###### Abstract
Higher obstacles pose a greater motor control and balance challenge for persons with PD as they approach the obstacle more slowly, with a wider BOS, and have a longer step duration over the obstacle; therefore, they spend an increased time in single-limb support with greater mediolateral instability compared to controls [42; 46]. These gait adaptations can lead to increased trip risk or loss of balance in persons with PD. The limited published research examining obstacle-crossing in persons with PD has focused primarily on gait spatiotemporal and kinematic measures. [PERSON] et al. [29] examined dynamic postural stability during obstacle crossing by analyzing COM, COP, and the margin of stability (MOS), i.e., the linear distance between the COM and BOS. It is notable that during locomotion, the COP is often defined as the BOS. These researchers reported that individuals with PD had decrements in gait performance and employed gait adaptations to reduce the mechanical demands to enhance their postural stability during this task. They also reduced their range of COM displacement in the anteroposterior (A/P) direction, demonstrated a larger range of COM displacement in the mediolateral (M/L) direction, and generally showed a reduced mean distance between COM and COP compared to healthy controls. At all points in the gait cycle, individuals with PD demonstrated significantly increased MOS compared to controls [29]. These findings reflect a conservative strategy to maintain dynamic balance during this challenging gait task. However, individuals with PD demonstrated reduced obstacle-crossing speed and increased single-limb stance time, which, combined with an increased M/L COM displacement, could predispose to increased fall risk, especially in the lateral direction. [PERSON] et al. [47] examined COM-COP inclination angles and the rate of change of inclination angles to reflect dynamic balance control during obstacle-crossing in persons with mild PD compared to age-matched controls. They found that the inclination angles and the rate of change of these angles in both the sagittal and frontal planes were greater in the individuals with PD compared to controls, and greater when the less-involved limb was leading compared to the involved limb (defined as the limb with initial motor symptoms), reflecting greater difficulty with balance control when the less involved limb was leading. [PERSON] and colleagues interpreted their findings by noting that the greater the inclination angle during this gait task, the more effort was needed to control balance; therefore, even individuals with mild PD demonstrated impaired dynamic stability when crossing obstacles. The theoretical model frequently utilized in clinical research to examine dynamic balance during gait, i.e., to examine the relationship between the COM and the COP, is the inverted pendulum model [48; 49; 50; 51] (Figure 1). The COM represents the line of gravitational pull on the body and the COP represents the position of the vertical ground reaction and is thought to represent the neurological response to changes in the position of the COM. Generally, when the medial horizontal distance from the COM to the COP is small, balance is expected to be maintained. If the COM is aligned with the COP but has a velocity directed outward, it may not be possible to maintain balance. On the other hand, balance may be maintained if the COM is beyond the BOS, but the COM velocity is directed inward [48; 49; 50; 51; 52]. The magnitude of movement of the COM in the mediolateral (M/L) direction is an important predictor of falls and has been identified as a more sensitive measure of postural instability during gait than spatiotemporal gait parameters [38]. [PERSON] et al. [48; 51] proposed a more nuanced variable to better understand the analysis of dynamic balance called the extrapolated center of mass (XCOM), which accounts for the position and velocity of the COM. Therefore, the trajectory of the COM is extrapolated in the direction of the COM velocity [48; 51].