Why does a curved spine cause back ache?

Most do not as they : Are usually not large enough in size to matter. Usually there is some other source of the pain although curves in the lower back can affect the spinal mechanics accelerating degenerative changes which can lead to back pain. A curve has to large enough otherwise to affect one's center of gravity causing the muscles to work harder at keeping you balanced leading to muscular pain.

It varies: In my experience, for two reasons. First, a spinal deformity can cause a muscular pain as the muscles try to hold the spine in a good position but battle a mechanical disadvantage. Second, the spine is loaded in an asymmetric fashion leading to early failure fo the discs leading to premature degeneration, herniations, and arthritis, all of which can certainly cause pain. Thank you.