A knowledge of the loads imposed on the spine during manual work and of the postures in which the spine is least stressed while performing a particular task are of considerable importance for the design of a safe and comfortable working environment. Spinal loads are intrinsically very difficult to measure and so are generally calculated from mathematical models of the spine. These models are based on assumptions of how the spine functions mechanically, and the forces calculated are dependent on the particular model used. A new model, which assumes that the spine functions in a similar way to an arch, is discussed. This model shows that spinal stresses are not as great as previously calculated using the traditional cantilever model and that, even when no external loads are being carried, the stress is strongly dependent on posture. The arch model re-interprets the role of intra-abdominal pressure and shows that it acts together with the lumbar lordosis to strengthen the spine. Maintaining a lumbar lordosis is then important when lifting heavy loads.