## 2 Materials and Methods
### Procedures
Following the calibration trial, each participant was given practice trials for walking the 10-m pathway at his or her self-selected pace to feel comfortable with the walkway and to assume his or her normal gait pattern. Participants were instructed to perform clean force plate strikes, defined as stepping in or near the middle of the force plate, during the walking tests. Data collection began after participants practiced self-paced forward walking until they were able to make clean force plate contacts without targeting the force plates, i.e., looking at them. Walking data were collected and processed for one gait trial at a comfortable speed to check for accurate thigh, femoral epicondyle, and tibial marker placement. Frontal and transverse plane hip and knee joint angle data were determined, graphed, and compared to the normal walking database to ensure correct marker placement. If participants' data fell within one standard deviation of the normal kinematic bands, the correct marker placement was verified, and testing continued. Following the kinematic check, forward walking data were collected until five clean force plate strikes were verified for both the right and left legs. To ensure the safety of the participants, a gait belt was available during the data collection process, but its use was up to the examiners' discretion following the initial screening. Rest breaks were given as necessary. Possible errors in joint angle and moment calculation following static calibration could occur if markers were placed incorrectly at anatomical locations or if anatomical markers moved during dynamic motions due to skin motion. In order to prevent these possible calculating errors, marker placement was checked frequently in between walking trials and during rest breaks. Three walking task conditions, which had been approved by the institutional IRB were collected for the larger study: (1) termination of gait, (2) performing a 90\({}^{\circ}\) turn, and (3) stepping over an obstacle. All data for all conditions were collected in one session for each participant. However, only turning data were analyzed and reported for the present study. For the walk-with-turn trials, participants followed a modified version of a procedure performed by [PERSON] et al. [15]. Prior to data collection, participants performed multiple practice trials at a self-selected speed. For left turns, participants walked in the positive y-direction (laboratory coordinate system) at a self-selected speed until reaching the second force plate, where they placed their right foot squarely inside the plate, took a 90\({}^{\circ}\) turn to the left, and continued walking in the negative x-direction (Figure 3). A cone was set on the corner of the force plate to mark the desired location of the turn. For right turns, the cone \begin{table}
\begin{tabular}{c c c} \hline \hline  & **PD (\(u\) = 11)** & **CON (\(u\) = 10)** \\ \hline Age (yrs) & \(67.5\pm 10.2\) & \(65.1\pm 7.4\) \\ \hline Gender (M:F) & 10:1 & 9:1 \\ \hline Height (cm) & \(1.8\pm 0.1\) & \(1.8\pm 0.1\) \\ \hline Mass (kg) & 90.5 \(\pm\) 17.5 & 86.4 \(\pm\) 12.6 \\ \hline BBS & \(53.4\pm 3.3\) & \(55.3\pm 1.6\) \\ \hline MoCA & 25.9 \(\pm\) 2.8 & 27 \(\pm\) 2.4 \\ \hline FOG-Q & 2.7 \(\pm\) 3.6 & Not Tested \\ \hline BBS = Berg Balance Scale, the highest score is 56 indicating no balance deficits. MoCA = Montreal Cognitive Assessment, the highest score is 30 indicating no cognitive deficits. FOG-Q = Freezing-of-Gait Questionnaire, the highest score is 24 indicating the most severe freezing of gait. FOG-Q was only used in the PD group. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Demographic means \(\pm\) SD for Parkinson’s (PD) and Control (CON) groups. was moved to the other side of the force plate in order for the participants to strike the force plate with their left foot and perform a 90\({}^{\circ}\) turn to the right. This procedure was repeated for right and left turns until five valid trials were obtained on each side.