###### Abstract
Anticipatory postural control and the ability to adapt to changes in postural demands in more complex mobility tasks such as walking and navigating obstacles deteriorate as the disease progresses, and this is a key intrinsic risk factor for falls in PD [29]. One hallmark functional limitation in persons with PD is the progressive gait decline with disease progression. Even in the early stages of PD, biomechanical changes in gait are evident, such as slowed speed, reduced step length and arm swing, and impaired gait automaticity [13; 30; 31; 32]. In a biomechanical analysis of gait kinematics in participants with PD as compared to healthy age-matched controls, [PERSON] et al. [33] reported large variability among the PD cohort, but common characteristics seen were reduced peak joint angles, reduced or absent heel rocker at foot strike, reduced ankle power generation at push off, increased stance phase knee flexion, and reduced hip extension during midstance. These findings were variable based on disease presentation and severity and reflect the slowed, flatfoot contact and shuffling gait classically seen in clinical gait evaluations of individuals with PD [32]. Dynamic postural control demands are high during walking as the center of mass (COM) travels outside of the base of support (BOS), i.e., the area within the perimeter created by every point of contact that the person makes with the supporting surface, during the gait cycle. The postural control must maintain a dynamic equilibrium while the BOS is moving. Impaired dynamic stability during gait tasks in persons with PD, particularly complex gait tasks such as changing directions and clearing obstacles, may underlie high fall risk as the individual attempts to adapt to environmental and task demands. Tripping and loss of balance during walking and obstacle crossing are commonly reported causes of falls in the PD population and often lead to injurious falls [34]. Previous research on obstacle-crossing in healthy older adults revealed that older individuals employ a more conservative strategy for this gait task compared to young adults, evidenced by slowed gait crossing speed, shorter step length, reduced step width, and limited separation of the center of pressure (COP) from the COM to minimize postural instability [35; 36; 37]. Older adults with postural instability demonstrate greater foot clearance over obstacles; have a greater risk of obstacle contact, especially with the trailing limb; and have increased frontal plane motion during obstacle crossing, all of which contribute to greater fall risk [38; 39]. Postural instability and conservative gait adaptations during obstacle-crossing are more pronounced in individuals with PD compared to healthy older adults. Individuals with PD demonstrated slower gait velocity, shorter steps, increased step width, slowed obstacle-crossing speed, and difficulty lengthening their step over the obstacle compared to healthy controls, despite demonstrating similar foot clearance over the obstacle [29; 40; 41; 42]. These gait alterations seen in PD are reflective of bradykinesia, a hallmark of this disease, and an attempt to maintain stability due to the postural threat of the obstacle-crossing task. Additionally, individuals with PD demonstrate increased step-to-step variability during the approach phase to an obstacle and plant their stance limb closer to the obstacle with significantly greater step length asymmetry than controls [43; 44]. There was also greater variability in the toe clearance over the obstacle in the PD cohort compared to healthy controls [45]. These findings in individuals with PD may reflect difficulty with gait control, rhythmicity, and adapting their gait during the obstacle-crossing task. The obstacle characteristics, e.g., height and depth, influence the necessary gait adaptations and postural stability demands.