Knee pain that occurs specifically when you bend or straighten your leg points to a mechanical issue within the joint. The knee is a complex hinge joint, relying on the precise interaction of the femur (thigh bone), tibia (shin bone), and patella (kneecap) to function smoothly. When this delicate system is disrupted by wear, injury, or alignment problems, the simple act of movement can generate pain. This discomfort is often a sign of increased pressure or friction within the joint during its motion.
The Mechanics of Knee Pain During Movement
The sensation of pain during flexion (bending) and extension (straightening) is governed by the biomechanics of the patellofemoral joint. The patella glides up and down in the trochlear groove on the end of the femur, facilitated by the articular cartilage covering the kneecap’s undersurface.
As the knee bends, the patella engages with the femur, and the load on the joint increases. This joint compression intensifies the deeper the bend, bearing forces up to six times body weight during activities like deep squats. Any irregularity in the cartilage or misalignment in the movement path increases friction under this growing load.
The movement of the kneecap is not uniform throughout the range of motion. At full extension, contact is minimal, but as the knee flexes, the patella articulates with the femoral groove. If the joint surface is damaged or the kneecap is pulled out of its intended path, concentrated pressure causes irritation and pain during both bending and straightening.
Patellofemoral Pain Syndrome and Tracking Issues
The most frequent cause of pain during knee movement, especially in active individuals, is Patellofemoral Pain Syndrome (PFPS), also called “Runner’s Knee.” This condition involves pain around or beneath the kneecap, exacerbated by activities that heavily load the joint in a flexed position. People often report a dull, aching sensation that worsens when climbing stairs, squatting, or sitting for long periods with the knees bent.
The core issue in PFPS is often improper patellar tracking (maltracking), where the kneecap does not glide centrally within the femoral groove. Instead, it may be pulled excessively to one side, frequently the outer (lateral) side, causing abnormal pressure and friction.
This misalignment is often caused by muscular imbalances in the hips and thigh, such as weakness in the hip abductors or quadriceps muscles. Weak muscles fail to stabilize the femur or control the kneecap’s movement, allowing it to track incorrectly and irritate the surrounding tissues and cartilage. Overuse or a sudden increase in physical activity can also overload the joint, leading to symptom onset.
Internal Structural Damage
Pain during movement can indicate physical damage to structures deep within the knee joint. Two significant internal causes are the wearing down of articular cartilage and tears to the menisci. These conditions cause pain because the joint surfaces can no longer move past each other without mechanical interference.
Osteoarthritis (OA)
Osteoarthritis (OA) is a progressive condition where the protective articular cartilage on the ends of the bones slowly erodes. As the cartilage thins, the cushioning effect is lost, and bones start to rub against each other, creating painful friction during bending and straightening. This bone-on-bone contact causes stiffness, often more pronounced in the morning or after periods of rest. The grinding sensation reported by some people is a physical manifestation of this uneven joint surface moving under load.
Meniscus Tears
Meniscus tears involve damage to the two C-shaped pieces of cartilage that sit between the femur and tibia, acting as primary shock absorbers and stabilizers. When a meniscus is torn, a fragment of the cartilage can become physically pinched or caught within the joint space during flexion and extension. This mechanical interference often results in a sharp, sudden pain. It may also be accompanied by distinctive symptoms like clicking, catching, or a locking sensation. A torn meniscus can restrict the ability to fully straighten the knee, as the displaced tissue blocks the normal range of motion.
When to Seek Professional Medical Advice
While many instances of knee discomfort improve with rest and temporary activity modification, certain symptoms require immediate consultation with a healthcare professional. Any sudden onset of severe pain, especially following a specific injury or trauma, requires prompt evaluation. If you are unable to put any weight on the affected leg, this suggests a serious injury, such as a fracture or a major ligament tear.
Other warning signs that indicate a structural issue or infection include:
- A visible deformity of the joint.
- A knee that feels unstable or gives way unexpectedly.
- Persistent mechanical symptoms, such as the knee locking up or catching.
- Inability to fully straighten the knee.
- Signs of infection, such as fever accompanied by warmth, redness, or severe swelling around the knee.