Persistent knee pain almost always comes down to one of a handful of causes: cartilage wear, muscle weakness, overuse of the tendons or cushioning around the joint, or a combination of all three. About 5% of the global population has knee osteoarthritis alone, and that number climbs steeply with age, accounting for nearly 83% of all cases in people over 60. But osteoarthritis isn’t the only explanation, and your age, activity level, and even the shape of your feet all play a role in which cause is most likely.
Osteoarthritis: The Most Common Culprit
If your knees ache most of the time regardless of what you’re doing, osteoarthritis is the leading suspect. This is the gradual wearing down of the cartilage that cushions the ends of your bones inside the joint. Once enough cartilage is lost, bone grinds closer to bone, producing stiffness, swelling, and a deep, persistent ache that tends to worsen over the course of the day.
Osteoarthritis affects women roughly twice as often as men. Global data shows a prevalence of about 6% in women compared to roughly 3.8% in men. By the time you reach your early 60s, about 1 in 6 people have it. By your early 80s, it’s closer to 1 in 5. If your knee pain came on slowly over months or years rather than after a single injury, and it feels worst after long walks or at the end of the day, this is the pattern to pay attention to.
Runner’s Knee and Kneecap Tracking Problems
Patellofemoral pain syndrome, commonly called runner’s knee, is one of the most frequent causes of front-of-knee pain in younger and middle-aged adults. It happens when your kneecap doesn’t glide smoothly in the groove at the front of your thighbone. Instead, the kneecap drifts slightly to one side, creating pressure and irritation on the soft tissues underneath.
The hallmark symptoms are pain that starts gradually, gets worse with stairs, squatting, or running, and flares up after sitting with bent knees for a long time (think long car rides or movie theaters). You might also notice popping or crackling sounds when you stand up or climb steps. The pain can show up in one knee or both.
What drives poor kneecap tracking is usually a strength imbalance. Weak quadriceps (the large muscles on the front of your thigh) and weak hip muscles allow the kneecap to shift out of alignment during movement. Problems with leg alignment from the hips down to the ankles can also push the kneecap too far to one side, or cause it to ride too high in the groove. This is why runner’s knee frequently responds well to targeted strengthening rather than rest alone.
Tendon and Bursa Problems
Two other soft tissue issues cause pain that can feel constant if you’re on your feet a lot: patellar tendonitis and knee bursitis. They’re easy to confuse because their symptoms overlap, but their locations differ.
Patellar tendonitis (sometimes called jumper’s knee) involves the tendon connecting your kneecap to your shinbone. The pain is typically sharp and localized just below the kneecap, and it worsens with jumping, running, or kicking. It’s an overuse injury, common in people who’ve recently increased their activity level.
Bursitis, on the other hand, involves inflammation of the small fluid-filled sacs that cushion the outside of the knee joint. These sacs reduce friction between bones, tendons, and ligaments. When they’re inflamed, you’ll notice swelling, warmth, and tenderness that can make it painful even to rest your knee against a surface. Bursitis often develops after prolonged kneeling or a direct impact to the knee. Both conditions can produce a grating sensation when you move the joint and limit your range of motion.
How Your Weight Affects Your Knees
Your knees bear far more force than your actual body weight. Walking on flat ground puts about 1.5 times your body weight through each knee. Climbing or descending stairs increases that to two to three times your body weight. So if you weigh 180 pounds, each knee absorbs up to 540 pounds of force every time you walk downstairs.
This is why even modest weight changes make a noticeable difference. Losing 10 pounds reduces the load on each knee by 30 to 45 pounds per step. Over the course of a day, that adds up to thousands of pounds of cumulative stress removed from the joint. For people with early osteoarthritis or chronic knee pain from cartilage wear, weight management is one of the most effective long-term strategies.
How Your Feet Can Cause Knee Pain
Your knees don’t work in isolation. The way your feet hit the ground directly affects what happens at the knee. Excessive foot pronation, where the arch of your foot collapses inward too much during each step, forces your shinbone and thighbone to rotate inward more than they should. This creates a knock-kneed alignment called dynamic knee valgus, which overloads certain parts of the knee joint.
Research has linked excessive pronation to both patellofemoral pain and the progression of knee osteoarthritis. If you notice that your shoes wear down unevenly on the inside edges, or that your arches are noticeably flat, your foot mechanics could be a contributing factor. Supportive insoles can help correct this pattern in some people, though not everyone responds to them equally.
Why Quad Strength Matters So Much
The quadriceps muscles on the front of your thigh are your knee’s primary shock absorbers. When they’re strong, they absorb impact forces before those forces reach the joint itself. When they’re weak, the cartilage, tendons, and ligaments take the hit instead.
Ongoing quadriceps weakness is associated with impaired knee biomechanics, slower functional recovery after injuries, and the progression of osteoarthritis visible on imaging. This connection is so well established that quadriceps strengthening is a cornerstone of nearly every knee rehabilitation program, whether for arthritis, ligament injuries, or general chronic pain. Simple exercises like wall sits, straight leg raises, and step-ups, done consistently over weeks, can meaningfully reduce pain by restoring the muscle’s ability to manage load.
What Happens When You Get It Checked
If you decide to have your knee pain evaluated, the first step is usually a standing X-ray rather than an MRI. Weight-bearing X-rays can reveal bone degeneration, fractures, and signs of osteoarthritis. Research from Washington University found that MRIs were unnecessary in about half of knee pain patients, with a simple X-ray providing enough information to guide treatment. MRIs can cost more than 12 times as much as an X-ray, and when significant arthritis is already visible, the MRI rarely changes the treatment plan.
MRIs become more useful when the X-ray looks normal but there’s suspicion of a soft tissue problem: a torn meniscus, a ligament tear, or damage that only shows up in cartilage and tendons. If your knee locks in place, gives way unexpectedly, or swelled up after a specific incident, those are signs an MRI might add valuable information.
Signs That Need Prompt Attention
Most chronic knee pain is manageable and not dangerous, but certain patterns warrant urgent evaluation. If your knee joint looks visibly deformed or bent at an unusual angle, if you heard a popping sound at the time of an injury, if the knee can’t bear any weight at all, or if it swelled up suddenly, those are reasons to seek care quickly.
Outside of acute injuries, call your doctor if the knee is badly swollen, red, warm to the touch, or if you also have a fever. Warmth and redness together can indicate an infection inside the joint, which requires prompt treatment. And if your knee pain is disrupting your sleep or making everyday tasks difficult, that alone is a good enough reason to get it looked at, even if nothing dramatic happened to cause it.