## 4 Discussion
Increased medio-lateral excursion of the COM and XCOM is a clinically relevant finding as it may be indicative of increased fall risk in the PD group. [PERSON] et al. reported that elderly fallers have significantly greater mediolateral COM motion when obstacle-crossing compared to healthy elderly controls [38]. Lateral instability during this gait task is a significant concern as lateral falls result in a greater incidence of hip fractures in both persons with PD and elderly fallers [10]. Several factors in PD may contribute to postural instability during the obstacle-crossing task; individuals with PD have bradykinetic(slowed) and hypokinetic (reduced amplitude) limb movements and anticipatory postural strategies [12; 18; 27; 28], which may adversely affect dynamic postural control and gait adaptability for this task. Additionally, individuals with PD have impaired visual-spatial, limb proprioceptive, and sensorimotor integration for balance [21; 22; 23; 67; 68]. The obstacle-crossing task requires spatial processing for successful task performance. Compensatory strategies to control balance may be employed to optimize foot clearance for both the leading and trailing limbs. A strength of our study design was that we examined MOS metrics throughout the two-step obstacle-crossing sequence as dynamic balance control demands are high throughout this sequence, i.e., not just during the lead limb stepping over the obstacle. Trips and loss of balance can occur throughout the two-step cycle as the individual attempts to clear the obstacle with the first step, control single-limb stance during the first step, and then maintain balance while clearing the obstacle with the trailing limb (second step). We analyzed the interactive effects of Event (in this two-step sequence) and Condition (PD vs. CON) to shed light on the differences in dynamic postural control between the two groups across the obstacle-crossing task sequence. Our results revealed significant differences for the COP-COM and COP-XCOM variables for both Event and Event interaction with Condition (PD vs. CON), whereas significant differences for UMAX-XCOM were only found for Event. These findings indicate that individuals with PD make different adjustments to control balance during this obstacle-crossing task than those of healthy controls. Previous research supports our finding that there are spatiotemporal gait changes seen in those with PD compared to healthy controls during the obstacle-crossing task, including slower gait velocity, shorter steps, increased step width, and slowed obstacle-crossing speed [29; 40; 41; 42]. These conservative strategies may be an attempt to reduce postural instability and fall risk during this task. Our findings provide further support for there being differences in dynamic postural control strategies in those with mild PD and healthy controls. These differences are seen at critical Events in the two-step obstacle-crossing sequence. We found significant interaction effects between Step and Condition for swing time, stance time, and step length. Post hoc analysis revealed that there was a significantly shorter step length for the second step in PD compared to the CON group, which was consistent with the findings by [PERSON] et al. [47]. One possible explanation for this finding is that individuals with PD may shorten step length on their trailing leg to control balance (and reduce COM A/P excursion) in this recovery step. It is uncertain whether the difference of 0.08 m (3.15 inches) is clinically meaningful. This information may provide insights into strategy training to enhance postural stability during this challenging gait task in people with PD. One other MOS metric that we examined was the COM-COP inclination angle, which is a spatial angle that accounts for both the A/P and M/L movement of COP relative to COM. We found a significant interaction effect for Event and Condition (PD vs. CON).