Reasons for performing study: Sacroiliac joint (SIJ) disease is associated with poor hindlimb action, lameness and poor performance in horses. However, little is known about the biomechanics of this low-motion joint. Objectives: To determine in vitro the capacities of movement of the SIJ in the sagittal plane, and to test the effect of a sacrosciatic and sacrotuberal desmotomy on its stabilisation. Methods: Six anatomical specimens underwent cycles of flexion-extension of the lumbosacral joint (LSJ) before and after desmotomy. Kinematic triads were linked rigidly to the sacrum, spinous process of vertebra L5 and iliac wing. Angles were measured using a joint coordinate system based on anatomical frames. Results: The LSJ underwent regular movements of flexion and extension (overall mean+or-s.d. range 23.4+or-1.6 degrees ). The only recorded movement of the SIJ was a nutation during LSJ flexion (overall mean+or-s.d. 0.8+or-0.5 degrees ). Desmotomy induced an increase of that nutation (overall mean+or-s.d. 1.7+or-0.2 degrees ). Conclusions and potential relevance: Movements of the SIJ were small and coupled only with the flexion of the LSJ. The ligaments surrounding the SIJ have a strong effect on the stabilisation of this joint. Due to the limited amount of movement, its biomechanical study in vivo seems to be difficult. Further in vitro studies would be useful to determine the role of each ligament, to better understand the clinical consequences of the tears frequently observed during necropsy.