Knee pain comes from a surprisingly wide range of sources: worn-down cartilage, torn ligaments, inflamed tendons, overuse, or even problems in your hip or lower back that send pain signals to the knee. The knee is the largest joint in your body and one of the most vulnerable, bearing forces of up to several times your body weight with every step. Understanding where the pain originates is the first step toward figuring out what to do about it.
How the Knee Is Built
The knee joint connects three bones: the femur (thighbone), the tibia (shinbone), and the patella (kneecap). Between the femur and tibia sit two wedges of tough cartilage called the menisci, which act as shock absorbers. A smoother type of cartilage lines the ends of each bone so they glide against each other with minimal friction.
Four major ligaments hold the joint together. The medial collateral ligament (MCL) runs along the inner side, and the lateral collateral ligament (LCL) runs along the outer side. Inside the joint, the anterior cruciate ligament (ACL) sits in front and the posterior cruciate ligament (PCL) sits behind it. These ligaments prevent the knee from shifting or rotating too far in any direction. Surrounding all of this are the quadriceps and hamstring muscles, tendons connecting those muscles to bone, fluid-filled sacs called bursae that reduce friction, and a network of nerves that can amplify or refer pain from other areas.
Damage to any one of these structures produces a different type and location of pain, which is why “knee pain” can mean very different things depending on the cause.
Osteoarthritis: The Most Common Chronic Cause
Osteoarthritis is by far the leading cause of long-term knee pain. By age 65, roughly 80% of people in the U.S. show some radiographic evidence of the condition. It develops as the cartilage cushioning the joint gradually wears away, leaving bone surfaces closer together and eventually rubbing against each other.
The hallmark symptom is pain that worsens with activity and improves with rest. Morning stiffness is common and can last an hour or more, with similar stiffness returning after any prolonged period of sitting. As the condition progresses, you may notice swelling, a grinding or crunching sensation when bending the knee, and a gradual loss of range of motion. On imaging, the joint space between bones narrows (especially on the inner side of the knee), and small bony growths called osteophytes develop around the edges of the joint.
Osteoarthritis tends to come on slowly over months or years. Risk increases with age, excess body weight, previous knee injuries, and repetitive occupational stress on the joint.
Ligament Injuries
Ligament injuries are a major source of sudden, acute knee pain, particularly in athletes. An ACL tear typically happens during a quick pivot, sudden stop, or awkward landing. People often hear or feel a pop at the moment of injury, followed rapidly by pain, swelling, and a feeling of weakness or instability in the knee. Bearing weight becomes difficult immediately.
MCL injuries, by contrast, usually result from a direct blow to the outer side of the knee that forces it inward. Pain concentrates along the inner edge of the joint rather than deep inside it.
Ligament sprains are graded by severity. A Grade 1 sprain involves minimal tearing and generally heals within a few weeks. A Grade 2 sprain means the ligament is partially torn, and recovery takes longer, sometimes several weeks to a couple of months. A Grade 3 sprain is a complete tear that may require surgical reconstruction, particularly for the ACL in active individuals. In a clinical exam, the Lachman test (where a doctor checks for abnormal forward movement of the shinbone) detects ACL tears with about 84% sensitivity and 92% specificity, making it one of the most reliable bedside tests for knee injuries.
Meniscus Tears
The meniscus can tear from a twisting motion, a deep squat, or simply from age-related wear. Meniscus tears share some symptoms with ligament injuries, like pain and difficulty bearing weight, but they produce one distinctive sensation: locking. Your knee may feel stuck, unable to fully straighten, as a torn flap of cartilage catches between the joint surfaces. You might also feel the knee “giving way” under you.
Unlike an ACL tear, which causes immediate instability and rapid swelling, a meniscus tear may swell more gradually and feel stiff rather than loose. The McMurray test, where a doctor rotates and extends your knee while feeling for a click, picks up tears in the inner meniscus about 88% of the time, though it’s less reliable for tears on the outer side (around 65% sensitivity).
Patellofemoral Pain (Runner’s Knee)
Patellofemoral pain syndrome is one of the most common causes of pain at the front of the knee, especially in runners, cyclists, and people who sit for long periods. The pain centers around or behind the kneecap and typically gets worse when climbing stairs, squatting, or sitting with bent knees for a long time.
The root cause is usually biomechanical. Your kneecap sits in a groove on the front of the thighbone, and the quadriceps muscles work together to keep it tracking smoothly through that groove as you bend and straighten. When the quads are weak or imbalanced, or when the hip muscles that control thigh rotation and stability aren’t doing their job, the kneecap drifts slightly off track. That poor tracking creates uneven pressure on the cartilage behind the kneecap, leading to pain and sometimes a grinding sensation. Strengthening both the quadriceps and the hip abductors is the primary treatment approach.
Tendonitis and Bursitis
Patellar tendonitis, often called jumper’s knee, causes pain right at the bottom of the kneecap where the patellar tendon connects the kneecap to the shinbone. It’s an overuse injury common in sports that involve repeated jumping or explosive leg movements. The pain tends to start as a dull ache after activity and, if ignored, progresses to pain during activity that can eventually interfere with daily tasks like climbing stairs.
Bursitis involves inflammation of the small fluid-filled sacs that cushion the knee. Prepatellar bursitis, the most common type, causes visible swelling directly over the front of the kneecap. It’s sometimes called “housemaid’s knee” because prolonged kneeling is a classic trigger. The swelling can be dramatic, making the area warm and tender to the touch, but the joint itself usually moves normally since the inflammation is outside the joint capsule.
IT Band Syndrome
The iliotibial (IT) band is a thick strip of tissue running from the outside of your hip down to the outer side of your knee. In runners, cyclists, and hikers, repetitive bending and straightening of the knee causes this band to slide back and forth across a bony bump on the outer thighbone. That friction creates inflammation and a sharp or burning pain on the outside of the knee. The pain is most intense at around 30 degrees of knee flexion, which is roughly the angle your knee passes through with every running stride. This is why the pain often kicks in at a predictable point during a run and disappears at rest, only to return at the same distance next time.
Infections and Inflammatory Conditions
Not all knee pain comes from mechanical problems. Septic arthritis is a joint infection that causes severe pain with rapid onset, along with swelling, warmth, and often fever. The pain is intense enough that using the joint becomes nearly impossible. This is a medical emergency because untreated infection can destroy joint cartilage within days. People with existing joint conditions like osteoarthritis, rheumatoid arthritis, or gout face higher risk.
Gout itself can strike the knee, causing sudden, intense pain and swelling when uric acid crystals accumulate in the joint. These flares often come on overnight and peak within 12 to 24 hours. The joint may look red and feel hot to the touch.
Pain That Starts Somewhere Else
Sometimes the knee isn’t actually the problem. Conditions in the lower back, like a herniated disc compressing nerves at the L3 or L4 level, can send pain radiating down the leg into the knee. Hip joint dysfunction, including arthritis or misalignment, can shift mechanical stress onto the knee or produce referred pain that the brain interprets as coming from the knee itself. This is one reason knee pain that doesn’t respond to typical treatments, or that comes with no visible swelling or structural damage, deserves a broader evaluation that includes the hip and spine.
Weight, Activity, and Other Risk Factors
Excess body weight is one of the strongest modifiable risk factors for knee pain across nearly all causes. Every extra pound of body weight adds roughly three to four pounds of force across the knee joint during walking. Over years, that additional load accelerates cartilage breakdown and increases strain on ligaments and tendons.
Sudden changes in training volume are another common trigger. Ramping up running mileage, starting a new sport, or returning to exercise after a long break can overwhelm tissues that haven’t had time to adapt. Muscle weakness, particularly in the quadriceps and hip stabilizers, leaves the knee relying more heavily on passive structures like ligaments and cartilage that aren’t designed to absorb those forces alone. Occupations that require frequent kneeling, squatting, or heavy lifting also raise the risk of both acute injuries and chronic wear.
Alignment issues in the legs and feet can play a role too. Flat feet, knock knees, or uneven leg length can shift stress unevenly across the joint, making certain structures work harder than they should over thousands of repetitions each day.