Reassessment of the role of intra-abdominal pressure in spinal compression

Abstract

Biomechanical models used to estimate loads on the lumbar spine often predict internal low back forces for heavy lifts that exceed known tissue tolerances, yet the particular lift caused no apparent damage to the lifter. To deal with this paradox, many researchers have incorporated some form of spinal compression alleviation from intra-abdominal pressure (IAP). The purpose of this work was to re-examine some of the issues involved in the feasibility of IAP to reduce spinal loads during stressful lifts. Questions remain over the trade-off between the beneficial tensile force on the spine, exerted via the diaphragm and pelvic floor when IAP is produced, and the undesirable compressive effects of abdominal muscular force required to maintain the pressure within the abdomen. Various strategies of modelling IAP and its effects on low back loading were employed, Three major differences between this and most previous models of IAP effects were the attempt to quantify the size of abdominal muscle forces and the utilization of a considerably smaller diaphragm cross-sectional area and corresponding IAP moment arm. Abdominal EMG recorded from rectus abdominis, external oblique and internal oblique generally indicated low levels of activity throughout the high loading phase of the lifts. However, model output predicted that the compressive forces generated by the abdominal wall musculature were larger than the beneficial action of those forces thought to alleviate spinal compression via IAP. These results suggest that modelling IAP as a force vector which produces a trunk extensor moment and lumbar disc compression alleviation, without accounting for the compressive effects of abdominal muscle forces required to produce the IAP, is incorrect. This does not exclude a possible role of IAP in assisting the trunk during loading, only that the role of IAP is not modelled properly at present. IAP may indeed play a role in spinal stabilization as yet not well understood.