Pain in the back of the knee when straightening the leg often happens when structures behind the knee joint become irritated or injured. This area contains ligaments, tendons, muscles, and fluid-filled sacs that help the knee move smoothly. When one of these structures is strained or inflamed, discomfort can appear as the leg approaches a fully straight position.
Several conditions can lead to this type of pain. In some cases, a ligament injury inside the knee can cause pain with movement or when putting weight on the leg. Inflammation of nearby tendons or muscles may also cause tightness and discomfort during extension. Another possible cause is a fluid-filled swelling behind the knee, sometimes called a Baker’s cyst, which can create pressure when the knee straightens.
The symptoms may vary depending on the cause. Some people notice stiffness, swelling, or a feeling of tightness behind the knee. Pain may appear after physical activity, repeated bending, or an injury. In some situations, the knee may feel weak or difficult to fully straighten.
Resting the knee and avoiding movements that worsen the pain can sometimes help reduce irritation. However, persistent pain, swelling, or difficulty moving the knee should be evaluated by a healthcare professional. An orthopedic specialist can examine the joint and determine whether imaging or treatment is needed.