Your knees are a complex hinge joint with many interconnected parts that allow your knee to bend, straighten, rotate, and bear weight. So, when your knee hurts to bend, it's important to understand what's causing the pain. Pain in the back of your knee when you bend may feel dull and achy or cause a twisting, burning, or pins-and-needles sensation. You might also notice that the pain gets worse with certain activities like sitting for prolonged periods of time, kneeling on the knee, or walking upstairs.
Knee arthritis is the most common cause of pain when bending your knee. Arthritis is a condition that occurs when the cartilage that protects your knee joint wears down, usually due to overuse, injury, or just age-related wear and tear. Depending on your type of arthritis, it can make your knee feel achy or sharp, and can result in swelling, stiffness, and difficulty bearing weight on your knee.
If you have a pain that starts behind your knee and becomes aggravated by activity, it's possible that you have a Baker's cyst, or a popliteal cyst. This is when excess fluid in the knee joint seeps out into the bursa at the back of your knee and causes it to swell. You'll experience pain and tightness in the front of your knee, below and around your kneecap, and you might hear a grinding or clicking sound when bending your knee. Occasionally the doctor will drain this fluid to reduce the symptoms and help your knee move more freely.