Variability in Measuring Physical Activity in Children with Cerebral Palsy.

Introduction: To establish the variability in the measurement of habitual physical activity (HPA) using the ActiGraph(R) GT3X+ accelerometer in children with cerebral palsy (CP).

Method: Repeated measures: Independently ambulant children with unilateral CP (n=30; aged 11y3m [2y4m]) completed standardized tasks over two consecutive days wearing an ActiGraph(R)GT3X+ accelerometer and heart rate monitor. Testing protocol comprised five minutes of seated rest (REST), walking at light (LW), moderate (MW) and vigorous (VW) pace, and rapid stepping on/off a step (STEP). Agreement was calculated between days using intra-class correlation coefficients (ICC; two-factor mixed agreement model). Minimum detectable difference was calculated (MDD=[SD[square root]1-ICC]x1.96x[square root]2). Performance variability: Participants (n=102) wore an ActiGraph(R)GT3X+ accelerometer for four days in the community. Activity counts were converted into activity intensity using uni-axial derived cut points to classify the time spent in moderate-to-vigorous physical activity (MVPA). Between-day intra-class reliability coefficients (R) and Spearman-Brown prophecy formula (ICCdesired/[1-ICCdesired])([1-ICCestimated]/ICCestimated) were calculated.

Results: Agreement between repeated measures was strong for light and MVPA (ICC=0.80). For MVPA, the MDD was 1412 counts[middle dot]minute-1. In the community, 345 days (87%) were recorded. Three days of monitoring produced acceptable variability estimates of MVPA (R=0.63-0.73). Spearman-Brown prophecy analysis estimated three days would achieve a reliability coefficient of 0.7, and 11 days would achieve 0.9.

Conclusion: Measurement of HPA using the ActiGraph(R) GT3X+ accelerometer is reliable under controlled walking and stepping conditions, as well as in a community environment in independently ambulant children and adolescents with CP.