Testing for Herniated Discs: Straight Leg Raise

Question: I just read an article about the straight leg raise test, but I still don't get it. How does raising my leg help the doctor know if I have a herniated disc? Could you please explain that?
—Walpole, NH
Female therapist raising patient's legAnswer: Yes, your doctor may use a straight leg raise test to determine whether your low back and/or leg pain is due to a lumbar herniated disc.

The test is easy to perform. In most cases, you will lie down on a table (sitting is a less common variation), and your doctor will lift your straightened leg into the air.

If you feel pain that travels down your leg when it's lifted to the 30° to 70° range, then the straight leg raise test is considered positive. That pain should replicate what you would describe as your typical leg pain. The radiating leg pain you feel is called sciatica, among the most common and painful symptoms of a lumbar herniated disc.

A straight leg raise test is used to help diagnose a lumbar herniated disc because the simple act of raising your leg stretches your spinal nerve root; doctors call this stretching excursion of the nerve. If you have a lumbar herniated disc, it should press on the stretched nerve root as your leg is raised above 30°.

If your doctor does a straight leg raise test and you have pain before your leg is at 30°, then it probably isn't a herniated disc pressing on the nerve. Before 30°, the nerve root isn't stretched, and it's the nerve root stretching in the straight leg raise test that brings the nerve closer to the herniated disc (if you have one). Therefore, pain before 30° means that there is something else besides a herniated disc pressing on your nerve.

A physical exam, which may include a straight leg raise test, is usually quite simple and is an important part of figuring out what is causing your back and leg pain.