Aching knees are most commonly caused by wear on the cartilage inside the joint, weakness in the muscles that support the knee, or inflammation from overuse. The specific pattern of your pain, when it shows up, and where exactly you feel it can narrow down what’s going on considerably.
Where and When You Feel It Matters
Knee aching generally falls into two categories: mechanical and inflammatory. The distinction is practical because each type behaves differently throughout the day and responds to different strategies.
Mechanical knee pain gets worse with activity and toward the end of the day. You might notice it most after a long walk, a workout, or hours on your feet. It can come with occasional locking or the knee giving way. Morning stiffness is usually minimal or absent. This pattern points toward cartilage wear, alignment problems, or structural issues like a meniscus tear.
Inflammatory knee pain behaves in the opposite way. It’s worst in the morning, with stiffness that can make simple tasks like getting dressed noticeably harder. Movement actually eases the symptoms, while sitting still for a while makes the joint feel stiff and “gelled up.” This pattern is more typical of arthritis driven by the immune system, like rheumatoid arthritis, or of a flare in osteoarthritis where the joint lining becomes irritated.
The Most Common Causes
Osteoarthritis
This is the single most frequent reason for persistent knee aching in adults over 40. The cartilage that cushions the ends of your bones gradually thins, so the joint doesn’t absorb shock as well. You’ll typically feel a deep, dull ache that worsens with weight-bearing activity and improves with rest. Over time, the knee may feel stiff after sitting for a while, swell slightly after use, or creak and pop during movement. Osteoarthritis tends to develop slowly over months or years, not overnight.
Patellofemoral Pain
If the ache is concentrated around or behind your kneecap, the problem may be how the kneecap tracks against the thighbone. Weakness in the hip and thigh muscles can allow the kneecap to shift slightly out of its groove during movement. This type of pain flares up when walking stairs, squatting, kneeling, or sitting with your knees bent for long periods. It’s one of the most common knee complaints in younger and middle-aged adults, especially those who’ve recently increased their activity level.
Tendon Irritation
The thick tendon just below your kneecap can become inflamed from repetitive jumping, running, or stair climbing. This produces a localized ache at the front of the knee that builds gradually over weeks and gets worse with activities that load the tendon, like walking downhill or lunging. There’s usually no swelling of the joint itself, just tenderness right over the tendon.
Meniscus Wear or Tears
Each knee has two C-shaped pads of cartilage (menisci) that act as shock absorbers. These can tear from a twisting injury or simply wear down with age. A torn meniscus often causes aching along the inner or outer joint line, sometimes with catching, clicking, or a sensation of the knee locking in place. Mild tears can produce a low-grade ache that comes and goes for months.
Referred Pain From the Hip
Sometimes the knee itself is perfectly fine, but a problem in the hip sends pain signals down to the knee. This is worth considering if your knee exam is normal but you also have stiffness or discomfort when rotating your hip inward. It’s especially common in adolescents and older adults with hip arthritis.
Why Body Weight Has Such a Big Effect
Your knees bear a multiplied version of your body weight with every step. A study published in Arthritis & Rheumatism found that losing just one pound of body weight removes four pounds of pressure from the knee joint. That means dropping 10 pounds takes 40 pounds of compressive force off your knees with each stride. For people carrying extra weight, this is one of the most powerful levers available for reducing knee aching. Even modest weight loss can shift the trajectory of cartilage wear over time.
Strengthening the Right Muscles
The muscles around your knee and hip act as natural shock absorbers. When they’re strong, they take pressure off the joint itself. When they’re weak, the cartilage, tendons, and ligaments absorb forces they aren’t designed to handle alone. The American Academy of Orthopaedic Surgeons identifies five muscle groups that directly reduce knee stress when conditioned:
- Quadriceps (front of the thigh): the primary stabilizer of the kneecap and the muscle group most closely linked to knee pain when weak
- Hamstrings (back of the thigh): balance the pull of the quadriceps and protect against hyperextension
- Glutes (buttocks): control how your thighbone rotates, which directly affects kneecap tracking
- Hip abductors (outer thigh): prevent the knee from collapsing inward during walking and stairs
- Hip adductors (inner thigh): stabilize the leg during single-leg activities
You don’t need a gym for this. Straight-leg raises, wall sits, bridges, and clamshells done consistently a few times per week can make a noticeable difference within four to six weeks. The key is consistency, not intensity. Low-impact activities like swimming, cycling, and walking on flat ground keep the joint moving without pounding it.
What About Glucosamine and Chondroitin?
These supplements are among the most popular over-the-counter options marketed for joint health, but the evidence is underwhelming. A systematic review published in Osteoarthritis and Cartilage looked at whether adding glucosamine (alone or with chondroitin) to an exercise program improved knee pain or physical function. The results showed no significant benefit compared to exercise alone. Pain scores and function scores were essentially the same whether or not participants took the supplements. Exercise itself remains the more reliable intervention.
When Imaging Helps
If your knee aching persists for several weeks despite rest, strengthening, and activity changes, your doctor will likely start with a standing X-ray. This is the most efficient first step because it can reveal bone changes from osteoarthritis and rule out fractures. The “standing” part matters: a weight-bearing X-ray shows how the joint space looks under real-world conditions, which gives a much clearer picture of cartilage loss than lying-down images.
An MRI is generally reserved for cases where the X-ray looks relatively normal but symptoms suggest a soft-tissue problem like a meniscus tear, ligament injury, or cartilage defect that wouldn’t show on X-ray. In many cases, a less costly X-ray provides enough clinical information to guide the next step, so MRI isn’t always necessary.
Red Flags That Need Prompt Attention
Most knee aching is gradual, manageable, and responds to conservative care. But certain patterns warrant faster evaluation. A knee that becomes suddenly hot, swollen, and intensely painful without any injury could signal a joint infection (septic arthritis), which causes severe pain with even the slightest movement. A knee that locks completely and won’t straighten may have a loose fragment or a significant meniscus tear blocking the joint. And persistent knee aching in an adolescent with a normal knee exam but pain when rotating the hip could indicate a hip condition called slipped capital femoral epiphysis, where the problem is actually at the top of the thighbone rather than the knee.