## 4 Discussion
The erector spinae muscle is a member of the superficial muscles of the trunk and has a long extension moment arm (MA) for the trunk, at 6.1 \(\pm\) 0.6 cm in the extension direction, and at 2.2 \(\pm\) 0.4 cm in the ipsilateral lateral flexion direction [21, 30]. However, the multifidus muscle is a member of the deep muscles of the trunk and has a shorter MA compared to the erector spinae muscles, at 5.5 \(\pm\) 0.7 cm in the extension MA and 1.1 \(\pm\) 0.1 cm in the ipsilateral lateral flexion MA [21, 30]. Considering that a previous study found a more stretched muscle-tendon unit when the change in joint angle was higher or muscle MA was longer [31], a higher shear modulus on the Wilson table compared to the flat table may be due to a change in the joint angle, but a greater change in the superficial muscle is due to the longer MA leading to more muscle stretch. Muscle elasticity can be influenced by many factors, such as physiological status, anesthesia status, and the use or non-use of muscle relaxants in consideration of clinical applications of the current findings. Due to a lack of studies associated with the assessment of material parameters in muscles under anesthetic conditions, it is uncertain how muscle relaxants and anesthesia would influence our results. Clinically, we determined that there are no differences between skeletal-muscle tone during general anesthesia with and without a muscle relaxant on spinal-surgery patients [32]. Isometric skeletal-muscle strength during general and spinal anesthesia was observed in humans. Using clinically applicable doses of bupivacaine, muscle strength was decreased by about 20%, whereas under general anesthesia (propofol and sevoflurane), muscle strength remained unchanged [33]. Additionally, the different effects of propofol, ketamine, and thiopental anesthesia on dorsal striated muscle microcirculation were observed in rats. The administration of thiopental and propofol induced the dilatation of arterioles and venules, but ketamine had the opposite effect at the induction period [34]. Depending on concentration, the same drug demonstrated different effects; in particular, thiopental demonstrated dose-dependentmicrovascular constriction [35]. Thus, the material properties of skeletal muscles during anesthesia could be different from those in the relaxed condition in our study. As such, this requires further investigation. Although this is the first study comparing the shear moduli of the erector spinae and lumbar multifidus muscles between flat and Wilson table conditions, few studies investigated the moduli of the erector spinae and lumbar multifidus muscles [24, 36, 37, 38, 21, 39, 40]. Our shear-modulus values for erector spinae 1, erector spinae 2, and multifidus muscles in the flat-table condition were comparable with those of previous studies in the range of approximately 14-18 kPa [36, 37, 38, 39]. There is a study that reported lower values than ours, between 5-7 kPa in the multifidus muscle [40]. SWE values can be influenced by transducer type (either linear or convex probes), the frequency of image acquisition, the angle between transducer and muscle fibers, and ROI location in the images [41]. In our case, the different values from a previous study may have come from the angle between transducer and muscle fibers since stiffness values are greater in the parallel direction rather than perpendicular to the muscle fibers [40]. Furthermore, the applied pressure to the ultrasound probe could influence the shear-elastic-modulus value [42]. Overall, our values and findings are reasonable and can be justified. Furthermore, they are supported by our good ICC values in shear elastic modulus from 0.805 to 0.942. Since all subjects were healthy males, further studies comparing gender differences and differences due to pathological conditions could be interesting to pursue.