## 2 Materials and Methods
### Experiments and Data Acquisition  
A diagnostic US device (RS85, Samsung Medison, Seoul, Republic of Korea) and a linear transducer array probe (LA2-14A, Samsung Medison, Seoul, Republic of Korea) with a frequency bandwidth of 2-14 MHz were used to acquire transverse B-mode scan images. The experiment was conducted with the participant in a supine position, and the transducer probe was placed in the center of the measurement area (5 cm above and 5 cm to the left of the navel) (Figure 1A). The measurement location was chosen based on information obtained from the pilot study in which the tendinous intersection was not observed in the US image [22]. The RA, SF, and DF were identified in the B-mode image (Figure 1B) and the corresponding area was marked with a blue pen on an absorptive marker on the skin (Figure 1A). The B-mode images were acquired without the absorber (No_PU). Subsequently, a transparent PU (Superfix BNG-#2003, BANDGOLD, Anyang, Republic of Korea) was cut into half the probe size (width: 30 mm, length: 10 mm, thickness: 0.05 mm) and attached to the right side of the measurement area to obtain B-mode images. The PU size and the location were determined to confirm the exact measurement location and to examine changes in signal intensity within the B-mode image. To prevent abdominal motion artifacts during the B-mode scanning, participants were asked to hold their breath for approximately 5 s, sufficient for localization and image acquisition. All participants underwent image acquisition for each attached and unattached absorber.  
The acquired US images were quantitatively evaluated based on the thickness, signal intensity, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) of the RA, SF, and DF. US images were acquired in DICOM format, and the thickness was measured using DICOM Viewer software (RadiAnt DICOM Viewer Version 2021.2.2 (64-bit), Medixant, Poznan, Poland, Eastern Europe). The thickness and signal intensity were measured at a vertical location one-quarter from the right edge of the B-mode image using the same evaluation protocol, corresponding to the central position of the absorber (as shown in the red vertical line in Figure 1). The thickness of SF and DF was measured where the vertical lines overlap, and the thickness of RA was measured at the bottom of SF and the top of DF. The signal intensity was measured at each location where RA, SF, and DF meet the vertical line, and the thickness of each tissue was measured using the full width at half maximum of the signal intensity within the vertical line. The signal intensity of the background (BG) was measured by setting the midpoint between DF and the bottom of the image.  
The signal intensity was measured in the area corresponding to the center of the absorber on the right side of the image using an image analysis program (Image) version 1.52a, free software, [[https://imagej.net/ij](https://imagej.net/ij)]([https://imagej.net/ij](https://imagej.net/ij)), accessed on 2 March 2024). The mean signals and standard deviations (SD) of the RA, SF, and DF were measured. In addition, a BG region (a circle with a diameter of nine pixels) was selected for the SNR and CNR calculations, and its SD was measured (Figure 1B). The CNR of SF and DF were determined in contrast to RA, corresponding to SF-RA and DF-RA, respectively.  
Using the mean signal intensity of the ROI and SD of the BG, the signal changes in the RA, SF, and DF with and without the absorber were compared with the SNR and CNR calculated using the following formulas:  
\[\text{SNR}=\frac{\text{Mean of signal}_{\text{ROI}}}{\text{Standard Deviation}_{\text{Background}}}, \tag{1}\]  
\[\text{CNR}=\frac{\text{Mean of signal}_{\text{ROI}\_A}-\text{Mean of signal}_{\text{ROI}\_B}}{\text{Standard Deviation}_{\text{Background}}}. \tag{2}\]