Juan 5 Disorder of Lumbar Region, section 2: Symptoms of lumbar pain, inability to bend forwards or backwards
腎主腰腳，而三陰三陽、十二經、八脈，有貫腎絡於腰脊者。
The kidneys govern the lumbar and legs. The three yin and the three yang, the twelve channels, and the eight vessels, connect to the branches of the kidneys at the lumbar vertebrae.
勞損於腎，動傷經絡，又為風冷所侵，血氣擊搏，故腰痛也。陽病者，不能俛；陰病者，不能仰，陰陽俱受邪氣者，故令腰痛而不能俛仰。
Where there is kidney damage caused by over-exertion, or injury in the channels and their branches caused by movement, while at the same time, wind and cold invade, attacking and fighting the blood and qi, this causes pain in the lumbar. Those with a yang disease cannot bend forward, while those with a yin disease, cannot raise their heads (bend backward). If both yin and yang receive deviant qi, this will cause pain in the lumbar, and an inability to bend forward or backward.
《養生方‧導引法》云︰伸兩腳，兩手指著足五指上。愈腰折不能低著，唾血、久疼愈。
The Yangsheng Recipes: The Daoyin Methods says, ’Stretch out both legs; attach your fingers to your toes, five on each foot. It will cure collapsed lumbar with the inability to bend down and spitting blood. Any long term pain will be healed.
又云︰長伸兩腳，以兩手捉足五指七通。愈折腰不能低仰也。
Again, it says, ’Stretch out both legs long; grab your toes, five on each foot, with both hands and do this seven times. It will cure collapsed lumbar with the inability to bend downward and rise upward.