Lumbar Spine Stiffness Is Increased by Elevation of Intra- Abdominal Pressure

Introduction lntra-abdominal pressure (IAP) is elevated during many functional tasks and has been argued to increase stability of the spine (Bartelink, 1957; Grillner et al., 1978; Daggfeldt and Thorstensson, 1997). Although several studies have shown a relationship between IAP and spinal stability (e.g. (Cresswell and Thorstensson, 1994; Cholewicki et al., 1999), it has been impossible to determine whether this augmentation of mechanical support for the spine is due to the increased IAP or the abdominal muscle activity associated with its production. In a recent study we have measured the magnitude of extension torque produced by an increase in IAP, which was produced in the absence of abdominal or back extensor activity (Hodges et al., 2001). This was achieved by electrically evoked contraction of the diaphragm by stimulation of the phrenic nerves in the neck. The aim of the present study was to use a similar method to determine whether elevated IAP increases spinal stiffness, in the absence of concurrent activity of the abdominal and back extensor muscles.