Knee arthritis flare up: what to do, how long it lasts and when to worry

What is a knee arthritis flare up?

Knee osteoarthritis rarely hurts the same amount every day. Most people have an ordinary level of ache and stiffness that rises and falls a little. A flare is a clear step up from that level which lasts at least a day, and it often comes on quickly.

Researchers in the UK asked people with knee pain to keep a daily diary for up to three months. During a flare, people were more likely to describe the pain as sharp, throbbing, stabbing or burning, and more likely to notice swelling and stiffness, or to limp. They were also more often woken by pain, took more painkillers and stopped some of their usual activities (Parry and colleagues, 2019).

A flare is not a sign that the joint has suddenly worn out. It is usually a sore, irritable knee that needs a few lighter days. It does feel alarming, though, so it helps to have a plan ready before the next one.

What triggers a flare?

A larger UK study followed adults who reported knee flares online and compared what they had been doing just before each flare with what they did at other times. The knee giving way (buckling) was the strongest trigger. Other activity triggers were squatting or kneeling, standing for long spells without a rest, lifting or moving heavy things, climbing ladders, moderate or hard exercise, and a slip, trip or fall.

Things outside activity counted too. A poor night's sleep, low mood, feeling angry or irritable, and cold, damp weather were also linked with a flare starting within the next 24 hours (Thomas and colleagues, 2021).

Some of those you can plan around, such as spreading a big gardening job over several days instead of one. Others, like the weather, you cannot. A knee that keeps giving way is worth an assessment. Strength and balance work can often reduce it, and a brace helps some people (see knee braces for arthritis).

What should you do during a knee arthritis flare up?

Think "less, not none". These steps are common physio practice, not a tested protocol.

1. Cut back the thing that set it off. Shorten walks, sit for jobs you would normally stand for, and leave the heavy lifting or the kneeling for a week.
2. Keep moving. Gentle movement every hour or two usually eases stiffness better than a day on the sofa. Short walks around the house count.
3. Drop to stage 1 exercises. Swap your usual strength routine for the lying and sitting exercises below.
4. Use heat or cold if it helps. Warmth often suits a stiff knee and cold a puffy one. Wrap either in a towel and keep it on for 10 to 20 minutes. Skip both on skin that has lost feeling.
5. Check your painkillers. NICE advises offering an anti-inflammatory gel (a topical NSAID) to people with knee osteoarthritis. A pharmacist can tell you whether one is safe alongside your other medicines. Never stop or change a prescribed medicine on your own.

A walking stick in the opposite hand from the sore knee can take some load off it for a few days. The 2019 US guideline from the American College of Rheumatology and the Arthritis Foundation strongly recommends a cane when knee arthritis is affecting walking, stability or pain enough to need one (Kolasinski and colleagues, 2020). Using one for a flare is common practice rather than something the guideline tested, and a physio can check the height and how you walk with it. Is walking good for knee arthritis covers how to build walking back up once the flare has passed.

How should you adjust your exercise in a flare?

Go back to stage 1 of the knee osteoarthritis program, whatever stage you had reached. Those exercises are done lying or sitting, so they keep the thigh muscles working and the knee bending without putting your body weight through it. Leave squats, step ups or split squats until the knee has settled.

Exercises that can help

These are the stage 1 exercises to fall back on during a flare, done lying or sitting so the knee carries no body weight. Start at the low end: many programs use 10 quad sets with a 5 to 10 second hold several times a day, 10 to 15 slow heel slides 2 or 3 times a day, 2 to 3 sets of 10 to 15 short arc quads with a 3 to 5 second hold once or twice a day, and 2 to 3 sets of 10 straight leg raises on each leg once a day. Your physio will adjust this.

Use the next-morning check to guide you. If the knee is no worse the morning after your exercises, the amount was fine. If it is sorer, stiffer for longer or more swollen, do fewer repetitions the next day. Our guide to the pain monitoring model explains how physios use this kind of check.

Once the pain is back to your usual level for a few days in a row, return to the stage you were on before. Many people find it easier to restart with fewer sets or a lighter version for the first week, then build up again. Exercises to avoid with knee arthritis shows how to make squats, lunges or stairs easier while you rebuild. Cycling on a light setting is often comfortable earlier than walking far; see cycling and swimming for knee arthritis, and best exercise equipment for knee arthritis for simple kit to use at home.

If you are pregnant and past the first three months, check with your physio before exercising flat on your back, as some of these exercises are done lying down. If you have high blood pressure, breathe steadily through every repetition and never hold your breath. After a knee fracture or knee surgery, including a knee replacement, follow your surgeon's or physio's plan, and start these exercises only when they have cleared you.

How long does a knee arthritis flare up last?

Two UK studies give the best figures so far. In the daily diary study, flares lasted a median of 8 days, with the shortest at 2 days and the longest at 30 (Parry and colleagues, 2019). In the larger online study, the median was 5 days, and half of all flares lasted between 3 and 8 days (Thomas and colleagues, 2021). So most flares settle within about a week or so, but a few take several weeks.

If a flare has not started to ease after about two weeks, or flares are coming closer together, book an appointment with your physio or doctor. That timing is a practice rule of thumb, not a research cut-off. Frequent flares usually mean the plan needs changing, for example a different balance of rest and activity, more strength work between flares, or help with sleep or weight. If you carry extra weight, weight loss and knee arthritis explains how much it can ease the load on the knee. If flares keep coming despite all of that, when to consider surgery covers that conversation.

When is it not a flare?

Most flares feel like your usual arthritis, only louder. A few things that look like a flare need a doctor, not a lighter week.

Infection in the joint is the one not to miss. A knee that is hot, red and swollen, or knee pain with a high temperature or feeling hot, cold or shivery, needs medical advice the same day, and an emergency department if you feel very unwell.

Crystal arthritis can look the same. Gout brings sudden, severe pain with hot, swollen, red skin over the joint; it most often affects the big toe, but the NHS lists the knees among the other joints it can affect. Pseudogout, caused by a different kind of crystal, is most common in the knee, and comes on with sudden, intense pain and a warm, swollen, tender joint (Cleveland Clinic). Neither can be told apart from an infected joint at home, so treat a sudden hot, swollen knee the same way: medical advice the same day.

A newly swollen, warm or tender calf or thigh is a different matter, because it can be a blood clot, and a bad flare that keeps you much less mobile than usual is one of the times clots are more likely. Get medical advice the same day, and call emergency services if you are also short of breath or have chest pain. Pain after a fall or twist, or a knee that locks, also needs medical advice the same day. Swelling that comes up within a few hours of a twist, fall or blow means going to an emergency department straight away, and not driving yourself. The warning list below gives the full set of signs.

Related exercise programs

• Knee osteoarthritis

Common questions

References

1. Parry E, Ogollah R, Peat G. 'Acute flare-ups' in patients with, or at high risk of, knee osteoarthritis: a daily diary study with case-crossover analysis. Osteoarthritis and Cartilage. 2019;27(8):1124-1128. https://doi.org/10.1016/j.joca.2019.04.003
2. Thomas MJ, Rathod-Mistry T, Parry EL, Pope C, Neogi T, Peat G. Triggers for acute flare in adults with, or at risk of, knee osteoarthritis: a web-based case-crossover study in community-dwelling adults. Osteoarthritis and Cartilage. 2021;29(7):956-964. https://doi.org/10.1016/j.joca.2021.04.007
3. National Institute for Health and Care Excellence. Osteoarthritis in over 16s: diagnosis and management (NG226). 2022. https://www.nice.org.uk/guidance/ng226
4. Kolasinski SL, Neogi T, Hochberg MC, et al. 2019 American College of Rheumatology/Arthritis Foundation guideline for the management of osteoarthritis of the hand, hip, and knee. Arthritis Care and Research. 2020;72(2):149-162. https://doi.org/10.1002/acr.24131
5. NHS. Gout. Page last reviewed 24 August 2023. https://www.nhs.uk/conditions/gout/
6. Cleveland Clinic. Pseudogout (chondrocalcinosis or CPPD). https://my.clevelandclinic.org/health/diseases/pseudogout-chondrocalcinosis-cppd

Written and checked by the PocketPhysio editorial team. Last updated 2026-10-16.

This page is general education, not a diagnosis or a personal treatment plan. See a physiotherapist or doctor for advice about your own situation.