Noises from the knees, what doctors call crepitus, have three common sources. The sharp, single pop that comes when you stand up or straighten the leg and that can't be repeated right away is a gas bubble: the joint fluid contains dissolved gases that form a bubble when the joint surfaces are pulled apart, and when the bubble collapses you hear a pop. It is exactly the same thing as cracking your knuckles, and completely harmless. The dull, repeatable pop or "jump" in the same place every time, often on the outside of the knee or hip, is a tendon or band sliding over a bony edge: usually harmless, sometimes irritating, and a sign that the structure is tight or the muscle balance is off. The fine, constant grinding or grating when you bend and straighten the knee under load, especially behind the kneecap on stairs and in squats, is cartilage against cartilage where the surface has become a little uneven: very common from age 30–40, usually without significance as long as it doesn't hurt, and in osteoarthritis one of several signs. The simple rule: noise without pain, swelling or locking never needs investigation. Noise with pain, noise that came with a pop at the time of an injury, and a knee that locks or gives way should be examined. Here is the whole picture, what you can do yourself, and what cupping the thigh can contribute.
Which sound is which
| Sound | Cause | Harmless? |
|---|---|---|
| A sharp pop when you stand up, straighten the leg or squat; can't be repeated for 20 minutes | Gas bubble in the joint fluid (cavitation) | Yes, always, if it doesn't hurt |
| A dull pop or jump in the same place every time you bend, often palpable under the fingers on the outside of the knee, the outside of the hip or behind the knee | Tendon or band sliding over a bony edge (e.g. iliotibial band, hamstring tendon, plica) | Usually; irritating and can chafe over time, addressed with mobility and strength |
| Fine grinding or grating behind the kneecap on stairs, squats, standing up | Uneven cartilage behind the kneecap, very common from age 30–40; more pronounced in osteoarthritis | Yes if painless; with pain and stiffness, see knee osteoarthritis |
| Grinding, sometimes felt as "gravel", after a long time sitting, better after a few steps | Joint fluid is more viscous at rest, cartilage "drier"; normalises with movement | Yes; move more often |
| A clear pop or crack at the time of an injury, followed by swelling within hours | Cruciate ligament, meniscus or joint capsule | No: seek care, see meniscus tear |
| A pop followed by the knee locking or not straightening fully | Loose cartilage or meniscus fragment getting caught | No: seek care |
| A pop with sharp pain and the knee "giving way" | Kneecap dislocating, instability | No: seek care |
Why it cracks more after sitting, and with age
Joint fluid is thixotropic: it becomes thinner and lubricates better when the joint moves, and more viscous when you sit still. That is why knees crack and grind most in the first steps after a film or a long flight, and least in the middle of a walk. Over the years the cartilage surfaces become a little more uneven, the amount of joint fluid somewhat less and the muscles around the knee often weaker, so the kneecap glides less smoothly in its groove. None of that in itself means something is wrong, and there is no relationship between how much a knee sounds and how much wear shows on X-ray. However, studies show that people with grinding and pain more often develop osteoarthritis, so that combination is worth taking seriously with early strength training.
Is cracking your knuckles bad for you?
No. The pop is a gas bubble in the joint fluid, and it takes about 20 minutes before the gas has dissolved again and the joint can be cracked anew. The best-known study is a doctor who cracked the knuckles of one hand daily for 60 years and not the other, with no difference in osteoarthritis; larger studies have confirmed that habitual knuckle cracking doesn't cause osteoarthritis. The only things seen are possibly slightly lower grip strength and mildly swollen joints in heavy crackers. The same applies to "cracking" your own neck and back: the pop is harmless, but repeated forceful twisting of the neck is not recommended, and the feeling of "needing" to crack comes from tension better addressed with mobility, warmth and cupping the muscles.
When to seek care
- The sound came at the time of an injury (twist, fall, tackle) and the knee swelled within a day.
- The knee locks, catches or can't be straightened fully.
- The knee gives way or feels unstable.
- The sound is accompanied by pain that doesn't pass in a few weeks, night pain, warmth, redness or swelling.
- Children and teenagers with popping knees and pain, especially during growth and a lot of sport.
If you have pain without noise, or want to understand where the pain sits: knee pain: where does it hurt and what it can be, and with swelling swollen knee and fluid on the knee.
What makes knees quieter and stronger
Strength in thighs and glutes
The kneecap is steered by the four thigh muscles, and if the inside (vastus medialis) is weaker than the outside the kneecap is pulled off track and grinds and aches more. Glutes and the outside of the hip in turn control how the knee tracks over the foot. Three sessions a week of squats to pain-free depth, lunges, step-ups and leg press or side-lying leg raises give measurably quieter and less tender knees in 6–8 weeks; in osteoarthritis it is the best documented treatment. Home workout without equipment and kettlebell and dumbbell exercises at home have plans.
Mobility in hip and ankle
Stiff hip flexors and calves force the knee to compensate. Stretch the hip flexor (see hip stretches) and calves daily, and work on ankle mobility (knee to wall with the heel on the floor). A tight iliotibial band on the outside of the thigh is the most common cause of the dull, palpable pop on the outside of the knee; read runner's knee and sensitive knees.
Move more often, warm up
Stand up every half hour and bend and straighten the knees 10 times before walking; cycle or walk 5 minutes before loading the knees on stairs or in squats. See warm-up before training.
Weight, shoes, surface
Every kilo of body weight becomes three to four kilos on the kneecap on stairs. Shoes with cushioning and a stable heel, and avoiding deep squats under heavy load if the knee grinds and hurts, reduce the load on the cartilage without you having to stop training.
Cupping the thigh and back of the knee
Cupping isn't something you do on the knee joint itself, but the thigh and the back of the knee are exactly where the cup is useful: tight, short structures on the outside and front of the thigh pull the kneecap off track and give both the pop and the grinding more room, and cupping releases them, increases blood flow in the muscle and makes stretching and strength training afterwards more effective. Oil the thigh, glide the dynamic cup with light to moderate pressure in long strokes from just above the kneecap up towards the hip, 3 minutes on the front and 3 minutes on the outside, and finish with the back of the knee and the calf with light pressure. Use the mini cup around the edges of the kneecap, never on the kneecap itself, 1–2 strokes per side. Stretch and do your strength exercises right after. A ready plan is in the cupping programme for sensitive knees and the programme for runners; the basics in the complete guide to cupping. Don't cup a swollen, warm or newly injured knee, varicose veins, or if you take blood thinners without having asked a doctor; see when to avoid cupping.
Other joints that pop
- The hip: a jump on the outside of the hip when you walk or stand up is "snapping hip", the iliotibial band sliding over the bony prominence; a pop deeper in the groin is usually the hip flexor tendon. Both harmless, both better with mobility and strength; see hip pain.
- The shoulder: grinding when you lift the arm is often the shoulder blade sliding against the ribcage or a slightly irritated bursa; harmless without pain. See shoulder exercises.
- Neck and back: pops when you turn the head are gas bubbles in the small facet joints; common and harmless, but repeated self-"cracking" doesn't solve the tension. See stiff neck and tight shoulders.
- The jaw: a pop when you open wide is the jaw joint's disc jumping; harmless if it doesn't hurt or lock, otherwise see a dentist.
- Ankle and toes: pops when you turn the foot are gas bubbles; a repeatable jump on the outside of the ankle can be peroneal tendons sliding out of their groove, usually after a sprain.
Frequently asked questions
Why does my knee crack?
Usually from gas bubbles forming and collapsing in the joint fluid when the joint surfaces are pulled apart (a sharp pop that can't be repeated right away), from a tendon sliding over a bony edge (a dull pop in the same place every time), or from slightly uneven cartilage behind the kneecap (a fine grinding on stairs and in squats). All three are harmless as long as it doesn't hurt or swell.
Is it dangerous when the knee cracks?
No, not if it is painless and the knee doesn't swell, lock or give way. There is no relationship between how much a knee sounds and how worn it is. Seek care if the sound came with a pop at the time of an injury and the knee swelled, if the knee locks or feels unstable, or if grinding comes together with pain and stiffness that doesn't pass in a few weeks.
Is cracking your knuckles bad?
No. The pop is a gas bubble in the joint fluid, and studies of people who have cracked their knuckles daily for decades show no increased risk of osteoarthritis. Possibly a slightly weaker grip in heavy crackers. Cracking your own neck is also harmless in itself, but forceful repeated twists are not recommended, and the tension that makes you want to crack is better addressed with mobility, warmth and cupping.
How do you stop knees from cracking?
Completely silent they rarely become, but quieter and less tender: strength train thighs and glutes three times a week (squats, lunges, step-ups, side-lying leg raises), stretch hip flexors, the outside of the thigh and calves daily, move every half hour and warm up before stairs and squats. Cupping the front and outside of the thigh releases structures that pull the kneecap off track and makes the training afterwards more effective.
Does cupping help cracking knees?
Indirectly: the cup isn't used on the knee joint but on the thigh, where tight muscles and the iliotibial band pull the kneecap off track and cause both pops and grinding. Three minutes on the front of the thigh and three on the outside with a dynamic cup, followed by stretching and strength exercises, makes the kneecap freer and the training more effective. It doesn't affect the cartilage, and doesn't replace examination if the knee hurts, swells or locks.
Related guides
- Kinesiology tape – how to tape knee, shoulder and back, and tape or cupping
- Baker's cyst and pain behind the knee – symptoms, causes, exercises and when to seek care
Written by Curam. Last updated 17 October 2026. This is general self-care information and does not replace medical care. For sudden, severe or unusual symptoms, contact your doctor. Talk to your doctor before cupping if you are pregnant, take blood thinners or have a condition affecting skin, blood vessels or blood.
