## 1 Introduction  
Fall incidence in adults aged 65 years and older is a critical health problem, as 28% report falling each year, resulting in 36 million falls per year (2020) in the United States [1]. Falls are significantly more prevalent in individuals with Parkinson's disease (IPD), as they are five times more likely to fall compared to healthy older adults [2]. Prospective studies estimate that between 45% and 65% of IPD fall each year, with 39% experiencing recurrent falls and a high rate of injurious falls [3; 4; 5]. Parkinson's disease (PD), the second most prevalent neurodegenerative disorder, is caused by a progressive loss of the dopaminergic neurons in the substantia nigra, which leads to dopamine deficiencies throughout the basal ganglia [6]. This progressive neurodegeneration leads to motor deficits in generating and executing voluntary movements, bradykinesia, rigidity, postural instability,and gait deficits [6; 7; 8]. PD-related gait impairments may include decreased gait velocity, hypokinesia (reduced step length and arm swing), shuffling gait with increased cadence, a narrower base of support (BOS), impaired gait rhythmicity and automaticity, freezing of gait (FOG), and impaired ability to perform dual tasks while walking [3; 9; 10; 11]. These gait deficits contribute to increased fall risk during walking in the home and community, such as when navigating turns, obstacles, and varied surfaces. As the disease progresses, the decline in gait and postural control mechanisms results in high fall risk, reduced functional independence, and a lowered quality of life. Falls are eight times more likely to occur during turns than straight-path walking in older adults [12]. Turns are an essential part of gait and often are the cause of falls, especially in the aging population with PD when considering the dynamic, unexpected, and fast turns of everyday life and the functional impairments experienced [4]. Previous research supports that IPD approach turns slower, turn wider with more steps, take longer to turn, turn at a slower peak turn speed, and are less accurate in their turns compared to healthy controls [13; 14; 15; 16]. Compromised turning strategies were reported even in individuals with very mild PD [13; 14; 15; 16]. Axial coordination and gait stability can be significantly disrupted in IPD due to increased muscle rigidity and anticipatory postural adjustment impairments [15]. Axial mobility, i.e., trunk rotation, intersegmental coordination, and dynamic changes in the BOS, are related to turning difficulty in IPD [13; 16]. Individuals with mild PD display \"en bloc turning\", an almost simultaneous rotation of their head and upper trunk instead of the cranial-to-caudal rotation pattern typically seen [16], which has been associated with lateral instability and reduced forward progression with extra steps during turning [15]. Interlimb temporal coordination, which is crucial for maintaining mediolateral (M/L) dynamic stability [17], is typically impaired along with gait rhythm in IPD and may be associated with increased instability during activities that require interlimb coordination [18]. Previous research on turning strategies during walking in IPD has focused primarily on temporal-distance gait parameters, such as step number, step length, and speed of turns; however, research that examines the dynamic control of balance during gait with turns is lacking. This research may provide insight into the mechanisms underlying postural instability during turning in IPD. Using an inverted pendulum (IP) model, derived biomechanical variables have been applied to assist in understanding the mechanisms of standing postural control and dynamic balance during gait and turning [19; 20] (Figure 1). In static situations, stability is maintained when the center of mass (COM) is constrained within the BOS [21]. Although, during gait, the COM vertical projection is outside of the BOS for the majority of the gait cycle, requiring complex balance control, healthy individuals can regulate it to maintain balance [9; 17].