It should be noted that there are two types of back pain; Complicated and Uncomplicated back pain.  If you are experiencing lower back pain or neck pain that shoots down the leg, you have complicated back pain. There are no muscles that run from the back to the lower leg so this can not be muscle pain.  There are also no ligaments that run from the back to the leg or the foot.  The only thing that runs from the back to the lower leg and feet are arteries, and the nerves.  Most likely you are having a nerve pain.  Most lower-back pain accompanied with shooting pain is due to disc damage.  When the material from inside the disc come out and puts pressure on the nerve it causes shooting pain down the leg.  In the picture below you can see the disc coming out, this is called disc bulge.  Disc herniation when the outer wall of the disc is broken.

If the damage disc or disc material puts pressure on the nerves, than you will have pain going down the leg as the nerves do.  In the picture below you can see the nervous system; from the brain to the spinal cord to the individual nerves going down the legs and arm.  So a disc putting pressure on the lower back nerves, it can cause pain down the leg.  This is sometimes called Sciatica.

Spinal Decompression Therapy may help with this condition.  It may allow pulling or slipping the discs back to its place.  This will alleviate the nerve from being impinged by the disc material.