Intra-abdominal pressure increases stiffness of the lumbar spine
Introduction
Intra-abdominal pressure (IAP) is increased during many tasks that load the spine, such as lifting (Hemborg et al., 1985) walking, (Grillner et al., 1978), jumping (Cresswell et al., 1993) and limb movement (Hodges et al., 1997). It has been argued that this pressure in the abdominal cavity contributes to the stiffness and stability of the spine (Bartelink, 1957; Grillner et al., 1978). In support, several studies have identified a relationship between the magnitudes of IAP and the forces imposed on the spine in a variety of static and dynamic tasks (Cresswell and Thorstensson, 1994; Grillner et al., 1978; Marras et al., 1985). However, others have argued that the pressures generated during functional tasks are insufficient to provide a significant increase in spinal stability (McGill and Norman, 1987). Yet, in a recent study, trunk stability—measured as an increase in instantaneous trunk stiffness in response to a sudden load release—was shown to increase when IAP was voluntarily elevated by contraction of the abdominal muscles (Cholewicki et al., 1999b). Although these data are consistent with the proposed mechanical role of IAP, it is impossible to determine whether it is the IAP or the abdominal muscle activity associated with its production that increases the stiffness and stability of the spine.
Several mechanisms have been proposed for IAP to augment spinal stability. First, IAP has been argued to influence spinal stability via the production of an extensor moment by exerting a force down on the pelvic floor and up on the diaphragm (Bartelink, 1957; Keith, 1923; Morris et al., 1961). Although this moment does not necessarily increase stiffness, antagonist flexor and extensor moments increase trunk stability (Cholewicki et al., 1999a). Thus, the opposing extension moment from IAP and flexion moment from abdominal muscle contraction may increase spinal stability and stiffness. It has been confirmed recently that increased IAP—in the absence of abdominal or erector spinae (ES) activity—produces a trunk extension moment (Hodges et al., 2001). In that study, IAP was increased by stimulation of the phrenic nerve to evoke contraction of the diaphragm. A second possibility is that increased stiffness of the abdominal cavity may limit intervertebral translation and rotation (McGill and Norman, 1987). Finally, IAP may prevent shortening of the abdominal muscles, thus maintain the hoop-like geometry of these muscles around the abdominal cavity and their ability to generate tension (McGill and Norman, 1993). It has not been established whether IAP alone can increase the stability or stiffness of the spine.
The aims of the present study were: (1) to identify whether increased IAP (without concurrent abdominal or back muscle activity) increases the posteroanterior stiffness of the spine, and (2) to determine whether any affect of IAP on spinal stiffness is consistent between different levels of the lumbar spine.
Section snippets
Experimental design
We produced an involuntary increase in IAP via a contraction of the diaphragm evoked by percutaneous stimulation of one or both phrenic nerves in the neck. Stiffness of the spine was measured from the force–displacement response to application of a force to a spinous process of the lumbar spine with the subject lying prone.
Subjects
Three healthy volunteer subjects participated in the experiment. The age, height and weight of the subjects were 30–44 years, 172–187 cm, and 58–87 kg, respectively. All
Results
When the diaphragm was electrically stimulated via one or both phrenic nerves, IAP increased by 1.8–5.9 kPa, which resulted in pressures equivalent to 27–61% of the pressure increase during a maximum voluntary pressurisation of the abdominal cavity with the glottis closed (Fig. 2). The maximum IAP achieved for each subject is presented in Table 1. When IAP was increased in this manner, the stiffness of the spine assessed by posteroanterior pressure applied to the L4 spinous processes, increased
Discussion
The results of this study provide evidence that increased IAP augments stiffness of the spine. These data are consistent with the hypothesis that modulation of IAP during functional tasks contributes to the mechanical stability of the spine. As pressure was increased involuntarily by electrical stimulation of the diaphragm, these data confirm that IAP without overt activity of the abdominal or back muscles, has a mechanical effect on the spine.
Conclusion
IAP—in the absence of abdominal and back extensor activity—augments stiffness of the spine. Thus, modulation of IAP provides an additional mechanism for the central nervous system to control spinal stability during functional activities and may simplify this control by providing a non-direction-specific increase in stiffness.
Acknowledgements
Financial support was provided by the National Health and Medical Research Council and Physiotherapy Research Foundation of Australia.
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