Quantification of acceleration as activity counts in ActiGraph wearable

The fast advances in computing and micro-electromechanical units (MEMS) engineering have established new alternatives for monitoring health in people’s cost-free-residing atmosphere above extended time durations. In specific, as wearable devices are able of quantifying patients’ behaviors continuously with nominal stress, they can give more complete, ecological and objective overall health facts than what is possible with regular in-clinic and questionnaire-based mostly assessments^1,2,3. For these causes, the use of wearable units for clinical uses has viewed rapid expansion in current yrs and is predicted to develop further in the approaching years⁴. 1 of the most applied outcomes furnished by wearable products is accelerometry-centered bodily exercise steps. Actual physical action steps have been utilised in clinical trials and revealed superiority in detecting treatment related variations over typical endpoints⁵. Development has also been produced in the qualification of electronic endpoints based mostly on counts of bodily exercise with both equally US and EU in respiratory and cardiovascular diseases^6,7.

Although the origin of the term “count” is not documented, it possible goes again to the modification and use of watches for measuring exercise, exclusively hyperactivity in little ones wherever pedometers have been also massive^8,9,10,11. This was accomplished by a winding body weight that was cost-free to pivot about its axis and as a result sensitive to acceleration though currently being related to the fingers of the check out by using gears, causing the minutes and hrs palms to tick when the observe experienced acceleration and thus enabling for looking through the accrued “counts” of actions from the enjoy the exact way as one particular would read through the time¹¹. As electronics know-how evolved, stable-point out analog-to-electronic converters became extensively out there and allowed for improved acceleration measurements and storing the action info in a digital structure inside the view^12,13,14. The to start with activity watch supplied by ActiGraph was model AM7164 (ActiGraph, Pensacola (FL), United states of america) that used a uniaxial piezoelectrical accelerometer in the type of a mechanical lever¹⁴. To eliminate artifacts unrelated to human motion, subsecond-amount measurements of acceleration (expressed in g) were being passed by an analog band-pass filter in advance of it was sampled at 10 Hz and quantized by an 8-little bit analog-to-electronic converter, consequently yielding 256 distinctive stages of acceleration. Each and every stage further than 128 is deemed to be 1 count. These had been then summed inside presented time intervals, or “epochs”, and the resulting counts were being stored on the gadget (for this reason, just about every epoch could lead up to (128 periods 10 times textitthe length of the epoch) counts to the total).

The counts unit is hence a measure that quantifies acceleration in just a time interval, or “epoch”, with a person epoch commonly remaining 10–60 s lengthy. The reliance on epoch-dependent counts was a requirement in earlier styles, thanks to the limitation of on-board storage and battery ability. While this limitation was conquer with components advancements and ActiGraph has been able to give multi-day uncooked data considering that 2010, electronic scientific measures and validation proof continue on to evolve about counts. For the reason that the algorithms that renovate the raw accelerometer details into counts change throughout equipment, quite a few of which have also been held proprietary, digital measures centered on counts are product dependent, creating it difficult to review success throughout scientific reports and establish reproducibility and validation evidence^15,16,17. The obscurity of the count algorithms has also led to the common misconception that “counts” is a universal unit of measurement that is the very same across gadgets, where even with an algorithm these as the 1 described underneath, variants in parameters these types of as the duration of every single epoch or the precision of the quantization method can lead to vastly distinctive counts. Thus, the advancements of electronic wellbeing science could be drastically facilitated with a better level of algorithm transparency which would let it to be made use of across multiple equipment.

ActiGraph devices (ActiGraph, Pensacola, FL, Usa) have been the most-made use of wearable accelerometry, or actigraphy, products for about two a long time, with far more than 20,000 papers released making use of ActiGraph products by the conclude of 2021. Even though the first solutions for computing counts in the before products have been posted¹⁴, the comprehensive algorithms and their evolution above the years have not been manufactured community. Quite a few experiments have tried to reverse engineer the ActiGraph counts algorithms or relate them to the counts algorithms of other devices^16,17,18,19. Whilst these research have been critically critical to attain additional comprehending of acceleration counts and how it relates throughout gadgets, they have been constrained by the absence of access to the ActiGraph firmware and application code and device documentation. In this write-up, we deal with this gap by presenting an overview of the counts algorithms of ActiGraph equipment along with a in depth description of the counts algorithm in each the ActiLife software package (ActiGraph, Pensacola, FL, United states) and the CentrePoint cloud service (ActiGraph, Pensacola, FL, Usa). An open-source Python package deal is also built readily available for use by the analysis community. By performing so, we hope to aid reproducibility endeavours and greatly enhance transparency in the subject of wearable accelerometry and accelerate its scientific use.