## 4 Discussion
### An Overview of the Research Methodology  
In the current literature, there is no complete consensus on the methodology for conducting studies of this type and how to collect data. Only three authors [19; 20; 25] decided to conduct a complex analysis that included kinematic parameters from three-dimensional motion-capture systems, ground reaction forces, and EMG signals. However, the most commonly used duo is the measurement of ground reaction forces combined with EMG [19; 20; 24; 25]. It makes it possible to observe how perturbation-induced CoP displacement affects muscle activity. A literature review shows that pulling [19; 20; 21; 23; 25] is a more frequently selected method for inducing perturbations during free-standing, than pushing [15; 24]. This inconsistency cannot be explained based on the authors' argumentation, since they did not specify why they chose this type of perturbation rather than another. However, it is reasonable to assume that this was for practical reasons since it is easier to construct a pulling system than a pushing one. It remains unclear whether the responses to pulling and pushing are the same, because, to date, no work has appeared utilizing both mechanisms to induce perturbations, which leaves space for further research. The situation is similar with the place of perturbation application. Most authors chose the pelvis [5; 15; 19; 20; 21; 23; 25], but none justified this decision. One may think it was motivated by its proximity to the body's center of gravity. However, the exact differences in responses to perturbations applied to the pelvis compared to the shoulder girdle are still unknown, and hence, it is not known whether they can be used interchangeably. [PERSON] et al. [15] were the only ones to conduct a study of the pelvic and shoulder rim perturbation responses. Perturbations were induced by pushing. For each condition, a different force (108.7 \(\pm\) 1.22 N vs. 122 \(\pm\) 1.3 N) was used for the shoulder and pelvis, respectively. They showed that displacements of body segments were by motion from proximal to distal. After shoulder and pelvis perturbations, the trunk had the highest excursion and moved before the lower extremities. The direction of trunk motion depended on its location. When the perturbation was at shoulder level, both the legs and the torso moved in the same direction as the perturbation, before returning to the vertical position. In response to the pelvic perturbation, the legs moved in the same direction as the disturbance, while the trunk moved on the opposite side. In addition, significantly higher CoP shifts were observed for perturbations at the pelvic level relative to the shoulders (12.5 \(\pm\) 0.2 cm vs. 11.6 \(\pm\) 0.2 cm). CoM displacements were much smaller in magnitude (6.4 \(\pm\) 0.2 cm vs. 6 \(\pm\) 0.1 cm) for the pelvis and shoulders, respectively. Based on this single paper, one may see that there are differences in reactions between pushing via pelvic and shoulder girdles. These findings may use confirmation by other authors. Yet, the question remains about the existence of such differences to pulling perturbations. The somatosensory system response may be different for pulls and pushes. In the reviewed papers, pulls occurred mainly with waist belts, which acted over a large area (the waist circumference). Pushes, on the other hand, were targeted and performed with a stick or bar. In our opinion, this stimulus setting would be significant in responding to this type of perturbation. However, none of the authors discussed this phenomenon. The neuronal activity of the left and right hemispheres of the brain is known to influence whether the left or right hand is dominant over the other [31]. Therefore, a similar relationship seems likely to occur when responding to induced perturbations. Unfortunately, none of the authors examined differences between responses to perturbations on the dominant and non-dominant (left/right) sides.