Pain behind the knee when bending can be a confusing symptom because the area, known as the popliteal fossa, contains many different structures that can be the source of discomfort. This diamond-shaped region at the back of the knee is a tightly packed space housing important muscles, tendons, nerves, and blood vessels. Because the knee joint relies on a complex interplay of these structures for movement, pain during the action of bending the knee, or flexion, can signal an issue ranging from simple soft tissue strain to a more intricate joint problem.
Causes Related to Muscles and Tendons
Muscles and their connecting tendons are frequent sources of pain behind the knee. The hamstring muscles, which run down the back of the thigh, attach near the knee and are placed under tension during flexion. Proximal hamstring tendinopathy or strain, while often causing pain higher up in the gluteal fold, can also refer pain to the back of the knee, particularly under load. This type of injury often stems from overuse, such as in running or hill climbing.
Another common muscular source is the gastrocnemius, the large calf muscle, which has two heads that originate just above the knee joint. Since this muscle crosses both the knee and ankle joints, a strain in its upper portion can cause pain that localizes directly behind the knee. This pain is especially noticeable when the muscle is contracted or stretched, and bending the knee can stretch this strained tissue, leading to sharp or aching pain.
The popliteus is a triangular muscle deep within the popliteal fossa that plays a role in “unlocking” the knee from a fully straight position. Issues with the popliteus muscle or its tendon (popliteus tendinopathy) can cause pain on the back and outer side of the knee. This pain is typically worse when bending the knee against resistance, walking downhill, or running, as the muscle is compressed or strained during these movements. The popliteus also helps pull the meniscus backward during bending to prevent it from getting pinched.
Structural and Fluid-Filled Causes
Pain upon bending can also originate from structures inside the knee joint or from fluid accumulation that is compressed during flexion. The most common fluid-filled cause is a Baker’s cyst, which is a swelling that forms when excess synovial fluid builds up. This cyst often results from an underlying knee problem, such as arthritis or a cartilage tear, which causes the joint to overproduce fluid. As the knee bends, the fluid-filled sac is compressed in the tight space behind the joint, leading to tightness, stiffness, or pain.
A tear in the posterior horn of the meniscus can also cause pain upon deep knee bending or squatting. The posterior horn absorbs significant force and can be torn due to trauma or degenerative changes. Forcing the knee into deep flexion can cause the torn portion of the cartilage to be pinched between the femur and tibia, leading to sharp pain, and sometimes a catching or locking sensation.
An injury to the Posterior Cruciate Ligament (PCL) can also result in pain deep within or behind the knee, especially when bending or squatting. PCL injuries typically occur from high-energy trauma, like a direct blow to the front of a bent knee. The damage and subsequent inflammation in the ligament, which is stressed when the knee is flexed, causes pain and can lead to a feeling of instability in the knee.
Immediate Steps for Pain Relief
For new or minor pain behind the knee, the R.I.C.E. protocol (Rest, Ice, Compression, and Elevation) is recommended. Resting the knee involves avoiding activities that aggravate the pain, such as deep squatting, kneeling, or excessive walking, to prevent further strain on the injured area.
Applying a cold pack to the painful area for 15 to 20 minutes every two to three hours helps to constrict blood vessels, which reduces swelling and provides temporary pain relief. Compression, using an elastic bandage, can help control swelling, but it must be applied snugly without causing numbness or increased pain. Elevating the leg above the level of the heart encourages fluid drainage and minimizes the buildup of swelling.
Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) can also be used to reduce pain and inflammation. Medications like ibuprofen or naproxen sodium are effective for managing discomfort associated with muscular strains or tendinopathy. Always consult with a pharmacist or doctor before beginning any medication regimen.
Identifying Serious Symptoms and Next Steps
Certain symptoms accompanying pain behind the knee require immediate medical evaluation to rule out potentially serious conditions. Signs of a Deep Vein Thrombosis (DVT) include severe swelling in one leg, warmth to the touch, and noticeable redness or discoloration of the skin below the knee.
Prompt professional attention is needed if you experience:
- Inability to bear weight on the leg.
- A sudden “pop” or tearing sensation followed by a feeling of joint instability.
- Signs of infection such as a fever or chills.
- Significant, rapid swelling that occurs right after an injury.
For persistent pain that limits daily activities, professional consultation is necessary. A physical therapist can help address muscle imbalances and movement patterns contributing to the pain. If a structural injury is suspected, an orthopedic specialist may be needed for advanced treatment planning.