This thesis investigated the relationship between the three-dimensional kinematics of the human back and spinal pathology. This required the development of a system capable of the in vivo measurement of spinal movement non-invasively and in three-dimensions. The opto-electronic CODA-3 Scanner proved unsatisfactory in this respect. The electro-magnetic 3SPACE Isotrak, however, was found to be an accurate and reliable system during a study of twisting in flexed postures. Available axial rotation was significantly increased in some degree of sagittal flexion suggesting that this may be a mechanism for intervertebral disc injury. At high degrees of sagittal flexion a reduction in available axial rotation was noted. In vitro tests on isolated lumbar motion segments confirmed the increase in axial rotation available in flexed postures shown in vivo, this was presumed to be due to an opening of the lumbar zygapophysial joints. Mechanical testing of lumbar interspinous and supraspinous ligaments showed them to be active only in the extremes of sagittal flexion and hence that they could be responsible for the reduction in axial rotation seen in vivo. The 3SPACE Isotrak was used in a clinical study of 80 normal and 43 pathologic subjects. In the normals ranges of motion were, in general, reduced with increasing age in both males and females although a significant increase in sagittal flexion occurred with increasing age in females. Male mobility significantly exceeded female in sagittal flexion but female tended to exceed male in extension, lateral bend and axial rotation. Opposite axial rotation occurred consistently upon lateral bend and vice versa, flexion also occurred on lateral bend but not axial rotation. There was widespread disruption to the primary and coupled movements of the back pain patients when compared to normal movement patterns but there was no clear distinction between the kinematic movement patterns of discrete patient groups. The small numbers in these patient groups warrant a further, more detailed, clinical study.