GRAVITATIONAL ARTIFACT IN FREQUENCY-SPECTRA OF MOVEMENT ACCELERATION - IMPLICATIONS FOR ACTIGRAPHY IN YOUNG AND ELDERLY SUBJECTS

Actigraphy, the long-term assessment of wrist movements by means of a small solid-state recorder, is widely used in a variety of human resea rch fields, among which sleep, circadian rhythms and aging. Actigraphs assess movement with the use of accelerometers, which sense accelerat ions resulting from muscle force as well as accelerations due;to chang es in the position of the sensor in the gravitational field. In the pr esent paper a method is described to minimise gravitational artefact i n movement assessment by calculating the instantaneous acceleration ve ctor from 3 perpendicular acceleration signals. It is shown that the p ower spectra of single axis acceleration signals are dominated by low- frequency components (+/-0.25 Hz) due to gravitational artefact. Spect ra of the instantaneous acceleration vector indicate that 'true' movem ent accelerations resulting from muscle force are present in a much wi der range: from 0.25 to 11 Hz. Wrist accelerations in elderly subjects were found to be of lower amplitude and frequency as compared to youn g subjects. It is furthermore shown that a bandpass filter of 0.25 to 2 or 3 Hz, as has been used in commercially available actigraphs, is f ar from optimal, and may even result in a positive bias for movement d etection in the elderly. This bias may underly contradictory findings in actigraphic studies on human aging. When a bandpass filter of 0.5-1 1 Hz is applied to a single-axis acceleration signal, the influence of gravitational artefact and bias are minimized, and the age-related de cline in activity is properly detected.