Pathological changes of the spine are induced by ageing and trauma due to mechanical overload or fatigue. The biomechanical consequences of these changes with respect to the structure, kinematics and strain patterns of the spine are reviewed. The pathological changes generally tend to lower the mechanical strength of the structure and limit the relative range of motion; at the same time the tolerance level for the onset of back pain is lowered. The interaction of spinal motion and pathology in producing low back pain is discussed on the basis of a diagrammatic representation proposed by Kirkaldy-Willis and Farfan