Pain experienced when bending and straightening the knee (flexion and extension) is a frequent complaint encountered by medical professionals. This specific symptom points toward a mechanical problem where structures within or surrounding the joint are being compressed, stretched, or irritated during movement. The knee is a complex hinge joint designed for this exact action, meaning the pain arises when the mechanics of the joint are disrupted. To understand the cause of the discomfort, it is helpful to look at the common conditions that interfere with this smooth process.
Understanding the Joint’s Role in Flexion and Extension
The knee joint is a modified hinge functioning through two primary articulations: the tibiofemoral joint and the patellofemoral joint. The tibiofemoral joint involves the large bones of the thigh (femur) and shin (tibia), which bear the body’s weight. The patellofemoral joint is where the kneecap, or patella, interfaces with a groove on the femur, called the trochlear groove. The patella acts like a pulley, significantly increasing the leverage and efficiency of the quadriceps muscles when straightening the leg. As the knee bends, the patella must glide smoothly downward and inward within the groove, gliding upward when the leg straightens. Pain during this motion suggests the patella is not tracking correctly, or that the surfaces it glides over are damaged or inflamed.
Primary Conditions Causing Pain During Movement
Patellofemoral Pain Syndrome (PFP)
Patellofemoral Pain Syndrome (PFP), often called “runner’s knee,” is a frequent diagnosis characterized by pain around or under the kneecap. This condition is typically caused by repetitive stress or poor alignment, leading to the kneecap rubbing against the thigh bone instead of gliding smoothly within its groove. The resulting irritation stresses the underlying soft tissues and can cause a dull, aching sensation that worsens with activities like squatting or climbing stairs.
Tendinitis
Irritation of the large tendons that facilitate bending and straightening is another common cause. Patellar tendinitis, or “jumper’s knee,” causes pain in the tendon just below the kneecap where it connects to the shinbone. Quadriceps tendinitis is located just above the kneecap, where the tendon connects the thigh muscle to the patella. Both are classified as overuse injuries, resulting from repeated, high-load activities that cause micro-trauma to the tendon structure.
Early-Stage Osteoarthritis
Early-stage osteoarthritis is a third significant cause, particularly in older individuals or those with previous knee injuries. This condition involves the gradual breakdown of articular cartilage, the smooth, protective tissue covering the ends of the bones. When this cartilage wears away, the friction between the bones increases during movement, producing pain and stiffness. The pain from early osteoarthritis is often described as stiffness upon waking or after sitting for a long period, worsening with prolonged activity or repetitive movements.
Pinpointing the Problem Through Symptom Location
Pain felt directly at the front of the knee, especially behind or immediately around the kneecap, suggests Patellofemoral Pain Syndrome. This anterior pain is frequently accompanied by a grinding, crunching, or crackling sensation, medically termed crepitus, as the joint surfaces move against each other. Tendinitis pain is more localized; it can be pinpointed to the tendon just below the kneecap (patellar involvement) or just above the kneecap (quadriceps involvement).
If the pain is felt primarily in the back of the knee, causes can include a Baker’s cyst, which is a fluid-filled sac that swells and causes pressure during full bending. Hamstring tightness or strain can also produce pain with deep bending, as these muscles attach near the back of the knee and are responsible for flexion. A mechanical sensation of the knee momentarily “catching” or “locking up” during motion suggests a physical obstruction within the joint. This may indicate a torn meniscus, where a flap of cartilage gets pinched, or a loose body floating in the joint space.
Immediate Care and Necessary Medical Consultation
Initial Self-Care
For mild, gradually developing pain, initial self-care focuses on reducing inflammation and modifying activity. The R.I.C.E. protocol (Rest, Ice, Compression, and Elevation) is the standard immediate approach. Resting the knee means temporarily avoiding activities that aggravate the pain, such as deep squats, running, or stair climbing. Applying ice for 15 to 20 minutes several times a day helps reduce swelling and discomfort. Gentle, low-impact movements like swimming or cycling are usually tolerated and help maintain joint mobility.
When to Seek Medical Attention
Specific symptoms, known as red flags, require immediate professional medical evaluation. You should seek medical attention right away if you experience:
- An inability to bear weight on the leg.
- Sudden and severe swelling.
- A visible deformity of the knee.
- A loud “popping” sound at the time of injury.
- A feeling that the knee is completely locked and cannot be moved.
Even if the pain is not severe, a consultation is advisable if the discomfort persists for more than a few days despite home care or if the pain begins to interfere significantly with daily activities.