Intra-Rater Test–Retest Reliability of Static Dual-Task Balance Measures in Untrained People with Parkinson’s Disease

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Background/Objectives: Dual-task (DT) motor assessment in Parkinson’s disease (PD) provides insight into functional abilities, yet limited evidence on its reliability may bias therapy results. This study aims to evaluate the intra-rater test–retest reliability of single and functional DT balance measures in untrained individuals with PD, and to determine the Standard Error of Measurement (SEM) and the Minimal Detectable Change (MDC). Methods: In this cross-sectional study, 22 untrained participants with PD performed a balance assessment twice, 30 min apart. The Romberg test was measured on a force platform under five conditions: single-task, two cognitive secondary tasks (visual, DTvisual; verbal, DTverbal) and two manual tasks (below 90° of shoulder flexion, DTlower; above 90°, DTupper). Test–retest reliability of Center of Pressure (CoP) outcomes was assessed using the Intraclass Correlation Coefficient (ICC) besides SEM and MDC. Results: Displacement, dispersion, force, swept area, and velocity of CoP movements showed excellent reliability (ICC > 0.75) under the single-task condition, except for anteroposterior displacement, which showed an ICC of 0.73. Swept area (ICC = 0.92), followed by CoP velocity (ICC = 0.90) and anteroposterior CoP force (ICC = 0.90), exhibited the highest reliability. Under dual-task conditions, reliability was more variable across outcomes and task types. CoP velocity showed the most consistent reliability (ICC = 0.74–0.92), whereas swept area showed lower reliability in all DT conditions, but DTupper (ICC = 0.90). Mediolateral CoP outcomes were more reliable than anteroposterior displacement and dispersion, while anteroposterior CoP force outperformed mediolateral force under DT conditions. Conclusions: Single-task static balance can be reliably assessed in untrained people with mild-to-moderate PD within a single session. However, under DT conditions, high variability across outcomes warrants cautious interpretation, highlighting CoP velocity and mediolateral excursion as relatively consistent outcomes.