Pain behind the knee when bending is known as posterior knee pain. This area, medically termed the popliteal fossa, contains muscles, tendons, nerves, and vessels that enable the knee to flex. When the knee bends, these tissues are compressed or stretched, often revealing an underlying irritation or injury. Understanding the specific structures involved helps determine the source of the pain.
Muscular and Tendon-Related Sources of Pain
The hamstring complex is a common source of pain when flexing the knee, particularly where the tendons attach near the joint. Hamstring tendinopathy (tendon inflammation) causes a sharp or pulling sensation because deep bending strains the irritated tissue. This pain is often localized to the posteromedial knee, especially during resisted flexion or activities like running.
The gastrocnemius muscle, which forms the calf, also crosses the back of the knee. A strain or tendinopathy here causes pain during knee flexion, especially when the ankle is pointed away from the body. The smaller popliteus muscle, located deep in the popliteal fossa, helps unlock the knee; its tendinopathy causes pain when the knee is bent against resistance.
Less commonly, injury to the Posterior Cruciate Ligament (PCL) can cause deep posterior pain exacerbated by knee flexion. The PCL prevents the shinbone from shifting too far backward. Inflammation from a PCL sprain can cause pain when the knee is fully bent or compressed.
The Role of Baker’s Cysts in Posterior Pain
A distinct cause of posterior knee pain is a Baker’s cyst, or popliteal cyst, a fluid-filled sac behind the knee. This cyst forms when the knee joint produces excess synovial fluid, usually due to an underlying issue like arthritis or a meniscus tear. The pain is characterized by tightness, pressure, or fullness, rather than a sharp, pulling feeling.
Deep knee bending compresses the fluid-filled sac, increasing internal pressure and causing discomfort. This mechanical compression explains why activities like squatting or prolonged sitting with the knee fully bent often aggravate the condition. The cyst is typically a symptom of a pre-existing joint problem. Addressing the root cause, such as arthritis or a meniscal tear, is necessary to prevent recurrence.
Initial Home Care and Self-Management Strategies
Initial relief should focus on reducing inflammation and avoiding aggravating movements. The R.I.C.E. principle is a starting point: rest from strenuous activity, apply ice to the back of the knee for 10 to 20 minutes, and use compression to minimize swelling. Elevating the leg above heart level also helps reduce fluid accumulation.
Activity modification requires temporarily avoiding deep knee flexion activities like squatting, lunging, or climbing stairs. Prolonged sitting with the knee bent should be avoided, as this increases pressure in the popliteal fossa. Gentle, non-weight-bearing stretches for the hamstring and calf muscles can help maintain flexibility.
Indicators for Professional Medical Evaluation
While many muscular or tendon irritations respond to home care, certain symptoms require professional medical evaluation.
When to Seek Medical Attention
- Severe, sudden pain with no apparent cause, or an inability to bear weight on the affected leg, suggesting a serious structural injury like a fracture or ligament rupture.
- Signs of infection, such as fever, accompanied by increased warmth, redness, or significant swelling around the knee.
- Numbness or tingling in the lower leg or foot, which could indicate potential nerve involvement.
- Pain that persists or worsens significantly after 48 to 72 hours of consistent home care, or swelling that rapidly increases.