Sprung back
Approximation of the spinous processes (‘kissing spines’) resulting in the development of a bursa between them has been indicated as a cause of low back pain.  ‘Sprung back’ is a term used to describe rupture of the supraspinous ligament after a sudden flexion strain applied to the spine with the pelvis fixed, as in falling on the buttocks with the legs out straight.  It is doubtful whether either of these entities is a cause of low back strain.