Supplementary Material for: Examining the Influence of Task Complexity on Dual Task Performance in Parkinson's disease and Healthy Older Adults.

Introduction: People with Parkinson’s disease (PwPD) often experience difficulty performing dual-tasks (DT), negatively impacting mobility, fall risk and quality of life. The influence of task complexity on DT performance remains unclear. This study examined the effects of gait complexity and cognitive/speaking task on DT effects (DTE) in PwPD and healthy older adults. Methods: Forty-one PwPD and eleven healthy older adults completed two gait tasks—straight walking (simple) and figure-8 walking with obstacle crossing (complex)—on a pressure-sensitive walkway, paired with four cognitive/speaking tasks: oral trail making, counting, diadochokinetic task, and spontaneous speech. Each combination was performed three times for 20 seconds. Single-task (ST) and DT performance were assessed for gait parameters (step length, step length coefficient of variation) and cognitive/speaking task response rate. Motor DTE (mDTE), cognitive DTE (cogDTE), combined DTE (cDTE), and modified attention allocation index (mAAI) were calculated. A 2×5 repeated measures ANOVA tested effects of gait complexity and cognitive/speaking task on raw gait metrics, and a 2×4 ANOVA tested effects on DTE metrics. Results: In PwPD, complex walking resulted in shorter step length and greater step length coefficient of variation compared to simple walking (ps < .001), whereas cognitive/speaking task did not affect raw gait metrics. For DTEs, cognitive/speaking task influenced step length-based mDTE, cDTE, and mAAI (ps < .05) and step length coefficient of variation-based mAAI (p < .05). In healthy older adults, gait complexity led to greater step length coefficient of variation (p < .001) while step length was unchanged. Cognitive/speaking task significantly influenced step length-based cogDTE, cDTE, and mAAI, as well as step length coefficient of variation-based cogDTE and mAAI (ps < .05). No significant interactions were observed. Conclusion: In PwPD, gait complexity influenced raw gait performance, whereas DTEs were more sensitive to cognitive/speaking task. In healthy older adults, gait complexity primarily affected step length variability, while DTEs were also influenced by cognitive/speaking task. These findings emphasize the value of diverse DT paradigms and the importance of assessing both raw performance and DTEs. Tailoring assessments to include a variety of motor and cognitive challenges may improve sensitivity in evaluating gait and fall risk, supporting more personalized interventions.