## 4 Discussion
The left belt was deliberately chosen for perturbation, as all participants were left-leg non-dominant, and targeting the non-dominant side was considered a safer approach. Notably, a review by [PERSON] et al. [48] highlighted that most studies have predominantly applied perturbations to the dominant (typically right) limb. Similarly, [PERSON] et al. [6] reported that the majority of previous research has perturbed the right limb. Therefore, the decision to perturb the non-dominant limb in the present study represents a distinctive and meaningful contribution to the existing literature. The first part of the study showed statistically significant differences for all kinematic and kinetic parameters between the treadmill gait of young and elderly subjects, which aligns with other publications [49; 50; 51]. During treadmill gait, the older individuals had significantly lower plantarflexion values, but not significantly lower maximum values of dorsiflexion ankle angle compared to those recorded in the young group. The older participants maintained significantly higher knee flexion in the support phase. During the swing phase, the knee flexion was significantly lower than that recorded in the young, resulting in a much smaller range of motion in this group. At the hip joint, the elderly group demonstrated significantly lower flexion and higher extension values compared to those noted in the young group. It is worth mentioning that the extreme lower limb torque values in treadmill gait were significantly lower in the elderly group. [PERSON] et al. [51] explained that these differences arise because older adults approach locomotion more cautiously, tending to stiffen their movements. Some explanations for this behavior can be found in a decrease in joint receptor activity, a reduction in stimulus processing abilities, and a deterioration of musculoskeletal viscoelastic properties. Additionally, it should be noted that the differences observed between groups are partly due to the significantly lower treadmill speeds in the older individuals' gait (1.2 m/s for young adults vs. 1.0 m/s for elderly participants). Despite different treadmill speeds between groups, the general shapes of kinematic and kinetic response curves were similar, though with significantly different magnitudes. In both age groups, three distinct recovery strategies were identified for both the perturbed and reactive limbs, primarily differentiated by vertical ground reaction force (vGRF) profiles. These were clustered using PCA and k-means methods, offering a novel classification approach not previously detailed in the literature. Statistical differences in parameter values and strategy frequency supported this categorization. Younger participants predominantly used Strategies I and II, whereas older adults favored Strategy II, possibly due to reduced proprioception or muscle power. Strategy I, more frequent in the young group, was characterized by joint angles and torques resembling unperturbed gait, indicating a more efficient and controlled recovery. In contrast, older adults relied less on this strategy, which may reflect age-related neuromuscular limitations. These findings provide insight into age-specific adaptations to perturbations and underline the need for individualized balance training interventions. In the elderly group, Strategy II was most frequently adopted, a surprising finding given that the extreme kinematic values were generally higher than in young subjects. However, Strategy II allowed moderate deviations from unperturbed gait, particularly for ankle dorsiflexion/plantarflexion, knee flexion, and hip extension torques, suggesting that older adults opt for responses that avoid functioning under extreme biomechanical conditions. For the reactive limb, Strategy I was most common in both groups (young: 33%; elderly: 41%). The young group then selected Strategy III (19%) and Strategy II (10%), while the elderly group showed a reversed order for Strategies II and III (16.67% and 14%, respectively).