There have been many questions and inquiries as to the origin of the term ‘count’ in relation to activity measurements. Below is an explanation of why ActiGraph uses this unit of measurement and what it represents in the physical world.
"Count" is an enduring term. Prior to solid-state analog-to-digital converter commercial availability, activity monitors utilized either the 'threshold crossing' or 'cycle count' activity measurement. The threshold crossing technique involved incrementing a "count" each time the magnitude of acceleration (activity) exceeded a given threshold. The cycle count approach produced a "count" when enough force was applied to move a mechanical lever through a full cycle (up and down). The latter of these two approaches is very similar in nature to the modern day pedometer measurement technique.
ActiGraph's original activity monitor, the 7164 model, utilized a mechanical lever capable of measuring the change in acceleration with respect to time (g/sec, where g is gravity or 9.806 m/s2). To suppress unwanted motion and enhance human activity, the acceleration signal was passed through an analog band-pass filter, the output of which yields a dynamic range of 4.26g/sec (+/-2.13g/sec) at 0.75Hz (center frequency of the filter). Using a sample rate of 10 samples-per-second, this filtered signal was then digitized into 256 distinct levels by an 8-bit solid-state analog-to-digital converter, producing 4.26g/sec per 256 levels or 0.01664 g/sec/count (each level is considered 1 count). When each filtered sample is multiplied by the sample window of 0.1sec, a resolution of 0.001664g/count is achieved.