## 4 Discussion
First, the force of the _rector spinae_, the major extensor of the spine, was analyzed. The results revealed muscle force increased in the course of pregnancy in both variants of the simulation. Comparing the values in late gestation to the pre-pregnancy condition, the changes in body mass required almost double its pre-pregnancy force (Variant I), while the increased pelvic tilt (Variant II) reduced its demands by \(\sim\)12%. An analysis of the _erector spinae_ components revealed that a deeper anterior pelvic tilt was accompanied by a larger muscular activation of the _ilicostasislumborum pars lumborum_. Finally, a \(\sim\)-5-fold increase in the force of the aforesaid muscle (from 3 N in P0 to \(\sim\)15 N in P3) was registered. The explanation by [PERSON] et al. [57] provides a rationale for the observation that half or more than half of the extension moment in the upright position affecting any lumbar segment stems from the thoracic fibres of the (musculus) _longissimus thoracis_ and the iliac-costal muscle of the loins. The remaining part of the moment stems in almost equal proportions from the lumbar fibres of these muscles and the multifidus muscle. The _transversus abdominis_, with its more horizontal orientation, is thought to contribute to spinal stability and increase the stiffness of the lumbar spine [58]. The activation of the _transversus abdominis_ increased as the pregnancy progressed. Its highest involvement was registered in P3 for the first and second variant (Figure 2b), respectively: 128 N (0.18 BW) and 112 N (0.16 BW); pelvic tilt reduced its activation. The pelvis endeavours to couple lumbar lordosis with hip extension in the upright position with a minimal expense of energy [59]. Generally, we observed the decreasing activation of the hip flexors (Figure 2c) and the increased activation of hip extensors (Figure 2d) in the course of pregnancy. The highest activity of the _biceps femoris longum_,registered in late pregnancy, was almost 3 times higher compared to its pre-pregnancy activation. An 18% increase in its muscular force was observed in Variant II compared to Variant I. Simultaneously, the activation of the _iliopsos_ was reduced in the advanced gravid state by 17% (Variant I) and 26% (Variant II) when compared to a non-pregnant condition. Due to a more anteriorly tilted pelvis, the activity of this hip flexor was reduced by 12% (P2) and 22% (P3). Knowledge of the myoelectric activity of the muscles helped [PERSON] et al. [28] in the estimation of the loading of the lumbopelvic complex during gestation. Their results showed a significant increase in the bioelectrical activity of the lumbar and pelvic extensor muscles (_erector guinea_ and _biceps femoris_) in the erect position in the third trimester of pregnancy. Our simulations of loads affecting the musculoskeletal system aimed to find solutions enabling the minimisation of muscular fatigue. Generally, in both variants, the results revealed increased muscular fatigue throughout pregnancy. However, the simulations related to Variant II showed lower values of the objective function (P2 = 0.14, P3 = 0.23) than those related to Variant I (P2 = 0.17, P3 = 0.30). The preceding may imply that an increase in pelvic anteversion reduces muscular fatigue. The human pelvis with the spine creates mechanisms to modulate posture [59]. In the erect position, an increase in the anterior pelvic tilt leads to an increase in lumbar lordosis angle [22]. The enlarged lordosis bends the sacrum forward, thus putting a strain on the joints, intervertebral discs, and ligaments of the lumbosacral spine. The movements compensate for the lack of the hyperextension described above in L5-S1 in the sagittal plane of the pelvis. This leads to the strain of the L5/S1 and L4/L5 segments of the lumbar spine [52]. The results of our simulations led to the understanding of the pregnancy-related loading conditions in the lumbar spine.