Function of the spine

In spite of the considerable effort which has been invested in attempts to understand the mechanism of human spines, substantial controversy remains, particularly in connection with assumptions which have to be made by those engaged in biological modelling. The hypothesis presented here is that the living joint has stress sensors driving a feedback mechanism, an arrangement which could react to imposed loads by modifying muscular action in such a way as to minimize stress at the joints and therefore the risk of injury. A theory of this kind gives an image of the spine not in terms of a spatial picture, as would a CAT scan, but in terms of stresses, forces and moments acting at the intervertebral joints. Calculations show that the erectores spinae alone cannot support more than about 50 kg; there must be some other mechanism to explain man's ability substantially to exceed that load. It is suggested that the interaction between the erectores spinae and the abdominals are of fundamental importance in the function of the spine; how they are co-ordinated during the lifting of weights is examined in detail. The theory resulting from this hypothesis is used to relate spinal injury and an injured subject's posture and behaviour. A mathematical formulation permits an objective evaluation of the spine, and a procedure for determining an automatic diagnosis of lumbar spine disabilities is proposed.