## 2 Materials and Methods
### Three-Dimensional (3D) Model Preparation  
The cortical and trabecular bones of the femur and pelvic girdle were thresholded and segmented in Mimics(r) (version 21.0, Materialise Software, Belgium) using an abdominal CT image, collected at Oulu University Hospital (Philips, 110 kVp, 105 mAs, 453 slices with 0.5 mm thickness, resolution 0.75 \(\times\) 0.75 mm). The location-dependent cortical thickness was obtained via a semi-automated slice-by-slice local thresholding procedure available in Mimics(r). The cortical thicknesses obtained, especially at the acetabulum (the thinnest cortical bone within the pelvis), were checked to make sure that they were in the typical reported range for the pelvic cortical bone [39; 40] (Appendix A, Figure A1). Since an abdominal CT image was used in this study, only the proximal part of the femur was reconstructed directly. The total length of the femoral shaft was estimated based on [PERSON] et al. [41], and the cortical bone of the femur was extended accordingly. The shape of the distal femur was not considered. Since the CT image used was not acquired with a contrast agent, accurate segmentation of the cartilage and the interpubic disc was not possible and they were added using 3-matic(version 13.0, Materialise Software, Belgium) software. The shapes of the cartiglages and interpubic disc were derived from the Human Biodigital(r) online platform [42], considering the position of the reconstructed bony parts of the model. Also, the thicknesses of the femoral (1.48 mm) [43], acetabular (1.31 mm) [43] and sacroiliac cartiglages (3.50 mm) [44], as well as the interpubic disc (22.72 mm) [38] were determined based on published data. Bones and cartiglages were subtracted from the primary model to obtain a reconstructed CT-based soft-tissue geometry [32; 45]. The soft tissue obtained was truncated in half and the impact side was retained to reduce the number of elements. According to our previous work, body configuration during impact can substantially affect the strain magnitude and distribution within the acetabulum [9]. The pelvic and femoral orientations during CT imaging are distinctly different from those related to a sideways fall. In the supine position, the femur is externally rotated, and the pelvis remains almost in the neutral standing position, whereas during a sideways fall the femur is adducted and the pelvis is laterally tilted. Therefore, to resemble the postures of the femur and pelvis during a sideways fall and to obtain a proper deformed soft tissue, the femur was adducted and rotated internally using a quasi-static (implicit) simulation in ABAQUS(r)/Standard (Appendix A, Figure A2a). The element size of the converged mesh was 4 mm (Appendix A, Figure A2b), comparable to the mesh size (3-10 mm) of [PERSON] et al. [13] for modeling the bulk soft tissue. The obtained mesh was exported to LS-Dyna as the soft tissue for the simulation of the impact (explicit). The body posture associated with the highest risk of acetabular fracture was selected based on our previous study [9]. The highest risk of acetabular fracture based on strain magnitude and concentration was obtained when the trunk flexion (sagittal plane), trunk tilt (coronal plane), femur internal rotation, and femur adduction angles were set to 0\({}^{\circ}\), 40\({}^{\circ}\), 15\({}^{\circ}\), and 5\({}^{\circ}\) (comparable to those used by [PERSON] et al. [13]), respectively (Figure 1a). A lumped mass-spring-dashpot system was used to represent the whole body except the reconstructed parts (bony parts and soft tissues) (Appendix A, Figure A3a). The segments' masses and the locations of the lumped masses for the 50 th percentile male model (weight: 76 kg, height: 168.67 cm) were adapted for the sideways fall body configuration, based on the work of [PERSON] [46] (Appendix A, Figure A4, Table A1). The stiffness of connecting springs and damping coefficient of the dampers were obtained from [PERSON] and [PERSON] [47], and [PERSON] [48], respectively (Appendix A, Table A2).