## 4 Discussion  
During their education in Poland, the majority of students use paper textbooks. Each student has one set of books, and it is impossible to leave textbooks at school due to homework and the need to study for tests. Additionally, in many schools, there are no lockers for pupils where they could leave books that are not needed for the next day or studying at home. The weight of the backpack is additionally increased by personal items carried by students not related to classes. The longitudinal arch of the foot in humans was formed in phylogenetic development, along with the verticalization of the body posture. The longitudinal arch in vertebrates is only observed in humans. Furthermore, its structure is subject to factors influencing \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{2}{c}{**Cop-Bars (mm2)**} & \multicolumn{2}{c}{**1-Bars (mm2)**} & \multicolumn{2}{c}{**r-Bars (mm2)**} \\ \cline{2-7}  & **S** & **O** & **S** & **O** & **S** & **O** \\ \hline \multicolumn{7}{c}{boys and girls (\(n\) = 100)} \\ \hline \(\mathbb{X}\) & 198.74 & 187.30 & 57.10 & 55.16 & 76.65 & 74.49 \\ \hline SD & 179.01 & 163.48 & 108.78 & 77.99 & 134.06 & 101.04 \\ \hline Me & 142.29 & 135.23 & 26.54 & 31.38 & 30.08 & 41.294 \\ \hline Q\({}_{1}\) & 76,84 & 84.10 & 15.15 & 14.65 & 15.34 & 19.31 \\ \hline Q\({}_{3}\) & 230.78 & 227.34 & 64.60 & 60.96 & 82.06 & 93.43 \\ \hline P & 0.907 & \multicolumn{3}{c}{0.364} & \multicolumn{3}{c}{0.618} \\ \hline d & 0.12 & \multicolumn{3}{c}{0.01} & \multicolumn{3}{c}{0.02} \\ \hline \multicolumn{7}{c}{girls (\(n\) = 54)} \\ \hline \(\mathbb{X}\) & 170.46 & 204.23 & 53.34 & 56.29 & 74.41 & 76.54 \\ \hline SD & 147.14 & 183.31 & 119.26 & 68.99 & 134.98 & 116.70 \\ \hline Me & 115.50 & 150.55 & 23.42 & 34.48 & 26.94 & 31.88 \\ \hline Q\({}_{1}\) & 74.94 & 93.47 & 14.19 & 16.61 & 14.69 & 19.57 \\ \hline Q\({}_{3}\) & 196.91 & 228.03 & 46.52 & 73.48 & 76.96 & 90.48 \\ \hline P & 0.204 & \multicolumn{3}{c}{0.020 \({}^{\star}\)} & \multicolumn{3}{c}{0.341} \\ \hline d & 0.36 & \multicolumn{3}{c}{0.02} & \multicolumn{3}{c}{0.02} \\ \hline  & \multicolumn{3}{c}{boys (\(n\) = 46)} \\ \hline \(\mathbb{X}\) & 231.93 & 167.42 & 61.52 & 53.82 & 79.28 & 72.09 \\ \hline SD & 207.16 & 135.91 & 96.16 & 88.16 & 134.42 & 80.06 \\ \hline Me & 154.44 & 124.84 & 36.75 & 25.80 & 40.23 & 44.93 \\ \hline Q\({}_{1}\) & 81.75 & 75.65 & 17.90 & 13.41 & 18.30 & 18.90 \\ \hline Q\({}_{3}\) & 308.78 & 210.78 & 68.05 & 47.48 & 86.14 & 102.30 \\ \hline P & 0.097 & \multicolumn{3}{c}{0.222} & \multicolumn{3}{c}{0.880} \\ \hline d & 0.40 & \multicolumn{3}{c}{0.07} & \multicolumn{3}{c}{0.06} \\ \hline \(\mathbb{X}\)—average, SD—standard deviation, Me—median, Q\({}_{1}\), Q\({}_{3}\)—quartiles 1 and 3, 5—resting measurement, O—measurement with a backpack, p—significance level of Wilcoxon Matched Pairs Test, cop-bars—the total surface area of the body’s center of gravity; lbars—the surface area of the left foot, r-bars—the surface area of the right foot, \({}^{\star}\)—statistical significance, d—effect size. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Distribution of the results of the stabilographic tests at rest and after loading with a backpack in the study group and the statistical significance of differences measured with the Wilcoxon Matched Pairs Test. the course of ontogenetic development. Significant changes in the size of the foot arch are attributable to the individual's body weight [15; 16; 17]. During this cross-sectional study, the authors investigated the relationship between the use of a backpack and changes in the distribution of forces on the plantar side of the foot and body balance in a group of children aged 11 to 14 years. Analysis of the results showed that wearing a backpack affects the arches of the foot and body balance. The main changes in the parameters tested after the application of external load were observed in the entire study population.