Age-stratified validation of the performance index (P-Index) as a metric for dual-task cost in functional mobility across the adult lifespan

OBJECTIVES: To evaluate the predictive validity of the Performance Index (P-Index), a metric that integrates functional mobility and accuracy of cognitive responses to quantify dual-task (DT) performance, for cognitive outcomes measured by the Mini-Mental State Examination (MMSE) across the adult lifespan during the instrumented Timed Up and Go (iTUG) test. STUDY DESIGN: This cross-sectional study included 247 participants from diverse age groups who completed single-task (ST) and dual-task (DT) iTUG tests in a semi-randomized order. The P-Index (range: 0-1) was calculated using iTUG completion times (measured with inertial sensors) and cognitive task accuracy (backward recitation of days of the week), with weighted components for time (W₁) and accuracy (W₂). MAIN OUTCOME MEASURES: The relationship between the P-Index and MMSE scores. RESULTS: The optimal weighting for predictive accuracy was W₁ = 0.6 and W₂ = 0.4, yielding a P-Index threshold of 0.90 with a sensitivity of 0.69 and specificity of 0.64 for detecting MMSE scores ≥ 26. Participants who reported depressive symptoms (OR: 1.19, p = 0.02) were less likely to achieve MMSE ≥ 26. Conversely, each additional medication (OR: 1.04, p = 0005) and each unit increase in the P-Index significantly improved the odds of higher MMSE scores (OR: 3.89, p = 0.002). SIGNIFICANCE: The P-Index reliably measures dual-task interference and serves as a strong predictor of cognitive performance across the adult lifespan. It shows potential as a practical tool for cognitive-motor assessments and early detection of cognitive decline.