The nerves that carry messages to and from your legs come from your low back. By checking your muscle strength, your deep tendon reflexes, and your sensation (feeling), your doctor can tell whether there is pressure on a nerve root coming from your spinal column. He or she can also tell which nerve root is involved.

Muscle strength

Muscle strength tests can detect true muscle weakness, which is one sign of pressure on a nerve root. (Sometimes leg weakness is actually due to pain, not pressure on a nerve.) Most people who have herniated discs that cause symptoms also have some nerve root compression.

Specific muscles receive impulses from specific nerves, so finding out which muscles are weak shows your doctor where nerve roots are being compressed.

See a picture of the lumbosacral region , from which nerve root compression usually originates.

Muscle strength tests include:

• Hip flexion. You sit on the edge of the exam table with your knees bent and feet hanging down. Then you lift your thigh up off the table while your doctor pushes down on your leg near your knee. (This test can also be done while you are lying on your back.) If your painful leg is weaker than the other leg, you may have nerve root compression at the higher part of your low back, in the area of the last thoracic and the first, second, and third lumbar vertebrae (T12, L1, L2, L3 region).
• Knee extension. While in the sitting position, you straighten out your knee while your doctor pushes down on your leg near your ankle. If your painful leg is weaker than the other leg, you may have nerve root compression at the second, third, or fourth lumbar vertebrae (L2, L3, or L4 region).
• Ankle dorsiflexion. While you are in the sitting position, your doctor pushes down on your feet while you try to pull your ankles upward. If there is weakness in one leg, the ankle will give way to the downward pressure. This is a sign of possible nerve root compression at the level of the fifth lumbar vertebra (L4 or L5 region).
• Great toe extension. While you are in the sitting position, your doctor pushes down on your big toes while you try to extend them (bend them back toward you). If there is weakness in one leg, its big toe will give way to the pressure. This is a sign of possible nerve root compression at the level of the fifth lumbar vertebra (L5 region).
• Plantar flexion power. You stand and rise up on your toes on both feet and then on each foot separately. Toe raises are difficult, if not impossible, to do if a particular nerve region is compressed. This is a sign of possible nerve root compression at the level of the first sacral vertebra (S1 region).