Quantification of Lumbar Stability by Using 2 Different Abdominal Activation Strategies

Stability analysis. This schematic depicts the process leading up to the calculation of stability. Abbreviations: DM, distribution moment; dL/dt, derivative of length with respect to time; EMG, electromyography.

A schematic representation of a simplified spine motion segment to show the concept of linear (muscles, ligaments, tendons) and torsional (intervertebral disks) springs working to sustain an applied load.

Because the moment is calculated as the product of force and perpendicular distance, moving the muscle attachment (A) closer to the joint (J) (as occurs during hollowing) shortens the moment arm of rectus abdominis. This has a large effect on the resulting potential energy, and, consequently, stability as well. Stability of a column with guy wires (or a spine with muscles) is a function of geometry (the location of the support attachments), stiffness, the “balance” in stiffness in all directions, and column imperfections, such as curvature. Hollowing reduces the potential energy and stability by narrowing the geometric base.

In the hollowing simulation, the moment arm of rectus abdominis was reduced by 5cm (“B” in left panel), when compared with the bracing condition (right panel), to account for the change in anatomic geometry. “A” defines the relaxed condition.

A composite of 2 trials of in vivo subject data (a 2-hand load trial chosen at random) shows that bracing had greater stability than an attempted hollowing. The ideal hollowing was not achieved. Ideal hollowing was tested in the simulations. Note the initial overactivation when trying to establish the hollow.

Lumbar stability increased by a significant margin (32%) under the simulated bracing condition. The hollow was conducted with 20% activation in both the lateral oblique and the transversus abdominis and 2% activation in both rectus abdominis and external oblique. This was considered feasible after observing our subjects. Most of the stability was obtained through the internal oblique. Abbreviations: BRC, bracing; HLW, hollowing; Stb, Stability.

Although the external load component contributes to lumbar compression equally among conditions, compression due to muscle activation differed between the simulation bracing and hollowing condition by as much as 500N. Note that the hollowing here included both the internal oblique and transversus abdominis as in figure 6. Abbreviation: Cmp, compression.

Stability index in simulation trial. A single, 2-hand load trial was processed, with 4 sensitivity tests, to isolate the role of different muscles. The bottom trace is actually 2 conditions using the transversus abdominis as the only active abdominal muscle (at 100% and 20% of MVC). Whether the transversus abdominis is active at 20% or 100% of MVC had little effect and those 2 plots are superimposed. The top trace is actually 2 conditions: (1) with all abdominal muscles active in a brace pattern (at whatever level the subject had activated them) and (2) with only the transversus abdominis removed. Only 1 line is visible because removing the transversus abdominis has almost no effect, and its plot overlays the full activation plot. The variability in the stability plot, including what stability is present, results from variable activity in the extensor (and other modeled) muscles. Abbreviations: abs, abdominals; TrA, transversus abdominis.

Stability index with abdominal muscles removed from simulation. This histogram shows the stability, for actual subjects, performing the brace and an attempted hollow (shown at the far right). Then, the right and left abdominal muscle pairs were removed from the analysis, each in turn, and the stability was recalculated. Although the removal of the transversus abdominis made little difference to stability, internal oblique was the most important in affecting stability, followed by external oblique and finally by rectus abdominis. Abbreviations: Brc, normal brace; BrEO, brace with external oblique; BrIO, brace with internal oblique; BrRA, brace with rectus abdominis; BrTA, brace with transversus abdominis; Hlw, normal hollow; HwEO, hollow with external oblique; HwIO, hollow with internal oblique; HwRA, hollow with rectus abdominis; HwTA, hollow with transversus abdominis.