Why we use standing x-rays for spinal analysis

The spinal column is a dynamic organ and the relationship between segments adjusts according to the load placed upon it by a combination of our posture and gravity. The two images shown below are of the same person. The one on the left shows the person lying down. The stress of gravity is less and the spine appears fairly normal. When standing, the spine must accommodate to a greater load and the instability becomes obvious.

Recumbent Image Standing Image
The lumbar curve appears fairly normal. You can see the problem more easily.
Note how L4 has moved backward w.r.t. L3.

-In the images above, the person is facing to the left.
-Recumbent means lying down.