Aliasing affects ActiLife software raw accelerometry to count conversion from different sampling frequencies

Accelerometry counts are widely used to quantify physical activity in an objective manner. ActiGraph™ accelerometers offer to record acceleration signal with different sampling frequency (fs). Nevertheless additional counts were shown to be computed by ActiLife software from acceleration signal with a sampling frequency fs>30 Hz compared to signal with default fs=30 Hz or multiple. This paper relies on the study of synthetic signals to point out the origin of this error and to recommend an adjusted method. A piecewise-frequency sinus time series (0-15 Hz) was generated at different sampling frequencies (fs=30, 50 and 100 Hz). The artificial acceleration raw signal was resampled to 30 Hz using different antialiasing lowpass filters before ActiLife count computation. The use of an antialiasing filter which did not properly attenuate aliasing replicas was found to induce aliasing frequencies within ActiLife bandpass filter which is the cause of extract activity counts. We were able to reproduce fictitious counts for acceleration around 10 Hz. A simple adjustment of antialiasing filter parameters allowed to avoid this problem. This study reproduces ActiLife counts processing from 50 and 100 Hz sampled signal. Count overestimations from fs=50 and 100 Hz signal were induced because of aliasing in the frequency bandwidth of the ActiLife count filter. This can be corrected by a relevant antialiasing filtering before ActiLife software processing or this can be done in high-level mathematical programing.