## 4 Discussion
Conversely, in the vertical direction, the delay of the treadmill during the Initial Contact phase exhibited the highest SampEn values (for \(m\) = 4, 6, 8, and 10), indicating irregularity. For \(m\) = 2, the highest irregularity occurred during treadmill acceleration in the Pre-Swing phase. Regarding the medial-lateral direction, the highest irregularity in CoM displacement was observed due to treadmill belt deceleration during the Mid-Stance phase, specifically for \(m\) = 2 and 4. However, other \(m\) values increased CoM irregularity during treadmill deceleration in the Pre-Swing phase. It is important to note that when examining extreme values, the highest irregularity (maximum value of SampEn) was observed for the CoM time series involving acceleration-type perturbations during the Pre-Swing phase, calculated for \(m\) = 2 in the vertical direction. Conversely, the highest order (minimum value of SampEn) was for the CoM signal for the anterior-posterior direction during the deceleration perturbation in the Pre-Swing phase (also calculated for \(m\) = 2). According to a study by Sloot et al. Sloot et al. (2018), the above results are adequately confirmed. Sloot et al. Sloot et al. (2018) were among the few researchers who conducted experiments utilizing identical equipment and settings, operating under conditions similar to those presented in this paper (perturbation intensity and treadmill-based accelerations and decelerations). The authors demonstrated that the applied perturbations appeared to have a limited effect on the gait pattern. Even at the highest intensities, the observed impact resulted in less than a 4\({}^{\circ}\) knee flexion or extension. Furthermore, there was a decrease of less than 4% in stride length and less than a 3.5% change in both stride time and stance phase, some of which were inherent to the perturbation itself. Notably, during deceleration, there was a 6.1% increase in step width. This adjustment might indicate an attempt to improve the base of support in the medial-lateral direction, compensating for the decrease in the anterior-posterior direction resulting from the deceleration Sloot et al. (2019). In contrast, during acceleration, step width remained unchanged, suggesting that the subjects were not affected by the perturbations. This study had several limitations, which also contribute value by showing the way for future research directions to complement the current approach. The first limitation refers to the way perturbations were triggered. They were applied solely on the left belt of the treadmill and occurred at intervals of every 10 s. The effect on gait regularity resulting from perturbations applied to the right treadmill lane or at different frequencies remains unclear. Consequently, in the case of young subjects, this setup allowed them to return to a typical gait pattern. Another limitation concerned the methodology used to calculate sample entropy. While different \(m\) values were considered, tolerance window \(r\) was not addressed. Ahmadi et al. [18] proposed a range of 0.1 to 0.3 times the standard deviation as potential \(r\) values. Unfortunately, this approach could significantly complicate the overall analysis. Additionally, a limitation exists in the exclusive analysis of the regularity of center of mass (CoM) displacements, while exploring trunk behavior might offer a more accurate determination. Readers may discover further limitations beyond those mentioned here; however, these will continually serve as new directions and sources of inspiration to enhance and complete this study and its approach.