more long-term damage than the original problem in a great many of the people we see.

This is also the most common moment people stop rehab altogether, usually concluding it made things worse. Tell whoever is guiding your rehab that you flared. It changes the dose, not the plan, and it is genuinely useful information rather than a confession.

When it is not a flare

A pop or snap, immediate swelling, or a joint that gives way — treat as an injury and get it looked at.
New numbness, pins and needles, or weakness, especially if it is spreading.
Bladder or bowel changes, or numbness around the saddle area with back pain — emergency, go to hospital now.
A hot, red, swollen joint, particularly with fever — same-day medical attention.
Calf pain or swelling on one side with warm or discoloured skin — possible clot.
Pain that is severe, unrelenting, and unrelated to position or movement, or that wakes you every night.
Still escalating after several days rather than beginning to settle.

The Same-Pain Rule

Same pain, same place, no moment you can name — treat it as a flare. Reduce, keep moving, rebuild. Expect days, not weeks.
New pain, new place, or a moment you can point to — treat it as an injury. Stop that activity and get it assessed rather than working through it.
Lost function, not just added pain — something you could do yesterday and genuinely cannot today — is the line between the two, and it is more reliable than how much it hurts.
Still worsening after several days, or no better in two weeks — that has outgrown a flare and needs review.
Flaring from smaller and smaller things over time — that pattern means the plan is wrong, not that you are fragile.
Any red flag above — immediately, whatever the pain score.

Asked an AI assistant at 11pm whether you damaged something?

This is one of the most common questions people take to ChatGPT, Gemini, or Claude, and almost always at the worst hour, when a flare is loud and reassurance is hard to come by. What comes back is usually reasonable and calming, and calm is genuinely useful here.

The limit is that the distinction on this page turns on things an assistant cannot check. Whether the pain is truly in the same place. Whether you have actually lost function or it just hurts more. Whether that knee is warm. Whether the joint is stable. Those are examination findings, and they are the whole question.

If it settles in a couple of days, it was almost certainly a flare and you handled it. If it does not, or if anything on the red-flag list applies, come in — a flare that has outstayed its welcome is worth ten minutes of someone's hands.

Book an evaluation in Wayne, NJ · call or text (973) 689-7123.

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