Introduction 34 19 vice versa 33 35 in vitro in vivo 1 9 11 36 10 27 3 4 18 2 8 12 20 22 28 24 30 15 Materials and Methods The 3-D Simulation Model et al. 15 8 4 17 1 xyz YZ xyz YZ 1 1 1 Figure 1 x y z 1 1  Table 1 1  Name of structure Number of elements Remark 1 M. adductor brevis 2 Upper and lower muscle 2 M. adductor longus 1  3 M. adductor magnus 3 Upper, middle and lower muscle 4 M. biceps femoris 1  5 M. coccygeus 1 Pelvic floor muscle 6 M. iliococcygeus 1 Pelvic floor muscle 7 M. pubococcygeus 1 Pelvic floor muscle 8 M. gemellus inferior 1  9 M. gemellus superior 1  10 M. gluteus maximus 2 Femur—sacrum and ilium muscle 11 M. gluteus maximus facia 2 Ilium—femur and trunk muscle 12 M. gluteus medius 3 Upper, middle, and lower muscle 13 M. gluteus minimus 3 Upper, middle, and lower muscle 14 M. gracilis 1  15 M. iliacus 1  16 M. longissimus 1  17 M. iliocostalis 1  18 M. multifidus 1  19 M. obliquus externus abdominis 2 Ventral and dorsal muscle 20 M. obliquus internus abdominis 2 Ventral and dorsal muscle 21 M. obturatorius externus 1  22 M. obturatorius internus 1  23 M. pectineus 1  24 M. piriformis 1  25 M. psoas 2 Upper and lower muscle 26 M. quadratus femoris 1  27 M. quadratus lumborum 5 Sacrum—rib12, L1, L2, L3 and L4 muscle 28 M. rectus abdominis 1  29 M. rectus femoris 1  30 M. sartorius 1  31 M. semimembranosus 1  32 M. semitendinosus 1  33 M. tensor fasciae latae 1  34 M. transversus abdominis 1  A Iliolumbar ligament 1 Transversal plane B Posterior sacroiliac ligament 1 Transversal plane C Sacrospinal ligament 1  D Sacrotuberous ligament 1  I L5-S1 joint 3 Shear (two directions) and compression II SI joint 3 Shear (two directions) and compression III Hip joint 3 Shear (two directions) and compression IV Knee joint 3 Shear (two directions) and compression V Pubic symphysis 1 Compression Simulations and Data Analyses 16 Lowering of the maximum vertical SIJ shear force must result in reduction of the total SIJ shear force (combination of vertical and horizontal shear); Ligament force must not exceed 250 N. A muscle was included for further analysis when it produced at least 15% of the maximum muscle stress during the simulation. For all muscles, the maximum muscle stress depended on the calculated minimum muscle stress (see optimization criterion 1 in the Appendix). Two muscle groups were analyzed separately: (1) the muscles that increased at least 80% in force after the first simulation step and (2) the muscles that increased at least 10 times in force after completion of the simulation series. Results 2 2 Table 2 Summary of the structures that stabilize the sacroiliac joints in terms of lowered shear Reduction of sacroiliac shear Initial N SIJ (vertical shear) 563 533 503 473 443 413 383 353 323 Structures N M. adductor longus 9 18 18 18 18 9    M. coccygeus  1 1 1 1 2 4 10 20 M. iliococcygeus  1 1 1 1 2 4 10 20 M. pubococcygeus       1 2 6 M. gluteus medius (lower) 7 11 13 14 15 27 44 30  M. gluteus medius (middle) 5 8 11 10 10 10 29 41  M. gluteus medius (upper)        42 90 M. gluteus minimus (lower)  3 3 3 3 4 7   M. gluteus minimus (middle) 5 10 11 11 11 5 10 12  M. gluteus minimus (upper) 8 17 18 18 17 5 11 16 25 M. iliacus 47 47 50 54 58 68 88 85 102 M. obliquus externus abdominis 21 18 17 14 12     M. obliquus internus abdominis 15 20 20 23 27 34 29   M. obturatorius externus 15 4 4 4 5 8    M. pectineus 8 18 18 18 18 6    M. piriformis 18 26 28 25 22     M. psoas (lower) 64 50 46 27      M. rectus abdominis 27 27 29 31 34 37 51 76 83 M. rectus femoris 34 34 36 39 42 49 64 62 50 M. sartorius  16 17 18 19 16 22 14  M. tensor fasciae latae 31 31 33 35 38 44 58 85 79 M. transversus abdominis 3 5 5 6 7 21 32 53 82 Iliolumbar ligament       53 250 250 Posterior sacroiliac ligament  26 49 62 73 147 250 250 250 Sacrospinal ligament  38 150 159 147 145 132 106 74 Sacrotuberous ligament 206 151 21       SIJ (compression) 92 121 130 142 154 229 473 607 633 SIJ (horizontal shear) −132 −141 −160 −154 −142 −154 −208 −226 −233 Total SIJ shear 579 551 528 497 465 441 436 419 398 Angle of SIJ reaction force (°) 81 78 76 74 72 63 43 35 32 Maximum muscle tension (kPa) 37 37 39 42 45 53 69 125 247 Included are those muscles that produced at least 15% of the maximum muscle stress after each simulation The muscles printed italic increased at least 80% in force after the first simulation step. The muscles printed bold increased at least 10 times in force after completion of the simulation series Figure 2 trunk 2 sacrotuberal lig. 2 sacrospinal lig. 2 transversus abdominis pelvic floor iliolumbar lig. http://www.primalpictures.com  2 2 2 Discussion 2 33 3 Figure 3 http://www.primalpictures.com  5 26 in vivo in vitro in vivo 25 in vitro 23 7 14 21 in vivo 33 24 32 31 13 Conclusions Effective stabilization of the SIJ is essential in transferring spinal load via the SIJ to the coxal bones and the legs. A biomechanical analysis of the upright standing posture showed that activation of transversely oriented abdominal M. transversus abdominis and pelvic floor, i.e., M. coccygeus and M. pubo- and iliococcygeus muscles would be an effective strategy to reduce vertical SIJ shear force and thus to increase SIJ stability. The force equilibrium in this situation induced loading of the iliolumbar and posterior sacroiliac ligaments. The M. transversus abdominis crosses the SIJ and clamps the sacrum between the coxal bones. Moreover, the pelvic floor muscles oppose lateral movement of the coxal bones, which stabilizes the position of the sacrum (the pelvic arc).