Intra-abdominal pressure and its role in spinal mechanics.

Intra-abdominal pressure has been suggested as a mechanism for reducing the compressive load on the spine. The cantilever model of the spine, on which the mechanism is based, is described and the nature of the intra-abdominal pressure response examined within this context. This model is also the basis for the proposal to use intra-abdominal pressure as an index of compressive stress on the spine, although the dependence on the model is not always acknowledged, and the validity of this approach is discussed. Experimental evidence relating intra-abdominal pressure to posture and loading is examined and compared with the predictions of the model and these are often found to disagree. A fundamental problem in spinal mechanics, that of explaining why the vertebrae are not crushed during the lifting of even relatively small loads, is shown to be unresolved. This suggests that a new model of the spine is required, within which there is a useful role for intra-abdominal pressure with the described characteristics. A model based on an analysis of the spine as an arch is proposed.