## 2 Materials and Methods  
### Musculoskeletal Model of a Pregnant Woman and Simulation  
The musculoskeletal system during pregnancy was developed using the AnyBody Modeling System environment (AnyBody Technology Inc., Aalborg, Denmark), containing the entire body model (Standing Model). The model comprises 69 rigid bodies representing the skeletal system, connected using kinematic pairs having the number of degrees of freedom corresponding to the mobility of individual joints. The muscular system is represented by 1000 linear elements reflecting muscular actions. The multisegment model of the lumbar spine is composed of the model of intra-abdominal pressure and 7 rigid bodies, i.e., 5 lumbar vertebrae L1-L5, pelvis and a part of the thorax; the model has 18 degrees of freedom [43]. In the AnyBody Modeling System software, the identification of loads in the musculoskeletal system consists of solving the inverse dynamic problem and, next, using static optimisation, estimating individual muscular forces [36; 43]. The applied model of the musculoskeletal system (Standing Model) has been validated many times by the authors of other works [44; 45; 46]. The pregnant female model (Figure 1) was developed by providing the Standing Model with changes in mass distribution and the position of the pelvis due to pregnancy found in the literature [18; 30; 31; 42]. The model in subsequent trimesters of pregnancy took into account: anthropometric data (body height, body mass, BMI, pelvic width), the distribution of masses of individual body segments, the additional mass appearing within the abdomen during pregnancy, change in the circumference of the abdomen, and the arrangement of individual body segments, especially the position of the pelvis in the sagittal plane. Considering the above-mentioned data, a model was developed for a woman before pregnancy and during subsequent trimesters of pregnancy, i.e., for the following weeks of pregnancy: 12, 25, 36. As pregnancy entails a non-uniform increase in body mass, the developed model included a method enabling the scaling of masses of body segments based on works by [PERSON] et al. [42]. The functions which scale masses of individual segments during successive weeks of pregnancy were defined. Based on data by [PERSON] [18], modeling the additional mass appearing within the abdomen during pregnancy, i.e., fetus mass (25% of additional mass), placenta mass (5.5% of additional mass), the mass of amniotic fluids (6.5% of additional mass) and the uterus mass (8% of additional mass) (Figure 1), a force vector corresponding to additional kilograms \"generated\" in given trimesters of pregnancy was applied to the centre of the rigid buckle segment [47]. The study takes into account that the center of mass of the extra pounds is approximately in the center of the abdominopelvic cavity. Therefore, the magnitude of the applied force is reduced proportionally to the increasing arm of the acting force (the distance from the center of the abdominal-pelvic cavity to the position of the buckle segment) with an increased abdominal circumference in subsequent trimesters of pregnancy. An increase in the midabdominal circumference during pregnancy was modeled by extending the length of the transverse abdominal muscle. The midabdominal circumference in the 2 nd trimester of pregnancy was increased by 8% and in the 3 rd trimester by 20% in relation to the initial data (after [PERSON] [18]). Only a few studies were found that analyzed the changes in the body's posture in the standing position during pregnancy [18; 48; 49]. Still, their results do not indicate a clear change in the position of the body segments. More often, scientists analyzed the change in gait of pregnant women in subsequent trimesters of pregnancy [18; 30; 31]. The results of their research are similar. Due to the lack of studies showing unequivocal changes in the position of body segments in a standing position during pregnancy, it was decided to use the data from the gait tests of women in subsequent trimesters of pregnancy.