Thanks, Old Man. Dear Old Man, The type and location of the pain you are describing makes me think of a herniated disc that is irritating one of the nerve roots in your back. While many people have low back pain due to a variety of different reasons, pain that shoots down the leg, especially past the knee, is a sign that more is going on. Due to arthritis, fractures, herniated discs, and other reasons, the nerves coming off the spine that travel to the legs get pinched. This causes pain, usually sharp, that travels down the leg.
Some people experience weakness in the legs as well.
If the pain is due to a herniated disc, it often gets worse with coughing, bending forward, and sitting for long periods of time. If the pain down the legs just began recently, and you are having no trouble urinating or having bowel movements, the main treatment is anti-inflammatories, physical therapy, manipulation and exercises to strengthen the back. This will show if you have pinched nerves that could be causing your symptoms and allows the surgeon to know where to operate, if needed. For most people with low back pain, surgery is not indicated and may make the pain worse. MRI's and X-rays are also rarely needed except in certain circumstances.