Dr Alan Stoddard writes that:

Another effect of the rigid section of the spine is that the child tries to keep supple to be as flexible as his or her friends but, because the rigid joints will not yield, it is the joints below which give way, leading to strained and sensitive ligaments there. The commonest site of such pain is in the lower lumbar area where more often than not no bony changes are seen on the X-ray.

(Talk Back, National Back Pain Association, January 1985, p3)