## 4 Discussion  
In this study, the skin thickness, muscle thickness, and shear elastic modulus of muscles around the lumbar spine in a spinal-surgery prone position on a Wilson table were investigated. Our hypotheses supported our findings that there were significant differences in the shear elastic moduli of the erector spinae and multifidus muscles between the flat and Wilson table postures. Posture was also found to have significant effects on muscle and skin thickness. Significant negative corrections were found between muscle thickness and shear elastic modulus of all muscles. Significantly higher shear elastic moduli in all test muscles (superficial and deep erector spinae muscles and the multifidus muscle) were found in the Wilson table condition compared to the flat-table condition. Significantly higher shear elastic moduli indicate that muscles became stiffer when they were on the Wilson table compared to the flat table. Elastography is a method used to characterize tissue mechanical structure and material  
Figure 7: Shear elastic modulus of erector spinae 1, erector spinae 2, and multifidus muscles while in flat (blue) and Wilson (red) table postures. Each box plot indicates 25 th (Q1), 50 th (Q2), and 75 th (Q3) percentiles of the data. Whisker indicates Q3 + 1.5 \(\times\) (Q3 \(-\) Q1) and Q1 - 1.5 \(\times\) (Q3 \(-\) Q1). *, \(p<0.05\). properties. Furthermore, SWE monitors changes in tissue acoustic impedance properties through the propagation of shear waves [27, 28]. Because both the erector spinae and lumbar multifidus muscles have roles in trunk ipsilateral flexion [21], the elongation of muscles caused by the Wilson table could be reflected by increases in the shear elastic moduli of erector spinae and lumbar multifidus muscles. Significant differences in spinal alignments at T1 and L5, but not at T12, between postures are related to spinal curvature changes caused by the Wilson table compared to the flat table (Figure 1). On the basis of our findings, spinal alignments at T12 were not necessarily changed due to the Wilson table. Such changes in spinal curvature have an impact on the elongation of the lumbar muscles, such as the erector spinae and multifidus. Therefore, significantly higher shear elastic modulus could be found in all test muscles. Our results are also supported by a previous study that compared shear elastic modulus across various trunk positions for developing stretching-based rehabilitation therapy [21]. In the study, significantly higher shear elastic moduli of the erector spinae muscle in the flexion-lateral flexion position were found compared to the rest or flexion positions, while a similar shear elastic modulus in the multifidus muscle was found among the flexion-lateral flexion position and flexion position; however, this was significantly lower in the rest position [21]. The other study evaluated paraspinal muscle stiffness using SWE by comparing several passive and active postures in healthy individuals. Different biomechanical behaviors were detected among all paraspinal muscles, depending on changing body position [29]. In particular, multifidus muscles stiffened proportionally more than the longissimus and liicocstalis did, probably to provide stabilization for the intervertebral joints. In the passive flexion posture, where a participant is laying down on the table with a 30\({}^{\circ}\) angulation at the level of the anterior superior iliac spine, shear-wave velocity of the multifidus at the first sacral vertebra and the multifidus at the level of the third lumbar vertebra were significantly increased compared to in the rest position [29]. In accordance with previous works, in this study, the shear elastic modulus between the two postures among muscles was different. This may have been due to the location of the muscles, where erector spinae 1 is the most superficial muscle compared to the erector spinae 2 and multifidus muscles.