Because the etiology of spondylolysis is not well understood, the authors performed an analysis of loads and stresses in human lumbar vertebrae to determine whether purely mechanical factors are likely to cause spondylolytic fractures in a normal spine. To perform these studies, modeling methods were applied. A mechanical system was developed to study muscle forces and reactions in joints of the lumbar spine. Next an optimization approach was applied to find loads on vertebrae and muscle forces. Finally photoelastic experiments were performed to find effective stresses and stress concentrations in low lumbar vertebrae. The analysis showed that the highest stresses appear in parts interarticularis. The results prove that factors of a purely mechanical nature are of fundamental importance in the etiology of spondylolysis.