What causes knee pain?

Knee pain is one of the most common reasons people see a GP. Most cases fall into a few main groups: injuries, overuse, arthritis, or structural problems that change how the knee moves.

The good news: many causes respond well to rest, physiotherapy, or targeted exercise. Knowing which category your pain fits helps your GP or physiotherapist recommend the right treatment.

You don’t need to guess at the cause yourself—your GP can narrow it down by asking about when the pain started, what makes it worse, and what you were doing when it began.

A sudden sharp pain often means an injury happened in that moment—a twist, a fall, a hard impact, or an awkward step. These injuries can damage ligaments (which hold the knee stable), cartilage (the smooth coating inside the joint), or the bone itself.

Common sudden injuries include:

• ACL or MCL tears (ligament damage from a twist or direct blow)
• Meniscus tears (cartilage damage, often with a catching or locking sensation)
• Patellar dislocation (kneecap moves out of place)
• Fractures (breaks in the bone, though less common)

If your knee swells badly, locks, or you cannot put weight on it, seek help from a GP or injury unit the same day.

Pain that builds over weeks or months usually means overuse, weak muscles, or wear-and-tear rather than a single injury. You may notice it starts when you repeat the same activity—running, cycling, or climbing stairs—without enough rest or strength in supporting muscles.

Gradual pain can also come from poor alignment (how your leg sits in the socket) or stiffness in your hip, ankle, or calf that changes how your knee moves.

Treatment at home often helps, and targeted exercises to strengthen weak muscles frequently resolve it over 6–8 weeks if you rest when needed.

Lingering pain weeks after an injury can mean the tissue is still healing, or that you have not yet regained strength and confidence in the knee. Some people favour one leg to avoid pain, which weakens the injured side and creates imbalance.

Other times, pain after injury signals that the damage was more complex than first thought—for example, a ligament tear that also damaged the cartilage underneath.

Physiotherapy speeds recovery by rebuilding strength and control. Your GP can refer you if the pain is not settling or if you want to return to sports safely.

Pain that came on without a clear moment of injury often puzzles people: “I did not fall or twist it, so why does it hurt?” The answer is usually overuse, weak muscles, or inflammation building up over time.

Other causes without injury include arthritis (joint wear), bursitis (fluid-filled sacs around the joint inflame), gout (a metabolic condition), or problems in nearby joints like your hip or ankle that change how your knee moves.

These are still serious—and still treatable. Your GP can help sort out what is happening by asking about your activities, how long the pain has lasted, and whether anything makes it better or worse.

Swelling (fluid around the joint) usually means inflammation—the body’s response to injury, overuse, or disease. Swelling can develop in minutes (after a sudden injury), hours (after overuse), or gradually (with arthritis).

When swelling arrives with redness, warmth, and feeling unwell, it may signal infection or inflammatory arthritis—seek help the same day. Otherwise, swelling usually settles with rest, ice, compression, and elevation.

Persistent swelling can come from a Baker’s cyst (fluid pocket behind the knee), ligament damage, or cartilage wear. Your GP can check what is causing it.

Pain in one knee usually comes from an injury, overuse specific to that leg, or weakness on one side (perhaps from an old injury or habit of favouring one leg).

Pain in both knees is more likely from a general cause: arthritis, age-related wear, weak quadriceps muscles, or alignment that affects both sides equally.

Sometimes pain starts in one knee and spreads to the other as you change your gait to protect the sore side, overloading the good knee. This is why physiotherapy and early treatment matter—they stop the problem spreading.

Knee pain becomes more common with age because the protective cartilage inside the joint wears down over decades—a process called osteoarthritis. You may also have weaker muscles, less flexible tendons, and slower healing.

That does not mean older people must accept pain. Gentle strengthening and stretching work at any age, and many people manage joint pain well with the right support and activity.

Some age-related pain is also caused by years of an old injury or overuse pattern finally catching up—for example, pain from a sports injury decades ago can resurface or worsen later.

Athletes and active people can hurt their knees in two ways: a single big injury (tackled hard, twisted on landing), or repetitive wear from high training load without enough rest or strength.

Running is the most common culprit—especially if you run too much, too fast, or too soon without building up gradually. Cyclists, footballers, and dancers see similar patterns.

The key to staying active is balancing your training with rest, strength work, and listening to your body when pain tells you to ease off.

Cartilage is the smooth, slippery coating inside the knee joint. When it wears away or tears, bone rubs on bone, causing pain and stiffness.

Cartilage damage can come from a single injury (a meniscus tear that damages underlying cartilage) or gradual wear over years (osteoarthritis). Unlike bone, cartilage does not heal itself, but pain can often be managed with physiotherapy, activity modification, and sometimes injections.

In rare cases, surgery helps, but most people improve without it if they work with a physiotherapist to strengthen the muscles that protect the joint.

Ligaments are tough bands that hold the knee stable. The four main ligaments are the ACL (stops the shin sliding forward), MCL (inside of the knee), LCL (outside), and PCL (back).

A twist or direct blow can tear these ligaments, causing immediate pain, swelling, and often the feeling that the knee is unstable or “giving way.” The ACL is the most commonly injured, especially in sports involving cutting or pivoting.

Many ligament injuries heal well with targeted physiotherapy and time. Surgery is only needed for severe tears or if the knee remains unstable.

Tendons are thick cords that connect muscle to bone. Around the knee, the patellar tendon (below the kneecap), quadriceps tendon (above), and hamstring tendon (back) can become inflamed (tendinitis) or tear from overuse or a sudden strain.

Tendinitis usually comes on gradually—pain with jumping, running, or straightening the leg—and responds to rest, ice, strengthening, and physiotherapy. Tears cause sudden severe pain and need urgent assessment.

Pain from tendon problems is often worse after activity, on stairs, or when squatting.

The meniscus is a C-shaped piece of cartilage that sits inside the knee and cushions the joint. A twist can tear it, often causing immediate pain and sometimes a catching or locking sensation.

You may be able to walk on a torn meniscus, but the knee often swells over hours or days. Some tears settle with rest, while others may need physiotherapy or, in rare cases, surgery to remove the damaged part.

Meniscus damage is common after sports injuries, but also happens from simple movements (like stepping awkwardly off a kerb) if the cartilage is already worn with age.

Arthritis means inflammation in the joint. The most common type around the knee is osteoarthritis (wear-and-tear of cartilage with age or overuse), but rheumatoid arthritis (an autoimmune condition) and gout also attack the knee.

Osteoarthritis develops slowly—you may notice stiffness when you first get out of bed, pain after activity, or swelling that comes and goes. Gentle strengthening exercises and weight management slow progression and ease pain.

Rheumatoid and gout come on more suddenly and usually affect other joints too. Your GP can do blood tests to tell them apart and recommend specific treatment.

Bursae are small fluid-filled sacs around the knee that cushion pressure. When they become inflamed (bursitis), the knee swells and feels tender in a specific spot—often below the kneecap, at the front, or on the inside.

Bursitis usually comes from overuse, repeated kneeling, or a direct blow. It settles well with rest, ice, and sometimes a strap to protect the area. Very rarely it needs draining if infection develops.

Gout is a metabolic condition where uric acid builds up and forms crystals in joints, causing sudden severe pain, redness, and swelling. It often strikes the big toe but can hit the knee.

Gout pain comes on fast—sometimes overnight—and the joint feels hot. If you have had gout before or it runs in your family, tell your GP. Attacks settle with treatment, and long-term management prevents future ones.

A Baker’s cyst is a fluid-filled sac that forms behind the knee. It usually develops because of wear inside the joint or a meniscus tear, and the body produces extra fluid to protect itself.

You may feel a lump or tightness behind the knee, or the cyst may be painless and you will never notice it. Most Baker’s cysts need no treatment—they settle as the underlying problem improves.

If a cyst is painful or limits your movement, your GP can discuss options like ice, compression, or (rarely) draining.

Weak or tight muscles around the knee—especially the quadriceps (front of the thigh), hamstrings (back), and glutes (buttocks)—can pull the kneecap out of its natural track or force the joint to move in strained ways.

Overuse happens when you do too much of one activity without enough rest or strength. Strengthening and stretching exercises are the main treatment and usually work well if you stick with them for 4–6 weeks.

Sometimes knee pain comes from problems not directly inside the joint: hip tightness or weakness that changes your gait, flat feet or high arches that affect how forces travel through your leg, or referred pain from your lower back.

Rarely, knee pain signals infection, a tumour, or bone disease, but these are uncommon and usually come with other red flags (fever, night sweats, unexplained weight loss, or pain that is not linked to activity).

Your GP will ask about your full history and may order imaging if the cause is unclear.

Your GP will ask: When did it start? What were you doing? Is it constant or does movement trigger it? Is there swelling, clicking, or locking? Have you had an injury?

They will examine your knee—checking how it moves, whether it is swollen or tender, and whether you can bear weight. Most of the time, this history and examination pinpoint the cause.

If diagnosis is unclear, your GP may arrange an X-ray (to check the bones and joint space), ultrasound (to see soft tissues like ligaments and tendons), or MRI (to get the most detailed picture). Blood tests help if arthritis is suspected.