General spectral characteristics of human activity and its inherent scale-free fluctuations

Abstract

Abstract Although actigraphy is commonly used in many research areas, the activity calculation methods are not standardized, therefore activity signals can be very different. The scale-free nature of daily human activity has been observed in different aspects; however, the description of its spectral characteristics is incomplete. The presence of 1/f noise in activity or acceleration signals was mostly analysed for short time windows, the complete spectral characteristic has only been examined in the case of certain types of activity signals. To explore the general spectral nature of human activity in greater detail, we have performed Power Spectral Density (PSD) based examination and Detrended Fluctuation Analysis (DFA) on multi-day-long, triaxial actigraphic acceleration signals of 42 healthy, free-living individuals. We generated different types of activity signals from these, using different acceleration preprocessing techniques and activity metrics. We revealed that different types of activity signals’ spectra generally follow a universal characteristic including 1/f noise over frequencies above the circadian rhythmicity. Moreover, we discovered that the PSD of the raw acceleration signal has this same characteristic. Our findings prove that the spectral scale-free nature is generally inherent to the motor activity of healthy, free-living humans, and is not limited to any particular activity calculation method.