BL-40, Wei Zhong (Middle of the Crook), is located in the center of the back of the knee. It’s the Ruler of the Lower Back, so I often use it to treat lumbar problems. It’s also great for deep knee pain and to cool the Blood* – try it if you’re overheated next summer, or if you’re suffering from hot flashes. Other good points for lower back pain: LV-3, GB-34, Du-4.
*Remember: The Chinese concept of “Blood” is different from your physical blood. I am not talking about your literal circulatory system. :)