## 4 Discussion  
Under pregnancy conditions, the anteriorly growing fetus puts higher demands on the musculoskeletal system. Therefore, we compared two variants of simulations to obtain more insight into how the body copes with pregnancy. Since one of the essential roles of the  
Figure 3: Values of loads in the musculoskeletal system generated within the lumbopelvic complex: (**a**) resultant forces in lumbar segments, (**b**) resultant, compression and shear force in segment L5-Sl, (**c**) resultant force in the hip joint. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Muscle Fatigue Function ***} \\ \cline{2-5}  & **P0** & **P1** & **P2** & **P3** \\ \hline
**Variant I** & 0.09 & 0.10 & 0.17 & 0.30 \\ \hline
**Variant II** & 0.09 & 0.10 & 0.14 & 0.23 \\ \hline
**Difference in the value of the** & & & & \\
**muscle fatigue function between** & 0 & 0 & 17.6\% & 23.3\% \\
**Variant I and II** & & & & \\ \hline \hline \end{tabular}  
* Muscle fatigue function—expressed by the value of the optimisation task objective function.  
\end{table} 
Table 3: Changes in the muscle fatigue function during measurement sessions: P0—before pregnancy, P1—in the first trimester, P2—in the second trimester, P3—in the third trimester of pregnancy. lumbopelvic structure is to support mechanical loads in the upright position, this study was designed to simulate the response of this complex to the growing fetus in pregnant women. The authors hypothesized that posture under pregnancy conditions may be adjusted to minimize the demand following obstetrical load. Due to an increase in anterior pelvic tilt in the course of gestation, we noted some reductions in muscle activation (e.g., erector spinae, transversus abdominis or iliospoas), muscle fatigue, or spinal loads (reaction forces). Thus, an increased anterior pelvic tilt may be recognized as a specific defence mechanism optimising the ergonomics of the musculoskeletal system in the gravid females. Our investigations also revealed an increased pelvic anteversion accompanied by an increased lumbar curve. The study by [PERSON] et al. [30; 31] revealed an increase in the body mass as pregnancy progresses, reaching \(\sim\)11.5 kg in late gestation (Table 1). The increasing weight of the anteriorly growing fetus produces the anterior tilt of the pelvis, which, in turn, enhances increased lordosis adaptation [21]. According to the simulations, in the first variant, there was a \(\sim\)2\({}^{\circ}\) decrease in lordosis in late pregnancy (P3) as compared to the pre-pregnancy state (P0). However, considering both mass gain and an increased anterior pelvic tilt (Variant II), the authors observed a \(\sim\)4\({}^{\circ}\) increase in the lumbar curve. Our observations confirmed the findings of [PERSON] and [PERSON] [21], who reported an angular increase of 7\({}^{\circ}\) in lumbar lordosis from early to late pregnancy. However, there is also a documented reduction of lumbar lordosis during pregnancy [18; 52] and the lack of its changes [28; 53]. Such inconsistency in the direction of lordotic change could result from the selection of women at different stages of pregnancy or from different measurement methods [28]. An increase in lumbar lordosis would generally be considered a biomechanically preferable accommodation of the spine to assist in absorbing greater vertical shock loading [54]. According to [PERSON] et al. [55], increased lordosis during advanced pregnancy stems from a compensatory backward lean to improve balance thanks to increased abdominal mass. As a possible explanation, others identified the muscular imbalance caused by overstretched weak abdominal muscles and strong back muscles [8; 56]. In addition to these physical changes, the hormonal effects of relaxin, progesterone and oestrogen lead to increased ligamentous laxity, compromising joint stability [1]. Therefore our simulation focused on analyzing the forces of the chosen muscles stabilizing the lumbopelvic complex.