## 2 Materials and Methods
### Measurement Protocol and Perturbation Specification  
The kinematics and kinetics parameters of the perturbed gait took place in a Gait Real-time Analysis Interactive Lab (GRAIL, Motek Medical B.V., Amsterdam, The Nether \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Groups** & **Age [Years]** & **Body Weight [kg]** & **Body Height [cm]** \\ \hline Young (\(\Upsilon\): \(n\) = 21) & 21.38 \(\pm\) 1.32 & 61.38 \(\pm\) 6.48 & 165.9 \(\pm\) 4.53 \\ Old (O: \(n\) = 21) & 67.75 \(\pm\) 4.7 & 73.36 \(\pm\) 11.06 & 161.5 \(\pm\) 5.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the participants (mean \(\pm\) SD). lands). The GRAIL includes an instrumented dual split-belt treadmill with pitch and sway (1000 Hz), a motion capture system (Vicon Metrics Ltd., Oxford, UK) (100 Hz), three video cameras, synchronized virtual reality environments, and a ceiling-suspended safety harness. The Human Body Model 2 (HBM2), based on 25 reflective markers (Figure 1A), captured the participants' motions. The D-Flow software v. 3.26 (Motek Medical B.V., Amsterdam, The Netherlands) was used for perturbation triggering and data acquisition. In this study, participants walked on a dual-belt treadmill at a fixed speed while wearing comfortable athletic footwear. The walking speed was set at 1.2 m/s for young adults and 1.0 m/s for older adults. This distinction was necessary because older participants tended to transition into a running gait at 1.2 m/s, while 1.0 m/s was too slow for younger adults and led to an unnatural gait pattern. Moreover, it is worth emphasizing that a review by [PERSON] et al. [6] addressed the topic of gait speed when studying the effect of perturbations on gait parameters, comparing older and younger groups. They showed that in most cases, the speed was self-selected and consistently differed between the two groups [23; 24]. Although the treadmill perturbations were not intended to induce falls, all participants wore a ceiling-mounted safety harness for protection (Figure 1B), which allowed the full range of motion and did not restrict movement. Unexpected mechanical perturbations were applied to the left treadmill belt during the toe-off phase of gait. Each participant completed  
Figure 1: **(A)** The front, side, and rear views of the marker configuration used in the Human Body Model are presented. The green markers indicate the anatomical reference points used to define the skeleton during the initialization process, as follows: XYPH—the sixphoid process of the sternum, LASIS and RASIS—the left and right anterior superior iliac spine, LPSIS and RPSIS—the left and right posterior superior iliac spine, LGTRO and RGTRO—the left and right greater trochanter, LLEK and RLEK—the left and right lateral epicondyles of the knee, LLM and RLM—the left and right lateral malleolus of the ankle, LMT5 and RMTS—the left and right fifth metatarsal, LTOE and RTOE—the tips of the left and right toes, LHEE and RHEE—the left and right heels. Additional markers, which are technical markers providing redundancy and robustness for the model, included the following: STRN—the jugular notch of the sternum, NAVE—the navel, T10—the 10 th thoracic vertebra, SACR—located midway between the LPSIS and RPSIS points, FLTHI and FRTHI—the left and right lateral thigh (positioned halfway between the greater trochanter and the lateral knee epicondyle), LATI and RATI—the left and right lateral shank (placed halfway between the lateral knee epicondyle and the lateral malleolus); **(B)** Ceiling-mounted safety harnesses. Perturbations were only applied to the left leg, which was the non-dominant, supporting limb for all subjects [25]. This decision was based on the assumption that applying perturbations to the non-dominant leg may pose a lower risk of imbalance or falls, particularly in older adults. While existing literature often focuses on the right leg and does not consistently validate left-right symmetry, some studies, such as those highlighted in the review by [PERSON] et al.