The fatigue strength of vertebral posterior elements was determined by
applying repetitive forces to the inferior joints between the articular
processes of human rhesus-monkey vertebrae. The results indicate that in
vivo stresses resulting from unusual activities for sustained periods can
induce fatigue fracture of the lumbar vertebrae. Comparison of these data
with those of other investigators suggests the use of ultimate strength as
a normalizing factor to correlate fatigue-strength data obtained under
different test conditions.