Whenever I see the doctor or physical therapist, they always do the same test on me: they lift my leg up off the table. I notice that sometimes this test is done while I'm lying down and sometimes when I'm sitting up. What is it they are testing? Is one of them doing it wrong?

It sounds like each examiner is conducting a straight leg raise test. It can be done in either position (sitting or lying down) but a recent study showed it is more accurate when lying down.

The test is designed to see if the cause of your pain is lumbar radiculopathy. Radiculopathy describes a condition in which one of the spinal nerves is irritated or compressed. This can result in pain we refer to as radicular pain. Radiculopathy can also cause weakness, numbness, or changes in sensation in the foot and leg.

Lifting the leg with the knee straight stretches the sciatic nerve. If there is a herniated disc pushing against the nerve, it won't be able to glide smoothly. The pressure from the disc may cut off the blood flow. It definitely increases the tension of the nerve.

The test tells the examiner that there is some structural abnormality causing the problem. Whether that is a disc, bone spur, tumor, or infection requires some additional testing.