Knees hurt for dozens of reasons, but most cases trace back to a handful of common culprits: wear and tear on cartilage, overuse from repetitive activity, acute injuries to soft tissue, inflammation, or simply carrying more body weight than the joint was designed to handle. Nearly 375 million people worldwide have knee osteoarthritis alone, and the number continues to climb. Understanding the specific cause behind your knee pain is the first step toward knowing what to do about it.
How the Knee Joint Works
The knee is essentially a hinge. It connects the thigh bone to the shin bone and opens and closes in one direction, much like a door hinge. That simple design supports an enormous workload. Your knees bear your weight every time you stand, walk, run, or jump. They stabilize your balance and absorb shock with every step.
Inside the joint, cartilage (including two C-shaped pads called menisci) cushions the bones so they don’t grind against each other. Four major ligaments hold the bones together and prevent the joint from moving in directions it shouldn’t. Tendons connect the surrounding muscles to bone, giving the knee its power. Fluid-filled sacs called bursae reduce friction between moving parts. When any one of these structures is damaged, inflamed, or worn down, the result is pain.
Osteoarthritis: The Most Common Cause
Osteoarthritis is the single most frequent reason for chronic knee pain, especially after age 50. It happens when the cartilage that cushions the joint breaks down faster than the body can repair it. In the early stages, cells in the cartilage try to keep up with the damage by producing more of the structural proteins that give cartilage its elasticity. Eventually, that repair process falls behind. The cartilage cracks, thins, and erodes, leaving less cushion between bones.
The pain typically comes on gradually and worsens with activity. Stiffness after sitting or resting for a while is common, and many people notice swelling that fluctuates from day to day. Over months or years, the discomfort tends to get progressively worse rather than better. On X-rays, the hallmarks are a narrowing of the space between bones, small bone spurs forming along the joint edges, and hardening of the bone just beneath the cartilage surface.
Overuse and Runner’s Knee
Patellofemoral pain syndrome, often called runner’s knee, is one of the most common causes of knee pain in teenagers and physically active adults. It produces an aching pain at the front of the knee, around or behind the kneecap. Activities that load the front of the joint trigger it: jogging, cycling in high gears, climbing stairs, squatting, or kneeling for long periods at work.
The underlying issue is repeated small-scale stress on the tissues around the kneecap, including the bands that hold it in place, the bone surfaces, and the tiny nerves nearby. Several factors raise the risk. Weak thigh or hip muscles can let the kneecap drift slightly out of its groove during movement. A leg-length difference, flat feet, knock knees, or unusually tight muscles around the hip and thigh all contribute. People who have recently started running, train competitively, or cover long distances are especially prone.
Meniscus Tears and Ligament Injuries
The menisci and ligaments are the knee’s internal shock absorbers and stabilizers, and tearing either one produces distinct symptoms.
Meniscus Tears
A torn meniscus usually causes pain on the sides or back of the knee, particularly when twisting or squatting. Many people can still walk on it initially, but over two to three days the knee gradually stiffens and swells. A hallmark sensation is the knee feeling “locked,” as if it can’t fully straighten. You may also feel like the knee could buckle if you put your full weight on it. Meniscus tears are especially common in men over 40, when the cartilage has become less resilient.
ACL Tears
A torn ACL (the ligament that prevents the shin bone from sliding forward) feels very different. People often describe hearing or feeling a distinct pop at the moment of injury, followed by immediate deep pain inside the knee and rapid swelling within hours. Weight-bearing becomes difficult right away, and the knee feels unstable or weak. Unlike a meniscus tear, the knee doesn’t typically lock, but it may feel like it could give out at any moment. ACL injuries are common in sports that involve sudden stops, pivots, or direction changes.
Inflammation: Bursitis and Gout
Not all knee pain comes from structural damage. Inflammation in the soft tissues or within the joint itself can be just as painful.
Bursitis happens when the small fluid-filled sacs that cushion the knee become irritated, usually from prolonged kneeling or repetitive pressure. The front of the knee swells and feels tender, and the pain worsens when you bend the joint or press on it directly. Gout, a condition caused by uric acid crystal buildup, can also strike the knee. Gout attacks often begin at night and escalate quickly. The joint becomes intensely painful, warm, swollen, and sometimes red or purplish. Even the light pressure of a bedsheet can be unbearable. Gout in the knee is more common in adults over 60 and can itself trigger bursitis in the joint.
Why Body Weight Matters So Much
The knee amplifies every pound you carry. Walking on flat ground puts a force equal to one and a half times your body weight on each knee. Going up or down stairs raises that to two to three times your body weight. Squatting to pick something up off the floor loads each knee with four to five times your body weight. For someone who weighs 200 pounds, that means each knee absorbs up to 1,000 pounds of force during a simple squat.
This is why even modest weight loss can make a meaningful difference. Losing 10 pounds removes roughly 15 pounds of force from your knees with every step, and up to 50 pounds of force during stair climbing. For people with early osteoarthritis, this reduction in joint stress can slow cartilage breakdown and significantly reduce daily pain.
How Knee Pain Changes With Age
The most likely cause of knee pain shifts as you age, which is useful to know when trying to figure out what’s going on.
- Adolescents (10 to 18): Growth-related conditions dominate. Osgood-Schlatter disease causes pain and a bony bump just below the kneecap during growth spurts. Patellar instability, where the kneecap slips out of its groove, is also common in this group.
- Active adults (20s to 40s): Overuse injuries take over. Runner’s knee, iliotibial band syndrome (pain on the outer side of the knee in runners and cyclists), and tendon problems from jumping sports are the most frequent diagnoses. Meniscus tears become increasingly common after 40.
- Older adults (50+): Osteoarthritis becomes the leading cause. Gout and pseudogout also become more likely after age 60, often striking joints already affected by arthritis.
When Knee Pain Signals Something Serious
Most knee pain is not an emergency, but certain patterns warrant prompt attention. A knee that swells rapidly after a trauma, especially with a popping sensation, suggests a possible ligament tear or fracture. Inability to bear weight, inability to bend the knee to 90 degrees, or tenderness directly over the kneecap or the bony bump on the outside of the lower leg are all criteria doctors use to decide whether an X-ray is needed after an injury.
Pain and swelling in the calf along with warmth or discoloration could indicate a blood clot rather than a joint problem. Cramping pain in the lower leg that comes on with walking and goes away with rest may point to reduced blood flow from arterial disease. Both of these deserve medical evaluation, as they involve the vascular system rather than the joint itself.
What Happens During Diagnosis
For chronic knee pain, the starting point is almost always an X-ray taken while you’re standing. Weight-bearing images give an accurate picture of how much space remains between the bones, something that can look falsely normal on an X-ray taken while lying down. If the X-ray looks normal but pain persists, or if it shows fluid in the joint, an MRI is typically the next step. MRI is better at revealing soft tissue problems like meniscus tears, ligament damage, and early cartilage changes that don’t yet show up on X-rays.
When an X-ray already shows clear signs of osteoarthritis, an MRI usually isn’t needed unless the symptoms don’t match what the X-ray shows, or a doctor suspects an additional problem like a stress fracture hiding behind the arthritis.