Accelerometer Cut Points for Physical Activity Assessment of Older Adults with Parkinson's Disease.

Objective: To define accelerometer cut points for different walking speeds in older adults with mild to moderate Parkinson's disease.

Method: A volunteer sample of 30 older adults (mean age 73; SD 5.4 years) with mild to moderate Parkinson's disease walked at self-defined brisk, normal, and slow speeds for three minutes in a circular indoor hallway, each wearing an accelerometer around the waist. Walking speed was calculated and used as a reference measure. Through ROC analysis, accelerometer cut points for different levels of walking speed in counts per 15 seconds were generated, and a leave-one-out cross-validation was performed followed by a quadratic weighted Cohen's Kappa, to test the level of agreement between true and cut point-predicted walking speeds.

Results: Optimal cut points for walking speeds ≤ 1.0 m/s were ≤ 328 and ≤ 470 counts/15 sec; for speeds > 1.3 m/s, they were ≥ 730 and ≥ 851 counts/15 sec for the vertical axis and vector magnitude, respectively. Sensitivity and specificity were 61%-100% for the developed cut points. The quadratic weighted Kappa showed substantial agreement: κ = 0.79 (95% CI 0.70-0.89) and κ = 0.69 (95% CI 0.56-0.82) for the vertical axis and the vector magnitude, respectively.

Conclusions: This study provides accelerometer cut points based on walking speed for physical-activity measurement in older adults with Parkinson's disease for evaluation of interventions and for investigating links between physical activity and health.