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Appendix

ActiGraph and Actical Physical Activity Monitors

A Peek under the Hood

JOHN, DINESH; FREEDSON, PATTY

Author Information
Medicine & Science in Sports & Exercise: January 2012 - Volume 44 - Issue 1S - p S86-S89
doi: 10.1249/MSS.0b013e3182399f5e
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Abstract

Since the early 1900s, accelerometers have been used to measure vibrations and detect motion in various industrial applications (2,4,5). Based on the idea that quantitative knowledge of the force acting on the human body during dynamic movement provides a comprehensive description of any physiological phenomenon, Cavagna et al. (1) developed a strain gauge accelerometer in 1961. This was one of the first accelerometer-based activity monitors. In the 1980s, activity monitors with technologically advanced piezoelectric ceramic sensors were introduced in the field of objective monitoring of physical activity (5). Since then, accelerometer-based activity monitors have gained prominence in physical activity measurement, and several are currently commercially available. In addition to piezoelectric sensors, current monitors are based on piezoresistive and capacitive technology. Commonly used monitors are the 7164, the GT1M, and the GT3X, manufactured by ActiGraph (Pensacola, FL), and the Actical, manufactured by Phillips Respironics (Bend, OR). This article provides brief descriptions of specific “electromechanical” aspects of the ActiGraph and Actical monitors that may help researchers further their understanding of these monitors. Although physical activity researchers can obtain information about several important features of the ActiGraph and the Actical from various open and closed sources, this article provides few of these facts in a freely available source.

ACTIGRAPH ACCELEROMETRY-BASED ACTIVITY MONITORS

AM7164.

When the AM7164 was introduced, ActiGraph described it as an electronic device that combined many different parts and pieces. The acceleration sensor in the AM7164 is a bimorph piezoelectric cantilever beam that has a seismic mass attached on one end. The opposite end of the beam is mounted on the electronic circuit board of the device. The bimorph beam comprises of two mechanically bonded synthetic lead zirconate titanate crystals. In response to acceleration, the seismic mass causes the sensor to bend and resonate in a direction parallel to the acceleration and produces a proportional signal. This analog electrical charge is filtered using a hardware-based (multiple semiconductor components) band-pass filter and then digitized by an 8-bit analog-to-digital converter (ADC). The signal also undergoes full-wave rectification. In other words, it is converted to absolute acceleration values. The filtered, digitized, and full-wave–rectified acceleration signals are then converted to activity counts.

GT1M and GT3X.

The AM7164 was replaced by the GT1M in the mid 2000s. ActiGraph stated that the GT1M was more cost-effective and had upgraded technology while providing output similar to that of its predecessor. The main difference between the AM7164 and the GT1M is that the former has a piezoelectric sensor that detects dynamic accelerations (resulting from motion), whereas the GT1M uses a capacitive accelerometer capable of detecting both static (e.g., force of gravity detected when stationary) and dynamic accelerations. Unlike piezoelectric accelerometers where a change in acceleration results in the production of a proportional electric charge, a capacitive accelerometer detects change in acceleration through changes in capacitance of the sensing element, in other words, variations in the sensor’s electric charge storage potential.

The acceleration sensor in the GT1M is the ADXL320 (Analog Devices, Norwood, MA), which is a 4 × 4 × 1.45-mm surface micromachined monolithic integrated circuit chip (polysilicon), dual-axis microelectromechanical system accelerometer. The ADXL320 has a full-scale range of ±5g and measures both static and dynamic accelerations. However, ActiGraph-specified restrictions allow the detection of only those accelerations that lie between 0.05g and 2g. Structurally, this capacitive accelerometer has two independent polysilicon fixed plates that act as electrodes, and between the two fixed plates is a parallel movable central plate. This plate is movable because it is suspended as a bridge structure using polysilicon springs that are attached to the sides of the structure. The suspended central plate forms two back-to-back capacitors with each fixed plate on either side. The three plates form a differential capacitor in which the deflection of the central plate in response to acceleration causes a change in the capacitance of the two back-to-back capacitors. The change in capacitance causes specific voltage changes to the existing electric flow, which is an analog signal that is proportional to the detected acceleration. Twelve-bit ADC and phase-demodulation techniques are used to amplify, digitize (sampling rate = 30 Hz) and full-wave rectify the signal and to determine the directions (vertical, anteroposterior and mediolateral axes) of the acceleration. Finally, the signal is filtered at a bandwidth of 0.25 to 2.5 Hz. This means that the acceleration signal is markedly attenuated if the frequency of acceleration peaks falls outside this range. ActiGraph’s rationale for using a band-pass filter is that most forms of human movement fall within this frequency range, and the filter might be useful for eliminating high-frequency artifact vibrations.

In mid 2009, ActiGraph released the GT3X containing the ADXL335 accelerometer (Analog Devices). The ADXL335 is a 4 × 4 × 1.45-mm triaxial capacitive microelectromechanical system sensor with a full-scale range of ±3g. The structure and theory of operation of the ADXL335 is similar to those of the ADXL320. Other than the accelerometer in the GT3X, all other specifications, including the filter and the ADC, are precisely the same as in the GT1M. In addition, although both the GT1M and the GT3X have the capability to measure static accelerations, only the GT3X provides inclinometer output. The GT3X uses vector magnitude data from all three axes and assigns a number from 0 to 3 to distinguish whether an individual is not wearing the monitor (number 0), standing (number 1), lying (number 2), or sitting (number 3).

ActiGraph prefiltered raw-mode output.

Unlike the AM7164, both the GT1M and the GT3X also provide output in the prefiltered raw mode. This is an important feature in the two capacitive activity monitors because the general consensus at the July 2009 “Objective Measurement of Physical Activity: Best Practices and Future Directions” conference was that researchers should begin using raw accelerations instead of activity counts when measuring physical activity. Before April 2010, the output in the prefiltered raw mode was a representation of the analog-to-digital quantization of accelerations sampled every 0.033 s (sampling frequency of 30 Hz) by the accelerometer. The analog-to-digital quantization values are discrete, unsigned digital numbers proportional to the voltage signal generated by the accelerometer in response to acceleration. Firmware modifications in April 2010 changed the output in the prefiltered raw mode to represent the actual G force sampled every 0.033 s.

To obtain these “raw-mode” variables, the analog-to-digital quantization is calculated using several parameters. These include the sensitivity of the ADXL320/335 accelerometers (174 mV·g−1 for each axis), the total number of levels in which a sampled acceleration can be classified (12-bit ADC enables 4096 (212) different levels), and the 0g offset of 1.5 V. The 4096 levels of the 12-bit ADC are spread equally across a 3-V reference voltage. Therefore, each ADC step is equivalent to 732.4 μV. Taking into account the 0g offset of 1.5 V, the voltage signal when the accelerometer detects an acceleration of +3g would be 2.022 V ([3g × 0.174 V·g−1] + 1.5 V). Thus, the analog-to-digital quantization equals 2760 [(2.022 × 1000 × 1000) μV / 732.4 μV]. The raw-mode output in G force units (e.g., 3g) is obtained in the following manner: 2.022 V (voltage signal from accelerometer) − 1.5 V (0g offset) / 174 mV·g−1 (sensitivity of accelerometer).

Although the ADXL320 and the ADXL335 originally have unequal sensitivities of 174 and 270 mV·g−1, respectively, ActiGraph states that the sensitivity of the GT3X accelerometer is scaled to match that of the GT1M to ensure comparability in output. However, it is unclear if a GT1M initialized in the prefiltered raw mode will measure a higher range of accelerations (−5g to +5g) than the GT3X, which only measures accelerations in the range of −3g to +3g. If this is the case, the output from the GT1M and GT3X in prefiltered raw mode will not be similar to each other. Output in the postfiltered raw mode is not a representation of the actual raw acceleration signal because it is subjected to transformation associated with manufacturer specifications (e.g., band-pass filtering).

Firmware updates.

Firmware is defined as a software programming code containing a set of instructions that enable communication between the hardware components of a device. The firmware in the GT1M and the GT3X is stored on the onboard flash read-only memory, and it performs several functions, including filtering the accelerometer signals. In comparison, this function in the AM7164 was performed by hardware components. Because the firmware is stored within flash memory in the GT1M and the GT3X, it can be erased and replaced with updated versions. Scheduled firmware releases by ActiGraph usually coincide with improvements that are made to the devices. For example, firmware version 03.01.01 for the GT3X was a scheduled release containing added support for the new 16-MB memory devices. Similarly, firmware release 04.01.00 in July 2008 for the GT1M enabled users to measure accelerations in the anteroposterior axis in addition to the vertical axis. On the other hand, unscheduled firmware releases are also made when “bugs” are identified and need to be fixed. A potential bug we observed in our laboratory was the presence of significant differences among activity counts in the anteroposterior axis from the GT3X and the GT1M (firmware 04.01.00) at different walking and running speeds (3). This problem was particularly evident at the speed that marked the transition from walking to running (3). A subsequent firmware release for the GT1M resolved this likely bug by restoring the similarity in anteroposterior axis activity counts during walking and running between the two monitors.

Although susceptible to programming errors, an advantage of having a firmware-based system is the reduction in possible errors due to the loss of calibration. As compared with the firmware filter in the GT1M and the GT3X, the drift and tolerance characteristics of each of the multiple semiconductor components in the AM7164 hardware filter contribute to the possibility of a loss in calibration. ActiGraph states that the GT1M and the GT3X do not require calibration. However, in our laboratory, we have observed that the GT1M and the GT3X can lose calibration (D. John, unpublished observation, December 2009). Researchers are encouraged to use the option available in the ActiLife software (ActiGraph, Pensacola, FL) that allows one to confirm calibration of the GT1M and the GT3X.

Because the GT1M and the GT3X are used to measure physical activity, a possible way for these monitors to lose calibration is from exposure to impact forces, such as being dropped. Several capacitive accelerometer manufacturers strongly caution against dropping the accelerometer onto hard surfaces and also furnish drop test details in the technical specifications. Although the electronics of the GT1M and GT3X are enclosed in a hard polycarbonate plastic casing, we were curious to determine whether dropping a calibrated GT1M on a hard surface would cause the ADXL accelerometer within to lose calibration. To answer this question, we dropped a calibrated GT1M a few times on a concrete floor from a height of approximately 2.0 m. We selected this height because we were not aware of the ADXL320 accelerometer’s absolute maximum rating for a drop test, and the height of 1.8 m was the absolute maximum rating height for a similar capacitive accelerometer that we use in our laboratory. Dropping a GT1M from this height did not cause it to lose calibration. However, we cannot say whether calibration would be compromised if the monitor was repeatedly exposed to similar forces.

Next-generation ActiGraph Monitor.

ActiGraph has developed the next-generation GT3X+, which will be physically smaller and have a greater data storage capacity (256 MB) than the current GT3X (16 MB). In addition, this device is water submersible, thereby enabling the recording of water-based activities like swimming. The accelerometer in the GT3X+ can detect accelerations ranging between −6g and +6g. This monitor will collect activity data in the raw mode only and provides researchers with the flexibility of selecting 1 of 10 different data sampling rates ranging from 10 to 100 Hz in multiples of 10 Hz (e.g., 10, 20, or 30 Hz).

ACTICAL ACCELEROMETRY-BASED ACTIVITY MONITOR

The Actical is a 37 × 29 × 9-mm omnidirectional monitor. The accelerometer in the Actical is the solid-state Murata Piezotite® (Kyoto, Japan) PKGS-LD-R series sensor. The sensor is a 6.4 × 2.8 × 1.2-mm ceramic package, which encloses a rectangular bimorph lead zirconate titanate piezoelectric element. Although the Actical has a bimorph piezoelectric sensor like the AM7164, the two sensors in these devices detect accelerations in distinctly different ways. The sensor in the AM7164 uses a seismic mass cantilever beam mechanism. On the other hand, the sensor in the Actical monitor is surface mounted on the printed circuit board and does not have a seismic mass mounted at one end of the bimorph piezoelectric element. In the Actical, vibrations in response to accelerations are transferred from the ceramic enclosure of the sensor to the piezoelectric element, thereby deforming the sensor and resulting in the production of a proportional charge. Although the sensor is omnidirectional, it is positioned within the Actical in such a way that when it is worn on the hip, the monitor is most sensitive to vertical accelerations. The voltage generated by the sensor is amplified and filtered (0.5–3.0 Hz) by analog circuitry and then digitized by an ADC at a sampling frequency of 32 Hz. In contrast to the ActiGraph GT1M and GT3X, the Actical does not provide the flexibility of providing data in raw mode or in any time epoch lower than 15 s. However, Phillips Respironics will be launching a new Actical monitor, which can provide output in 1-s epochs and has increased memory capacity (256 KB).

FUTURE OF ACTIVITY MONITORS

Currently, the field of objective physical activity monitoring is still in its formative years. However, the introduction of accelerometry-based activity monitors has provided researchers with reasonably accurate and reliable monitors, as well as preliminary methods to estimate physical activity and energy expenditure in the free-living environment. Currently, scientists are working on developing novel monitors that measure accelerations at multiple locations on the body and also measure other physiological variables (e.g., respiratory rate and volume). Some researchers also are working on incorporating activity monitors into everyday devices like cellular phones. In addition, advanced energy expenditure estimation (using activity monitor output) techniques such as artificial neural networks and classification tree modeling are being used. The introduction of superior data processing methods and improved wearable activity monitors will help to advance the field of activity monitoring.

This work was supported by National Institutes of Health grant UO1-CA130783.

The authors report no conflicts of interest.

The authors thank Dr. David R. Bassett for his contributions to this article.

REFERENCES

1. Cavagna G, Saibene F, Margaria R. A three-directional accelerometer for analyzing body movements. J Appl Physiol. 1961; 16: 191.
2. Karcher J. A piezo electric method for the instantaneous measurement of high pressures. J Franklin Inst. 1922; 194 (6): 815–6.
3. Sasaki JE, John D, Freedson PS. Validation and comparison of ActiGraph activity monitors. J Sci Med Sport. 2011; 14 (5): 411–6.
4. Walter P. The history of the accelerometer. Sound Vib. 1997; 31 (3): 16–23.
5. Wong TC, Webster JG, Montoye HJ, Washburn R. Portable accelerometer device for measuring human energy expenditure. IEEE Trans Biomed Eng. 1981; 28 (6): 467–71.
Keywords:

ACTIVITY MONITOR; ACCELEROMETER; PHYSICAL ACTIVITY

©2012The American College of Sports Medicine