Numbness is very common in Qigong still meditation. When you sit for a long time, your circulation slows down, reducing the blood supply to your legs. This is very common with beginners. You should not continue your meditation once concentration is affected. Stretch your legs and massage the bottom of your feet, especially the Yongquan in the center of the sole of the feet, to speed the recovery of circulation. If you meditate regularly, you will find you can sit longer and longer without your legs becoming numb. Your body adjusts the blood supply to fit the new situation. After six months of regular practice, you should be able to sit at least thirty minutes without any problem. If your lower back feels stiff, adjust your posture by sitting on a higher cushion or a chair with back support.