## 4 Discussion  
[PERSON] et al. [33] reported that most falls are caused by trips or slips, and older adults are at greater risk of falling than young adults. Both tripping (fall direction-forward) and slipping (fall direction-backward) lead to disruption of the gait pattern [34]. When a person trips, their foot unexpectedly encounters an obstacle or uneven surface, often resulting in  
Figure 12: Percentage of strategies adopted for perturbed limb (left), and reactive limb (right) in young and older group. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & \multicolumn{2}{c}{**Ankle [Nm/kg]**} & \multicolumn{2}{c}{**Knee [Nm/kg]**} & \multicolumn{2}{c}{**Hip [Nm/kg]**} \\ \cline{2-7}  & \(\mathbf{min}-\mathbf{df}\) & \(\mathbf{max}-\mathbf{pf}\) & \(\mathbf{min}-\mathbf{flx}\) & \(\mathbf{min}-\mathbf{df}\) & \(\mathbf{max}-\mathbf{pf}\) & \(\mathbf{min}-\mathbf{flx}\) \\ \hline  & \multicolumn{7}{c}{**Young**} & & & \\ \hline I–n & \(-\)0.18 (mean) & 0.13 (min) & \(-\)0.29 (min) & 0.89 (max) & \(-\)0.24 (min) & 1.13 (min) \\ II–n & \(-\)0.05 (min) & 0.73 (max) & \(-\)0.44 (max) & 0.7 (mean) & \(-\)0.67 (max) & 1.29 (mean) \\ III–n & \(-\)0.23 (max) & 0.14 (mean) & \(-\)0.38 (mean) & 0.65 (min) & \(-\)0.53 (mean) & 1.44 (max) \\ \hline  & \multicolumn{7}{c}{**Elderly**} & & & \\ \hline I–n & \(-\)0.1 (mean) & 0.12 (mean) & \(-\)0.28 (max) & 0.19 (mean) & \(-\)0.26 (mean) & 1.28 (max) \\ II–n & \(-\)0.22 (max) & 0.48 (max) & \(-\)0.27 (mean) & 0.57 (max) & \(-\)0.44 (max) & 1.16 (mean) \\ III–n & 0.04 (min) & \(-\)0.08 (min) & \(-\)0.08 (min) & 0.02 (min) & \(-\)0.09 (min) & 0.23 (min) \\ \hline \end{tabular}
\end{table} 
Table 5: Differences between the means of the extreme values adopted in Strategies I, II, III, and those for gait on a treadmill without perturbation (n) for kinetic parameters of the reactive limb with the indication of their spread (min, mean, max) relative to each other. For negative values, the comparison was for absolute values. a sudden change in the foot alignment. Slipping happens when the friction between the foot and the walking surface is insufficient, causing the foot to slide [35]. Individuals often make rapid changes in their kinematics and kinetics to recover from trips or slips [35; 36; 37]. It involves a broad range of compensatory and protective movements. Some common strategies include: stiffening the muscles and joints to maintain stability and minimize movement upon impact, exaggerated stepping or arm movements, extending the arms or legs in an attempt to reach out and grab onto something for support or cushion the fall, or relaxing the body to reduce the risk of injury by allowing the body to absorb the impact more smoothly [1; 38]. Several methods have been employed in previous studies to evoke tripping-like responses during gait, including treadmill belt acceleration [39; 40; 41; 42], ankle rope pull (break and release) [43], and dropping an obstacle on the belt [44]. Although treadmill belt acceleration does not incorporate a physical obstacle, it simulates the overall forward rotation of the trunk and stepping [41; 45]. It is crucial to understand how treadmill belt acceleration affects gait parameters to assess how overloaded the body is during perturbation recovery. There are few studies in which perturbations are induced during the Pre-Swing phase. For instance, [PERSON] et al. [46] examined both types of anterior-posterior perturbations--belt acceleration and deceleration--during this critical phase. In contrast, [PERSON] et al. [6] focused exclusively on belt deceleration perturbations during the Pre-Swing phase. Similarly, perturbations during this phase were also studied by [PERSON] et al. [47] and [PERSON] [28], who extended their analysis to include responses during the Initial Contact and Midstance phases as well. The present study aimed to analyze the effects of external perturbations involving the acceleration of the left treadmill belt during the Pre-Swing phase on gait kinematics and kinetics in both young and older adults.