Intra-abdominal pressure and abdominal wall muscular function: Spinal unloading mechanism

Abstract

Background: The roles of antagonistic activation of abdominal muscles and of intra-abdominal pressurization remain enigmatic, but are thought to be associated with both spinal unloading and spinal stabilization in activities such as lifting. Biomechanical analyses are needed to understand the function of intra-abdominal pressurization because of the anatomical and physiological complexity, but prior analyses have been over-simplified.

Methods: To test whether increased intra-abdominal pressure was associated with reduced spinal compression forces for efforts that generated moments about each of the principal axis directions, a previously published biomechanical model of the spine and its musculature was modified by the addition of anatomically realistic three-layers of curved abdominal musculature connected by fascia to the spine. Published values of muscle cross-sectional areas and the active and passive stiffness properties were assigned. The muscle activations were calculated assuming minimized muscle stress and stretch for the model loaded with flexion, extension, lateral bending and axial rotation moments of up to 60 Nm, along with intra-abdominal pressurization of 5 or 10 kPa (37.5 or 75 mm Hg) and partial bodyweight (340 N).

Findings: The analysis predicted a reduction in spinal compressive force with increase in intra-abdominal pressurization from 5 to 10 kPa. This reduction at 60 Nm external effort was 21% for extension effort, 18% for flexion effort, 29% for lateral bending and 31% for axial rotation.

Interpretation: This analysis predicts that intra-abdominal pressure produces spinal unloading, and shows likely muscle activation patterns that achieve this.

Figure 1

Three layers of abdominal musculature, dorsal muscles and lumbar spine as represented in the analytical model. Rectus abdominis is considered to be embedded within the middle layer (internal oblique). Axis dimensions are in mm.

Figure 2

Spinal loading averaged over the six intervertebral levels from T12 to S1 with 5 kPa and 10 kPa abdominal pressure in efforts in four principal moment directions (`Baseline' model).

Figure 3

Activation of trunk muscles in response to four different levels of external loads for efforts in four principal moment directions, as predicted by the analytical model. (a) Dorsal muscles (b) Abdominal muscles. For each panel the horizontal axis gives the magnitude of the effort (moment) in Nm. In the legend, `Lt' = left side muscles; `Rt' = right side muscles.

Figure 3

Activation of trunk muscles in response to four different levels of external loads for efforts in four principal moment directions, as predicted by the analytical model. (a) Dorsal muscles (b) Abdominal muscles. For each panel the horizontal axis gives the magnitude of the effort (moment) in Nm. In the legend, `Lt' = left side muscles; `Rt' = right side muscles.

Figure 4

Comparison of muscle percent activation with 10 kPa and 5 kPa IAP, for 60 Nm extension effort. Each point on the graph represents values for one of the symmetrical muscle pairs used in the analysis. 'Abdominal wall' = obliques and transversus; 'Rectus' = rectus adominis; 'Dorsal Muscles' = 90 pairs of extensor muscles, including psoas.