Purpose is to measure the ability to flex the L-S spine [patients with ankylosing spondylitis have fused vertebrae that can’t flex]
Identify the “Dimples of Venus” [a bona fide medical term, despite the pictures below], which lie directly over the sacroiliac joints, & mark a dot between them at midline.
Measure 15 cms up the spine & mark another dot.
Have the patient flex the spine as much as possible. The dots should separate by at least an additional 5 cms. If not, order a sacroiliac film [not an L-S Spine series].
Of course, severe back pain due to a simple strain may preclude full flexion. In that case, measure again when they’re all better.