## 4 Discussion
The aim of their study was to assess the variability of selected stabilographic parameters caused by increasing external load of a school backpack in children aged 10 years. The same backpack was used for the measurements, with its design allowing for a symmetrical distribution of the weight. The school backpack in the subsequent tests accounted for 0%, 5%, 10%, and 15% of the body weight of the examined person. The mean variability of the static analysis indices in the studied group of students ranged from 16.5% for the COP path length to 55.6% for the difference in the load distribution between the right and left feet. The mean variability of the parameters of dynamic gait analysis ranged from 4.7% for the left limb stride length to 39% for lateral symmetry. The authors found differences in load distribution in the standing position for the heel and forefoot areas, both in static and dynamic conditions. Increasing the load led to the differentiation of the arch parameters of the right and left feet with increasing pressure on the area of the forefoot and heel [23]. Similar results were obtained by [PERSON] et al., who analyzed the effect of backpack load on the magnitude of ground reaction force in children aged 6 to 13 years. The analysis also showed that after applying a backpack weight of 5.2 kg, the load on the forefoot area increased [24]. The examinations discussed in the present study showed that after loading with a backpack weighing 5 kg, the impact on the forefoot and toes area decreased. Another example of research dealing with the backpack effect is the study by [PERSON] et al., who analyzed the effect of a 4 kg backpack load on the gait pattern and postural sway. They examined a group of 12 children aged 7-10 years without any neurological or orthopedic problems. The load of the backpack, averaging 15% of the body weight, led to a decreased walking speed, shortened walking speed, shortened stride length, and an extended double-support phase. School backpacks weighing 4 kg caused changes in children's gait, muscle activity, posture, and stability. Due to the weight of the backpack, the center of mass shifted back, and the children's posture became less stable [9]. Similar results were also obtained by [PERSON] and [PERSON]. These scholars found that carrying a backpack weighing 15% of the child's body weight changes the position of the head and increases the normalized value of the anterior-posterior shear force. Increasing this force may lead to the development of postural disorders and defects [25]. In addition to heavy books and notebooks, students carry different personal items in their backpacks that increase the backpack's weight. Excess weight in the form of a schoolbag during this period of life changes the body posture of school-age children. A partial solution to the problem of too heavy a backpack may be the introduction of digital textbooks by schools. In their 3-year research, [PERSON] et al. showed that the arch of the feet in children shifts towards the physiological norm with age, and the relationship with body weight and height and the weight-height index (BMI) is minimal [26]. Similar results were obtained by [PERSON] et al., who analyzed the differences in the arch of the foot and body balance in three age groups of boys training football. It was observed that the total length of the longitudinal arch of both feet in the boys studied tended to flatten in direct proportion to their age. There was also a statistically significant increase in body balance with age [27]. The above results are consistent with the results contained in this paper and another work by [PERSON], in which the same research methodology was used in a group of children aged 8 to 10 years [28]. Comparison of both papers shows that there are fewer statistically significant differences observed with age, both in the parameters related to the load on the plantar side of the foot and body balance. Based on the previously mentioned studies by [PERSON] et al.