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

Abstract

The scale-free nature of daily human activity has been observed in different aspects; however, the description of its spectral characteristics is incomplete. General findings are complicated by the fact that—although actigraphy is commonly used in many research areas—the activity calculation methods are not standardized; therefore, activity signals can be different. The presence of 1/f noise in activity or acceleration signals was mostly analysed for short time windows, and the complete spectral characteristic has only been examined in the case of certain types of them. 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 several-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 the spectra of different types of activity signals 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 the 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.

Hierarchical organization of human physical activity

“…On these grounds, we assume that the interplay between internal and external driving forces controlling human daily physical activities is manifested in a non-random, hidden structure of ultradian actigraphy signals. However, the main tool used nowadays for evaluating actograms is still Fourier analysis, which—though effective in revealing periodic components 26 —normally fails to identify stochastic structures that seem to be prevalent on the ultradian scale.…”