Nerve Function Tests for Evaluating Low Back Problems

Overview

Nerves carry messages back and forth from your low back and legs. By testing how well your muscles, reflexes, and sense of feeling are working, your doctor can tell whether there is pressure on a nerve root in your spine. Your doctor often can also tell which nerve root is involved.

Muscle strength tests

Muscle strength tests can detect muscle weakness, which is one sign of pressure on a nerve root (nerve root compression). Specific muscles receive messages from specific nerves, so finding out which muscles are weak shows your doctor where there is pressure on a nerve root.

The vertebrae of the spine are divided into regions: cervical (C), thoracic (T), lumbar (L), sacrum (S), and coccyx. Within each region, each vertebra is numbered. For example, the cervical vertebrae are C1 through C7. Doctors use these numbers to talk about the results of your muscle strength tests. 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 pressure on a nerve root at the higher part of your low back, in the area of the last thoracic and 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 pressure on a nerve root 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 point your feet upward. If there is weakness in one leg, the ankle will give way to the downward pressure. This is a sign of possible pressure on a nerve root at the level of the fourth or 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 pressure on a nerve root 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 pressure on a nerve root at the level of the first sacral vertebra (S1 region).

Sensory testing

Testing your sense of feeling helps your doctor find out what nerve root may have pressure on it. Testing may include touching your skin lightly with a cotton ball or pricking it lightly with a pin. Because it's easier to imagine the feeling if you can see the test being done, close your eyes during the testing.

The vertebrae of the spine are divided into regions: cervical (C), thoracic (T), lumbar (L), sacrum (S), and coccyx. Within each region, each vertebra is numbered (for example, C1 through C7).

Below is a list of areas that send messages through specific nerves:

• The front of your thighs sends messages through nerve levels L1, L2, L3 and L4.
• The inside of your lower leg (your knee to the inner ankle and arch) sends messages through L4.
• The top of your foot and toes send messages through L5.
• The outside of your ankle and foot sends messages through S1.

Reflex testing

Tendons attach your muscles to your bones. Reflexes are little movements of the muscle when the tendon is tapped. A reflex can be decreased or absent if there is a problem with the nerve supply. To test your reflexes, your doctor will use a rubber hammer to tap firmly on the tendon. If certain reflexes are decreased or absent, it can show your doctor that there is pressure on a nerve root. Not all nerve roots have a reflex associated with them.

The vertebrae of the spine are divided into regions: cervical (C), thoracic (T), lumbar (L), sacrum (S), and coccyx. Within each region, each vertebra is numbered. For example, the cervical vertebrae are C1 through C7. Doctors use these numbers to talk about the results of your reflex tests.

There are two common tests.

Patellar tendon reflex.

You sit on the exam table with your knee bent and your foot hanging down, not touching the floor. Your doctor will use a rubber hammer to tap firmly on the tendon just below your kneecap. In a normal test, your knee will extend and lift your foot a little. A decreased or absent reflex may mean that there is compression in the L2, L3, or L4 region.

Achilles tendon reflex.

You sit on a table with your knees bent and feet hanging down, or you may be asked to lie down on your stomach with your legs straight and your feet off the edge of the exam table. Your doctor will use a rubber hammer to tap firmly on the Achilles tendon, which connects the muscle at the back of your calf to your heel bone. In a normal test, your foot will move as though you were going to point your toes. A decreased or absent reflex may mean that there is compression in the S1 region.