## 4 Discussion
### Kinematic and Kinetic Differences between Older and Younger Adults  
It has been observed that older adults display greater trunk flexion during balance recovery reactions compared to younger adults [33; 34]. An unexpected surface drop leads to a reflex overaction of the back muscles, resulting in greater trunk flexion [35]. This trunk flexion following an unexpected surface drop poses a challenge to the dynamic stability of forward-directed locomotion [36]. The trunk represents a significant portion of the total body mass, accounting for more than 50%, and its position above the ground is relatively high [37]. Thus, excessive trunk flexion during balance recovery negatively affects balance control, potentially increasing the risk of falls [38]. We observed that older adults exhibited greater hip extension of the leading (perturbed) leg during the SLS phase compared to younger adults. This finding suggests that older adults employ biomechanical strategies that involve increased hip extension to counteract the disadvantage of greater trunk flexion for better balance control. From the biomechanical perspective, the greater trunk flexion observed during the single-limb stance phase may lead to higher passive tension on the hamstring muscles, and this increased tension could result in greater activation of the hamstring muscles in older adults [39]. Increased trunk flexion while walking is also associated with an increase in mechanical energy demand generated by hip extensor torque [40]. Trunk flexion alters the alignment and position of the body's CoM, shifting it forward [41]; therefore, hip extensor muscles are required to generate more torque to counterbalance the forward movement of the body's CoM and maintain postural stability [42]. Given that the increase in hip extension serves as a compensatory mechanism to counteract the anterior pelvic tilt associated with trunk flexion [42], this prominent hip extension can be considered as a movement strategy employed by older adults to improve the dynamic stability of forward-directed locomotion, which is closely related to the risk of falls. However, the higher amplitude of CoM oscillation in the vertical direction observed in older adults during the balance recovery process in the SLS phase indicates that greater trunk flexion and hip extension can negatively impact vertical balance, leading to increased postural sway in the vertical direction. In addition, increased demand for AP balance control, resulting from the control of greater trunk flexion [43], may disrupt ML balance control during the balance recovery process. When the leading (perturbed) leg steps onto the surface drop platform, the CoM shifts towards the perturbed leg and continues moving toward the lateral side of the perturbed leg (see Figure 3A). To maintain postural stability in the ML direction, it is essential to reposition the CoM radially from the lateral side towards the inside of the base of support, which is the center of the body in the frontal plane. This L-to m displacement of the CoM is important in preventing potential loss of balance in the ML direction [44]. We found that older adults exhibited a delayed latency in initiating the L-to m displacement of CoM compared to younger adults following perturbation onset. This delayed latency was found to be positively correlated with the degree of trunk flexion. In other words, greater trunk flexion means that more time is required for the CoM to return from the lateral to medial side during the SLS phase, which is crucial for maintaining ML balance control. This finding provides important insights into the relationship between trunk flexion control in the AP direction and hip abduction for ML balance control. Younger adults who exhibited relatively less trunk flexion during the balance recovery process demonstrated earlier initiation of L-to m CoM displacement with greater hip abduction torque when compared to older adults.