Straight Leg-Raising (SLR):  Most lumbar disc protrusions compress one of the spinal nerves that make up the sciatic nerve, which runs down the back of the leg.  By raising the leg straight, the sciatic nerve is stretched and moves downward a bit. If it’s impinged by a disc protrusion pain will increase down the leg.  The amount of SLR tolerable is proportional to the amount of impingement of the nerve root.

Crossed Straight Leg-Raising:  In some cases, SLR of the opposite (non-painful) leg causes increased pain in the painful leg, due to a location of the disc protrusion medial to the nerve root.  This sign is diagnostic of a disc protrusion.

Bent Leg-Raising;  This is a check for a false-positive SLR because with the knee bent, no stretch is placed on the sciatic nerve.  This should not cause increased leg pain in the presence of a disk protrusion.