Backache. You should be alerted if for several months or years you feel an obvious stiffness in the lower back and sacrum, and a strong limitation of mobility in the lumbar spine or the entire spine. This restriction of the mobility of the spine may also be a sign of psoriatic arthritis. The patient’s ability to turn on both sides, to the right or to the left, and to bend forward or back. The ill person usually turns or bends over the whole body, with a “straight back.”