In vivo measurement of the effect of intra-abdominal pressure on the humanspine

In humans, intra-abdominal pressure (IAP) is elevated during many everyday activities. This experiment aimed to investigate the extent to which increa sed IAP-without concurrent activity of the abdominal or back extensor muscl es-produces an extensor torque. With subjects positioned in side lying on a swivel table with its axis at L3, moments about this vertebral level were measured when IAP was transiently increased by electrical stimulation of th e diaphragm via the phrenic nerve. There was no electromyographic activity in abdominal and back extensor muscles. When IAP was increased artificially to similar to 15% of the maximum IAP amplitude that could be generated vol untarily with the trunk positioned in flexion, a trunk extensor moment (sim ilar to6 Nm) was recorded. The size of the effect was proportional to the i ncrease in pressure. The extensor moment was consistent with that predicted from a model based on measurements of abdominal cross-sectional area and I AP moment arm. When IAP was momentarily increased while the trunk was flexe d passively at a constant velocity, the external torque required to maintai n the velocity was increased. These results provide the first in vivo data of the amplitude of extensor moment that is produced by increased IAP. Alth ough the net effect of this extensor torque in functional tasks would be de pendent on the muscles used to increase the IAP and their associated flexio n torque, the data do provide evidence that IAP contributes, at least in pa rt, to spinal stability. (C) 2001 Elsevier Science Ltd. All rights reserved .