## 2 Materials and Methods
### Experimental Protocol  
Physical examination included measures of height, weight, gross lower extremity range of motion, manual muscle testing of all major lower extremity joints, and screening integrity of nerve roots L4 and L5. Anthropometric measurements were taken for knee, \begin{table}
\begin{tabular}{c c c} \hline \hline  & **CON (\(n\) = 9)** & **PD (\(n\) = 9)** \\ \hline
**Age (yrs.)** & 65.6 \(\pm\) 7.2 & 65.5 \(\pm\) 9.6 \\
**Gender (M:F)** & 9.0 & 9.0 \\
**Height (cm)** & 182.7 \(\pm\) 9.6 & 181.8 \(\pm\) 8.3 \\
**Mass (kg)** & 83.6 \(\pm\) 12.6 & 92.9 \(\pm\) 15.4 \\
**BBS** & 55.2 \(\pm\) 1.5 & 54.7 \(\pm\) 1.6 \\
**MoCA** & 26.7 \(\pm\) 2.1 & 25.8 \(\pm\) 2.9 \\
**FOG-Q** & Not Tested & 1.5 \(\pm\) 0.9 \\ \hline \hline \end{tabular} Data presented as mean \(\pm\) standard deviation. BBS = Berg Balance Scale, where the highest score of 56 indicates no balance deficits. MoCA = Montreal Cognitive Assessment, where the highest score of 30 indicates no cognitive deficits. FOG-Q = Freezing Gait Questionnaire, where the highest score of 24 indicates the most severe freezing of gait. Only the PD group was tested for FOG-Q.  
\end{table} 
Table 2: Demographics for control (CON) and Parkinson’s disease (PD) groups. Figure 2: Participant flow diagram. Of the 13 Parkinson’s disease and 11 control participants recruited, 9 for each group were included for data analysis. ankle, and elbow width, wrist and hand thickness, leg length (distance from the anterior superior iliac spine (ASIS) to medial malleolus), pelvic width (distance between bilateral ASIS), pelvic height (perpendicular distance from a line connecting the right and left ASIS and the superior edge of the pubic tubercle), and pelvic depth (measurement from ASIS to posterior superior iliac spine). Pelvic measurements were utilized to estimate the location of each subject's hip joint center [59]. All participants wore tight-fitting shorts and their preferred athletic shoes. Physical examination measurements and marker placements were conducted by an examiner who had received three months of prior training (and who was supervised during data collection by the laboratory director). Forty spherical markers (14 mm diameter) were placed on anatomical landmarks according to a modified version of the full-body PIG model. Modifications of the PIG model included additional markers on the heads of the 5 th metataralsals and placement of the thigh and tibial markers (as opposed to using thigh and tibial wands) on the mid-lateral thighs and tibiae. Refer to [PERSON] et al. [53] for a detailed description of anatomical marker placement. Markers were placed by a single examiner to eliminate inter-examiner variability and checked between walking sessions for any displacement [53]. Testing procedures included the following walking conditions: (1) self-selected pace; (2) walking with 90-degree turns at a normal pace; (3) termination of gait; and (4) walking with an obstacle-crossing task. Dynamic balance variables were analyzed for the 90-degree turn and during a gait cycle before the 90-degree turn in prior studies. This study focused on data analysis of dynamic balance during the obstacle-crossing task. Following marker placement, a static calibration trial was performed in which participants were asked to stand still while assuming the anatomical position (arms slightly abducted and feet shoulder width apart) for approximately four seconds. The medial femoral condyle and medial malleolar markers were removed following the completion of the static calibration trial, before walking tasks. 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 hip and knee joint angle data were determined, graphed, and compared to the laboratory normal walking database (within one standard deviation) to ensure correct marker placement.