## 4 Discussion  
This study aimed to explore biomechanical variables reflective of dynamic postural control, specifically margin of stability metrics using 3D motion analysis, during walking and obstacle crossing in persons with Parkinson's disease. Our study is also unique in that we examined gait and balance variables during the complete two-step sequence (leading limb and trailing limb step cycles) during this obstacle-crossing task. The primary purpose of this study was to determine if there were differences in dynamic balance control during self-paced walking and obstacle-crossing between persons with mild-to-moderate PD and age-matched controls. Our objective was to shed light on postural control strategies and compensations during walking with obstacle-crossing in the PD group as compared to healthy controls. Tripping or loss of balance during walking and obstacle-crossing is a common cause of injurious falls in persons with PD [34]. Falls can have costly consequences in PD, leading to fear of movement, activity restrictions, and reduced independence. Injurious falls often result in hospitalization, admission to nursing homes, and increased mortality [3; 6; 10]. Falls in persons with PD occur during complex gait tasks that require increased attention to the task and divided attention between gait adaptability and control of balance. Examination of dynamic balance control during obstacle-crossing in persons with PD has been inadequately studied in previous research, which has mainly focused on spatiotemporal gait parameters and primarily examined single leading-limb gait parameters. We sought to gain insights into how individuals with PD control dynamic balance during this challenging gait task, which may provide information pertaining to strategies to minimize falls in this population. Qualitative analysis of the MOS metrics over the two-step sequence during obstacle-crossing revealed that the COM remained medial to the BOS throughout the gait task for both the PD and CON groups. This finding is consistent with the results reported by [PERSON] et al. [66] and may reflect a conservative strategy to minimize the risk of a lateral fall during this challenging gait task. Individuals with PD, however, demonstrated significantly greater mean values for COM-COP M/L at first double support for step one and COP-XCOM M/L at second double support for the second step as compared to controls. These time points in the two-step obstacle-crossing sequence represent critical transition points as the trailing limb moves over the obstacle and the individual continues forward gait progression. These findings may indicate a tendency for postural instability at these transition points during walking and obstacle-crossing in people with PD. Increased mediolateral excursion during these points in the step cycle may also reflect the possible use of a compensatory strategy to optimize the safe clearance of the trailing foot over the obstacle. Our findings agree with previous research [29; 66]. [PERSON] et al. reported that medio-lateral excursion was 21% greater in the PD group than in the control group during obstacle-crossing, and [PERSON] et al. [29] reported that this M/L excursion was 36% greater in their PD cohort compared to controls. Consistent with our findings, [PERSON] et al. [29], who also examined COM M/L across Events in the two-step sequence, reported significantly increased M/L COM excursion in the PD group at the double-limb support Event for both the leading and the trailing limb. It was interesting that we did not find significant differences in COM-COP M/L or COM-XCOM M/L between groups at the midstance (MST1, MST2) points in the two-step sequence during obstacle-crossing, which is the most unstable point in the gait cycles (steps one and two). This finding may indicate that individuals in our PD cohort who were in the early disease stage were successful at controlling balance during the single-limb stance demands of this obstacle-crossing task.