Anterior displacement in spondylolisthesis is a slow process. The forces contributing to anterior shear arise from the vertical load on the spine above the lesion, from activity in the muscles of the spine and trunk, and from the effects of movements. The shear is normally resisted by the annulus fibrosus, compression of the facets at the apophyseal joints, and by tension in the anterolateral layers of cortical bone in the pars interarticularis. The anterior shear-strength of the disk is not known though Cyron and Hutton have shown that it is a major contributor to the total stiffness; in the early stages of spondylolysis it provides the only resistance. The capacity of the pars for resisting tensile forces anteriorly has been studied in tests of single vertebrae in which posterior forces were applied to the inferior articular processes. Forces at failure at L4 and L5 have been reported at up to 2500 N by Cyron et al. and up to 5800 N by Lamy et al. with means of 1800 N and 3200 N respectively. The mean posterior displacement of the inferior facets reported by Cyron et al. was 6.5 mm at failure. Too little is known of the mechanical factors in the etiology of spondylolysis for a definition of the postures and movements of the body which put the lumbar neural arch at greatest risk. While a combination of fatigue-failure and high strain-rate of forces induced in extended postures seems likely, further investigations are needed to elucidate the pathogenesis in many cases.