## 5 Conclusions  
The purpose of this study was to determine if there were differences in dynamic postural control during self-paced walking and obstacle-crossing between persons with mild-to-moderate PD and healthy controls. This study examined margin of stability variables to characterize dynamic balance control during the obstacle-crossing task and secondarily examined spatiotemporal gait parameters during the two-step sequence for obstacle-crossing. Our findings may be reflective of postural instability at key transition points during stance while walking and obstacle-crossing in individuals with PD. Greater medio-lateral excursion of COM in the PD group as compared to healthy controls may be a red flag for lateral fall risk during this task; however, further research is needed across PD disease severity. The results of our study may reflect a conservative strategy employed by individuals with PD to minimize postural instability during this challenging gait task. We suggest that the margin of stability metrics may be useful for the biomechanical analysis of dynamic postural control to elucidate mechanisms underlying postural instability in individuals with Parkinson's disease.  
Conceptualization, G.A., M.A., C.D. and D.L.; methodology, G.A. and L.H.; software, G.A. and L.H.; validation, G.A., M.A., C.D. and D.L.; formal analysis, G.A., C.H., D.W.Z., L.H., M.A., C.D. and D.L.; investigation, G.A.; resources, G.A.; writing--original draft preparation, M.A., C.D. and D.L.; writing, G.A., C.H., L.H. and D.W.Z.; visualization, G.A., C.H., L.H., D.W.Z., M.A., C.D. and D.L.; supervision, G.A., C.H. and D.W.Z.; project administration, G.A., M.A., C.D. and D.L.; funding acquisition, G.A. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement**: The study was conducted under the Declaration of Helsinki and approved by the Institutional Review Board of Grand Valley State University (protocol code 17-267-H; date of approval: 07_26_2017) for studies involving humans.  
**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.  
**Data Availability Statement**: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to authors' change in institutional status and inability to access data from the biomechanics laboratory.  
**Acknowledgments**: The authors of this study would like to thank all the participants and their families who volunteered their time to contribute to the work of this study. We also acknowledge [PERSON] (Hromi LLC), a biomedical illustrator.  
**Conflicts of Interest**: The authors declare no conflicts of interest. The funders (all resources were obtained internally) had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.  
Appendix A Mean (Standard Error) for Spatiotemporal Gait Parameter during Normal Self-Selected Walking  
The level walking data shown in the table below were collected before the experimental walking procedures, i.e., gait termination, turning, and obstacle-crossing. However, since we were most interested in only statistically analyzing the spatiotemporal gait parameters related to obstacle crossing we provide the level self-selected spatiotemporal gait parameters only as additional descriptive demographic data.