## 4 Discussion  
Individuals with PD have potential postural instability when making 90\({}^{\circ}\) turns because they spend a greater percentage of time with the COM and XCOM outside of their BOS [14]. Currently, there is limited research using 3D motion analysis to examine the MOS in relation to dynamic stability in IPD. There are also no studies at this time that have analyzed the MOS using the marker placement on the foot that this study utilized. Therefore, MOS variables were examined in this sample of the PD population during gait and 90\({}^{\circ}\) turns to identify possible differences compared to healthy, age-matched controls. All ST parameter variables were not significantly different between the CON and PD groups. Also, all dynamic balance variables were not statistically different between groups, although MS COM-COP inclination angle, MS COP-COM A/P, FDS U\({}_{\text{MAX}}\)-XCOM A/P, and MS U\({}_{\text{MAX}}\)-XCOM A/P demonstrated marginally significant differences. Three of these dynamic balance variables demonstrated marginally significant differences in the midstance phase of gait. This is important because MS is the single-limb support phase of the gait cycle, which is the most unstable phase of gait [32]. All of the dynamic balance variables that demonstrated marginally significant differences besides the COM-COP inclination angle were in the A/P direction. There is no prior research on PD gait during turns that examines these dynamic balance variables in the A/P direction, suggesting that these findings are novel and may be clinically relevant. Prior research supports the idea that a flexed posture, typically seen in IPD, is associated with a posterior COM during stance, which could lead to instability [9]. Increased cadence, decreased stride and step lengths, and decreased velocity in IPD may also affect the dynamic variables in the A/P direction [3; 9; 10; 11], although the ST findings in this PD sample did not support group differences in these ST variables during gait with turns. These novel findings may help clinicians gain a better understanding of dynamic balance in the A/P direction in this population in order to decrease fall risk. In examining the ST parameters, velocity was the only variable that demonstrated a marginally significant difference (\(p\) = 0.011) between groups. The mean velocity for the CON group was 1.02 m/s, whereas, for the PD group, it was 0.87 m/s. The lack of a significant difference between groups in gait velocity may be related to three things: (1) both groups were instructed to target the plate to strike as part of their turn; (2) the relatively mild manifestation of the disease in the PD group; and (3) relatively greater variability (i.e., SD) in gait velocity in both groups with a smaller sample size. It was expected that turn velocity may be slower in IPD as this turn task may reflect underlying motor deficits such as gait hypokinesia, bradykinesia, difficulty adapting gait such as executing direction changes, and impaired execution and sequencing of motor plans [3; 9; 10; 11]. Slower velocity during turning was also expected, as slower peak turn speeds were reported in prior studies in individuals with mild and severe PD during 180\({}^{\circ}\) turning in similar gait trials and within the Timed Up and Go (TUG) test [13; 16]. Perhaps the relatively mild disease stage in our sample may explain no significant differences in ST parameters between groups. As mentioned previously, the COM-COP inclination angle may be a useful measure when examining dynamic balance [15]. Analysis of the COM-COP inclination angle at FDS, MS, and SDS revealed that MS was the only point in the gait cycle that demonstrated a marginally significant difference between groups. Of all the dynamic balance measures examined in this study, the COM-COP inclination angle at MS was the only variable with a \(p\)-value (\(p\) = 0.006) approaching the significance level (\(p\) < 0.0028).