## 4 Discussion
Greater inclination angles in the frontal plane were previously reported as a good indicator by which to distinguish elderly fallers from non-fallers [38]. In contrast to [PERSON] et al. 's results [47], we did not find greater inclination angles in the PD group compared to controls. Our post hoc analysis revealed a greater COM-COP inclination angle for the CON group at SDS2 (second double-limb support for step two). This finding could suggest that the healthy controls were more dynamically moving their center of mass as they transitioned to second double-limb support and gait continuation during the obstacle-crossing task. Our finding of no significant difference in inclination angle at other Events during obstacle-crossing may be due to the relatively mild disease stage for our PD cohort, or it could indicate a conservative strategy used by individuals with PD to limit the excursion of their COM. Differences in inclination angle findings between our study and the [PERSON] paper may be due to methodological differences as we examined the inclination angle at different Events in the two-step sequence gait cycle. Our findings support the contention that analyses of MOS metrics, specifically COM-COP M/L, COM-XCOM M/L, and UMAX-XCOM M/L, are useful to assess dynamic stability during walking and obstacle-crossing and may provide insights into postural control strategies used by individuals with PD during this challenging gait task. This present work has laid a foundation from which to elucidate postural instability in persons with mild PD during this task. In addition to the small sample size, our study has several additional limitations that may have influenced our results. Subjects were instructed to target the first and second force plate with their steps during the obstacle-crossing task, which may have affected their gait strategy or step adjustments as they completed the task. Since most individuals, especially those with a fear of falling, use vision to safely clear an obstacle, we encouraged the use of vision throughout the task. Participants in our small PD cohort had mild disease severity, no freezing of gait, and no previous fall history (an exclusion criterion for the study was two or more falls in the past year). Therefore, our findings cannot be generalized to individuals with moderate-to-severe PD disease or frequent fallers who have more marked gait and balance deficits. Further research is warranted to examine dynamic balance control during obstacle-crossing in those in the middle-to-late disease stage and those with freezing of gait characteristics, as these individuals are at a much higher risk of falling during challenging gait tasks. [PERSON] et al. found that individuals with PD with greater disease severity demonstrated greater postural instability during an obstacle-crossing task than those with mild PD [66]. Understanding the mechanisms underlying this postural instability may help to guide treatment strategies for fall risk reduction in those with PD. Finally, only a few studies have utilized MOS variables to characterize dynamic balance control during obstacle-crossing in PD [29; 47; 66]. Identifying which variables are the most meaningful and sensitive indicators of postural instability is needed in future studies. [PERSON] et al. [69] and [PERSON] et al. [70] have suggested that [PERSON] et al. 's [48; 50] original work on the margin of stability for static postures may need to be modified for use in gait, so additional work is needed in that area. Furthermore, [PERSON] et al. [71] recently reported that body size was correlated with the margin of stability in both mediolateral and anteroposterior directions. They proposed that expressing stability margins as an impulse should be considered in future work, which may be necessary for additional research involving individuals with PD.