It happens so quickly.  It happens when you are doing the most pedestrian movement or simply reaching to pick up a pencil.  But when it happens - 'oh lord have mercy'!  A spasm in the back can be earth shattering and leave you sobbing on the couch as you are stuck, immobilised, face down with your derrière stuck up 'in the air', clutching at your mobile to call someone to come to your aid.