Technical reliability assessment of three accelerometer models in a mechanical setup.

PURPOSE: To determine which of the three most commonly used accelerometer models has the best intra- and interinstrument reliability using a mechanical laboratory setup. Secondly, to determine the effects that acceleration and frequency have on these reliability measures.
METHODS: Three experiments were performed. In the first, five each of the Actical, Actigraph, and RT3 accelerometers were placed on a hydraulic shaker plate and simultaneously accelerated in the vertical plane at varying accelerations and frequencies. Six different conditions of varying intensity were used to produce a range of accelerometer counts. Reliability was calculated using standard deviation, standard error of the measurement, coefficient of variation, and intraclass correlation coefficients. In the second and third experiments, 39 Actical and 50 Actigraph accelerometers were put through the same six conditions.
RESULTS: Experiment 1 showed poor reliability in the RT3 (intra- and interinstrument CV > 40%). Experiments 2 and 3 clearly indicated that the Actical (CVintra = 0.5%, CVinter = 5.4%) was more reliable than the Actigraph (CVintra = 3.2%, CVinter = 8.6%). Variability in the Actical was negatively related to the acceleration of the condition, whereas no relationship was found between acceleration and reliability in the Actigraph. Variability in the Actigraph was negatively related to the frequency of the condition, whereas no relationship was found between frequency and reliability in the Actical.
CONCLUSION: Of the three accelerometer models measured in this study, the Actical had the best intra- and interinstrument reliability. However, discrepant trends in the variability of Actical and Actigraph counts across accelerations and frequencies preclude the selection of a superior model. More work is needed to understand why accelerometers designed to measure the same thing behave so differently.