Relation between intra-abdominal pressure and lumbar stress: effect of trunk posture.

Abstract

The recording of intra-abdominal pressure (IAP) variations has been proposed as a tool for evaluating the biomechanical stress imposed on the low back in working tasks. Literature data are however controversial as regards the relationship between IAP and lumbar stress, some studies showing that IAP increases with trunk flexion while others indicate that IAP is greater when the trunk is kept erect. The present study was aimed at a quantification of this relation for sagittal-plane lifting activities performed at various inclinations of the trunk. Fifteen young males lifted loads ranging from 49 N to 147N from four standing postures: trunk erect and trunk flexed at 30, 60 and 90 degrees from the vertical. The force exerted at the hands, as well as intra-abdominal and intrathoracic pressures were recorded throughout the lift and the L4/L5 moment was calculated using a static biomechanical model. Results showed that the IAP-moment relation is influenced both by the trunk angle and by individual characteristics. Peak IAP increases per unit increase in lumbar moment were higher in the erect posture than in the flexed ones, and in each posture were inversely related to the subject's stature and maximal oxygen uptake. In the flexed postures, no significant general IAP-moment relation could be drawn from the 15 subjects1 data, whereas at the individual level, the relation was significant for two thirds of the subjects. The study thus shows that IAP cannot be used as an index of biomechanical stress, unless the individual IAP to lumbar moment relation is considered. It is suggested that the differences in pressure response found between erect and flexed trunk postures were related more to differences in the leverage moment acting at the shoulder than to differences in trunk angle.