## 1 Introduction
Recently, several biomechanical measures of balance during movement, e.g., walking or turning, have been described [15; 19; 21]. The COM-COP inclination angle represents the spatial relationship between the COM and COP and has been used to evaluate gait stability during 180\({}^{\circ}\) turning with IPD [15]. The position of the extrapolated COM (XCOM) is another variable that initially measured stability in static situations [21] but has also been used to determine dynamic stability in a recent study [19]. The XCOM accounts for the velocity of the COM [21]. The difference between the location of the XCOM and COM positioning is greater in less stable positions [21]. Balance can be predicted by knowing the XCOM and COP relative positions [19]. For example, generally, at the time of foot contact, walking stability is maintained when the COP is positioned at a specific distance behind and lateral to the XCOM [19]. The margin of stability (MOS) is the distance between the XCOM and the BOS. It is \"proportional to the impulse needed to unbalance an individual\" and has been validated for its application to examine and describe both static and dynamic balance/control [21; 22; 23]. When the BOS is decreased during walking, the MOS will decrease, and the XCOM may demonstrate a greater excursion [21]. A larger MOS may reflect increased dynamic stability, whereas a smaller MOS may reflect poorer balance control [17]. The MOS has been an effective variable in measuring dynamic stability [17; 21] during gait in healthy individuals as well as those with neurological conditions [19; 24], such as multiple sclerosis [24]. There is no previous research that has assessed the MOS during gait and turning in IPD, despite the known high fall risk of IPD during these activities. Therefore, the MOS may be useful to characterize dynamic balance control in IPD during turning activities. Dynamic balance during walking with turns is impaired and often poses a high fall risk for IPD, which has been identified in previous research [13; 14; 15; 16]. However, there are gaps in the literature regarding the underlying biomechanical deficits in the turning strategies for IPD. Few studies have examined biomechanical variables related to dynamic balance control in this population, such as COM-COP inclination angle and MOS, during walking and turning to identify possible underlying biomechanical deficits. Specifically, there is a lack of research that examines the relationship between MOS and dynamic stability in this population. Due to the limited studies on dynamic control of balance at any stage of PD, specifically regarding anteroposterior (A/P) and mediolateral (M/L) MOS, there may be features of gait and postural control during turns that are currently unknown. Therefore, the purpose of this exploratory study was to examine the dynamic balance control in individuals with mild to moderate PD and healthy individuals while walking while performing 90\({}^{\circ}\) turns utilizing computerized three-dimensional gait analysis. Specifically, we examined the A/P and M/L margins of stability and their effect on the dynamic balance during turning in IPD and healthy controls. These results may help inform researchers and clinicians about factors that contribute to fall risk during walking with turns in IPD.