The knee: ligaments, menisci, cracking knees and the evidence on squats

Two joints, four ligaments, two menisci

The thigh bone meets the shin bone in the main joint and the kneecap in the front one. Two cruciate ligaments cross inside the joint, two collateral ligaments brace the sides, and two menisci spread the load.

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It bends to about 140 degrees

Healthy adults bend the knee about 133 to 142 degrees. Walking needs less than 90, and stairs and chairs 90 to 120.

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Cracking is common

About 4 in 10 people have noisy knees, including about a third of people whose knees don't hurt. The NHS says painless clicking is normal.

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Squats suit healthy knees

Reviews of the biomechanics found no evidence that squats harm healthy knees, a body-weight knee bend loads the knee about as much as walking, and knee-loading exercise didn't harm cartilage in trials.

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Knees over toes is normal

Letting the knees travel forward probably works the thigh muscles harder, and blocking it shifted load to the hips and back. The evidence comes from small lab studies.

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Strong thighs protect knees

Weak thigh muscles predicted knee arthritis, and injury-prevention warm-ups halve injuries to the anterior cruciate ligament (ACL) in sport.

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Two joints in one

The knee is the largest joint in the body. In the tibiofemoral joint, the two rounded ends of the thigh bone (femur) sit on the flatter top of the shin bone (tibia). In the patellofemoral joint, the kneecap (patella) glides in a groove at the front of the femur. The fibula, the thin bone on the outside of the shin, isn't part of the knee joint.

The kneecap, the largest sesamoid bone in the body, sits inside the quadriceps tendon. It increases the thigh muscles' leverage and shields the joint. The knee is mainly a hinge, with small amounts of rotation and gliding.

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Four main ligaments

Two cross inside the joint and two brace its sides.

• Anterior cruciate ligament (ACL): inside the joint; it stops the shin sliding forward on the thigh and limits some inward rotation. It doesn't heal well once torn.
• Posterior cruciate ligament (PCL): inside the joint and nearly twice as thick as the ACL; it stops the shin sliding backward, especially with the knee bent. It's the strongest ligament in the knee.
• Medial collateral ligament (MCL): on the inner side; it resists the knee bending inwards (knock-knee force). Its deep part is attached to the inner meniscus.
• Lateral collateral ligament (LCL): a cord on the outer side, from the thigh bone to the top of the fibula; it resists the knee bending outwards.

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The menisci

Two C-shaped wedges of fibrocartilage sit on top of the shin bone, about 65% to 70% water. The inner (medial) one covers about 60% of its side and is tethered to the MCL, so it moves less and tears more often; the outer (lateral) one is almost circular, covers about 80% and moves more.

They carry about half the load on the inner side of the knee and 70% on the outer side, absorb shock and steady the joint. Only the outer 10% to 30% has a blood supply; the rest is fed by the joint fluid.

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Tendons, capsule and bursae

The quadriceps tendon and the patellar tendon run over the front, and the capsule and ligaments wrap the back. At least 12 bursae, small fluid-filled cushions, protect the knee, including in front of and below the kneecap and on the inner side just below the joint. The quadriceps are the knee's main muscular stabilisers.

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Bending and straightening

In the CDC (US Centers for Disease Control and Prevention) study of 674 healthy people, adults bent the knee about 133 to 142 degrees lying on their backs. Straightening goes to zero, and on average about 2 degrees past it; a few people went 17 to 19 degrees past straight.

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Turning and unlocking

The shin can turn a little on the thigh, more easily with the knee bent, and the soft tissues limit it. As a fully straight knee starts to bend, the popliteus muscle unlocks it with a small twist.

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What everyday life uses

In 20 healthy adults averaging 67, walking and slopes needed less than 90 degrees of bend, stairs and chairs 90 to 120, and getting out of a bath about 135. Squatting with the heels up needed about 157 degrees.

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How range changes with ageStrong evidence

Knee bend was about 5 degrees smaller at 45 to 69 than at 20 to 44 in the CDC data, and a US national survey found most knee movements only 3 to 5 degrees smaller at 65 to 74. A big loss of knee movement isn't just age and is worth getting checked.

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Very bendy knees

A few people's knees bend well back past straight. If your joints are very flexible and painful, our hypermobility guide covers the strength work that helps.

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How common it isEmerging evidence

A 2025 review of 103 studies (36,439 people) estimated knee crepitus, the clicking, cracking or grinding you hear or feel, in 41% of people overall, 36% of people whose knees don't hurt and 81% of knees with osteoarthritis, all at very low certainty. In 214 runners checked by a clinician, two-thirds of those who'd never had knee surgery had it.

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Where the noise comes fromEmerging evidence

The gas-bubble explanation comes from MRI (magnetic resonance imaging) films of finger joints, where a crack came with a gas cavity forming in the joint fluid, not from studies of knees. In the knee, noises may also come from soft tissue moving over bone, and grinding can come with cartilage changes. These explanations are inference for the knee.

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Painless clicking is normalStrong evidence

The NHS says painless clicking is normal. It lists a knee that locks, gives way or clicks painfully as a reason to get advice.

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It's linked with arthritis, but it isn't a diagnosisEmerging evidence

Crepitus was strongly linked with arthritis on X-ray (about 3.8 times the odds), with low to very low certainty. In 888 women aged 45 to 60 without arthritis, it went with early changes behind the kneecap on MRI, not in the main joint.

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What it predictsEmerging evidence

In 3,495 middle-aged and older adults at risk of knee arthritis, more frequent crepitus predicted later painful arthritis, up to about 3 times the odds for knees that were always noisy. But more than 75% of the new cases were in people who already had arthritis on X-ray without symptoms.

In 112 young adults a year after ACL reconstruction, crepitus went with cartilage damage behind the kneecap but didn't predict it getting worse over 5 years.

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It doesn't predict a knee replacementMixed evidence

In 4,566 people from the same large study, crepitus didn't predict who had a knee replacement over 3 years.

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Our take

Painless cracking needs no treatment. If your knees crunch and you're over about 45, treat it as a reason to keep your thighs strong, not to stop using your knees. Get advice if the noise comes with pain, swelling, locking or the knee giving way.

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Reviews find no harm to healthy kneesEmerging evidence

A review of more than 164 articles judged worries that deep squats damage cartilage or cause arthritis unfounded, and warned that very heavy half and quarter squats might be worse for the knee in the long run than deep squats with sensible loads. Another review called the parallel squat not harmful to healthy knees, but preferred it to deep squats for athletes. The reviews disagree on depth, and there are no long-term trials.

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Squats load the knee about as much as walkingEmerging evidence

In 5 people with instrumented knee replacements, a body-weight knee bend loaded the knee about 2.5 times body weight, against 2.6 for walking and 3.2 to 3.5 for stairs.

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Loading exercise didn't harm cartilageEmerging evidence

In 9 trials of people at risk of, or with, knee arthritis, knee-loading exercise wasn't harmful to cartilage, though the evidence was low quality. In healthy animals, moderate daily exercise had no effect or a good effect on cartilage; only very high doses harmed it.

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Exercise eases knee arthritisStrong evidence

Across 44 trials, exercise reduced knee arthritis pain by about 12 points out of 100 and improved function by about 10. NICE (the National Institute for Health and Care Excellence) makes exercise a core treatment.

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Kneecap stress rises towards 90 degreesEmerging evidence

Stress behind the kneecap rises as you bend from straight towards 90 degrees, so shallow to medium squats suit a sore kneecap at first. Deeper than that, the tendon wraps onto the thigh bone and the contact area grows, which lowers the stress.

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Thousands of hours of kneeling or squatting at work is differentEmerging evidence

Physically demanding jobs were linked with knee arthritis across 80 studies, and the odds rose by about 26% for every 5,000 hours of kneeling or squatting at work. That's about two and a half years of full-time work spent kneeling or squatting, not a few sets a week.

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Our take

Squats are one of the best knee-strengthening exercises. Choose a depth you can control without pain, build load gradually, and squat shallower for a while if the front of your knee hurts.

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What the lab studies showEmerging evidence

In 7 weight-trained men squatting to parallel with a bar equal to their body weight, blocking the knees from passing the toes cut knee torque by about a fifth, but raised hip torque about tenfold and made the trunk lean further forward. In 30 people, restricted squats changed how the trunk moved and curved, and the authors advise against strict coaching to keep the knees back. These are small lab studies of one exercise.

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Why forward knees work the thigh harderEmerging evidence

Moving the shin forward increases the bending force at the knee and the work the quadriceps must do; leaning the trunk forward does the opposite and loads the hips and back. Raising the heels lets the shin travel further. This part reasons from mechanics rather than from measured outcomes such as injuries.

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Our take

Knees travelling past the toes is normal, and you need it for deep squats and stairs. It probably works the thigh muscles harder, which is the point when you want stronger knees. If the front of your knee is sore, limit how far it travels forward for now, then build it back.

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Weak thigh muscles predict knee arthritisEmerging evidence

In 5 cohort studies (5,707 people), people with weaker knee extensors (the quadriceps) had about 1.65 times the odds of developing knee arthritis. That's an association, but it fits the trials showing that strengthening eases knee arthritis.

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Injury-prevention warm-ups halve ACL injuriesStrong evidence

Combining 8 meta-analyses, injury-prevention programmes cut all ACL injuries by about half in athletes, and non-contact ACL injuries in women by about two-thirds. There wasn't enough data on men alone.

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Meniscal tears are common without symptomsStrong evidence

In 991 people aged 50 to 90, a torn or damaged meniscus showed on MRI in 19% of women in their 50s and up to 56% of men aged 70 to 90, and 61% of those with a tear had had no knee pain, aching or stiffness in the past month.

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Running isn't bad for kneesEmerging evidence

Recreational runners had less knee or hip arthritis than people who didn't run, in 17 observational studies. Our joints hub covers running and knees in more detail.

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Weight matters for arthritic kneesStrong evidence

For people with osteoarthritis who are living with overweight or obesity, NICE says weight loss will improve quality of life and physical function and reduce pain, and that any amount helps, though losing 10% of body weight is likely to be better than 5%.

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Strengthen your thighs and hips

Our editorial starting point: 2 or 3 days a week, with 2 to 3 sets of 8 to 12 of a squat pattern (sit-to-stands, then goblet squats), a single-leg move such as step-ups, and a hamstring move, adding load before adding sets. Doses come from each move's page and our workouts.

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Build depth and forward knee travel over weeks

Add range gradually: split squats with the front knee travelling forward, then heel-elevated squats. If the kneecap complains into the next day, go shallower or lighter next time.

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If a knee aches, use the pain rule

Our knee pain guide uses the pain-monitoring rule from knee trials: rate pain from 0 to 10 during and after exercise, aim for 0 to 3, and treat up to 5 as acceptable if the knee is back to its usual level by the next morning. If it isn't, drop back a step, not to nothing.

Warm up properly for sport

If your sport involves jumping, cutting or landing, an injury-prevention warm-up such as our knee control warm-up is the best-supported way to protect your ACL.

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Stay active

Walking, cycling and recreational running all suit healthy knees. With knee arthritis, NICE pairs strengthening with general aerobic fitness.

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Knee pain

Our knee pain guide covers when to see a doctor, how much pain is fine while you train, and a staged plan.

Around the kneecap

Runner's knee is pain around or behind the kneecap, jumper's knee is pain in the tendon just below it, and Osgood-Schlatter is a painful bump below the knee in active teenagers.

Arthritis and replacements

Exercise is the first treatment for knee arthritis, and our knee replacement guide covers recovery stage by stage.

Ligaments and menisci

ACL tears, MCL sprains and meniscus tears each have a guide covering rehab, recovery times and where surgery fits in.

The outer knee and the back of the knee

IT band syndrome causes outer knee pain in runners. A Baker's cyst is a swelling behind the knee, and a burst one can look like a blood clot.

Very flexible or knock knees

Knees that bend back past straight can be part of hypermobility, and our knock knees guide covers what's normal and what exercise can change.

Get advice from NHS 111 now

If your knee is very painful; you can't move it or put any weight on it; it's badly swollen or has changed shape; it locks, gives way or painfully clicks (painless clicking is normal); or you have a very high temperature, or feel hot, cold or shivery, and there's redness or heat around your knee, which can be a sign of infection. In the US, contact your doctor or urgent care.

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See a GP

If knee pain doesn't improve within a few weeks.

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A painful, swollen calf

Throbbing pain in one leg, usually in the calf or thigh, with swelling, red, blue or darkened skin around the painful area, or swollen veins: ask for an urgent GP appointment or get help from 111. If you also feel short of breath or have chest pain, call 999 (911 in the US). A burst Baker's cyst can look similar, so get it checked either way.

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While you exercise

Stop a move that causes sharp pain, makes the knee give way or leaves it swollen afterwards, and drop back a step if pain hasn't settled by the next morning (our editorial advice, as in our knee pain guide).

"Cracking knees mean arthritis"Emerging evidence

Crepitus is common in knees that don't hurt and isn't a diagnosis. It is more common with arthritis, especially behind the kneecap.

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"Squats are bad for your knees"Emerging evidence

Not supported for healthy knees, and squats are a standard part of knee rehabilitation.

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"Never let your knees go past your toes"Emerging evidence

In small lab studies, blocking forward knee travel shifted load to the hips and back. With a sore kneecap, limit it for a while, then build it back.

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"Deep squats wreck knees"Emerging evidence

Reviews of the biomechanics found no evidence of harm to healthy knees, though they disagree on the best depth for athletes.

Sources 8, 34

"Running wears out knees"Emerging evidence

Recreational runners had less knee and hip arthritis than people who didn't run, in observational studies.

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"A meniscus tear on an MRI explains knee pain"Strong evidence

Tears are common in people over 50 without symptoms: most people with a tear in one large study had no knee pain in the past month.

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"Rest protects arthritic knees"Strong evidence

Exercise reduces knee arthritis pain. It may hurt a little at first, which NICE says to expect.

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