Course:KIN366/ConceptLibrary/Accelerometers

Movement Experiences for Children
KIN 366
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Instructor: DR. Shannon S.D. Bredin
Email: shannon.bredin@ubc.ca
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Brief Overview

Using objective measures like accelerometers to study physical activity has become more customary with researchers, as shown by the vast number of research articles using the device (Evenson, 2009). Accelerometers are advantageous because they eliminate several factors including language or literacy difficulties, recall bias, and social desirability bias present with self-report measures of physical activity (Evenson, 2009). They are available in many different shapes and sizes, and are now increasingly popular and found in all sorts of things like pedometers, video game controllers, and cell phones (Adriana, 2010). When measuring physical activity, they are typically attached to an individual’s waist, wrist, ankle, or even their shoe (PARCPH, 2015).

Definition

According to the Encyclopedia Britannica (2015), an accelerometer is an “instrument that measures the rate at which the velocity of an object is changing (ie. its acceleration).” Because acceleration cannot be measured directly, an accelerometer “measures the force exerted by restraints that are placed on a reference mass to hold its position fixed in an accelerating body” (Encyclopedia Britannia, 2015). Acceleration is then computed by using the relationship between restraint force and the acceleration that can be found using Newton’s second law: force = mass x acceleration (Encyclopedia Britannia, 2015).

History

The concept of an accelerometer was initially designed to validate universal gravity and other Newtonian physic phenomenons (Adriana, 2010). The Atwood Machine, developed in 1783 by an English physicist named George Atwood, was regarded as the first accelerometer (Adriana, 2010). The first resistance-based accelerometer was introduced by Burton McCollum and Orville Peters in 1923 and became commercialized in the United States in 1927 (Walter, 2013). In 1943, Per Bruel and Viggo Kjaer produced and sold the first commercial piezoelectric accelerometers, which were designed to focus on sound and vibration measurements (Walter, 2013). As it turns out, much of the technology and gadgets we see today still utilize piezoelectric accelerometers as the principle source for measurement (Walter, 2013).

How They Work

Accelerometers measure body movement in terms of acceleration. They measure acceleration along a given axis, using any of a number of technologies including piezoelectric, micromechanical springs, and changes in capacitance (PARCPH, 2015). There is a slight improvement in validity estimates when accelerometers with more than one axis are used (Troiano, 2005). Movements are measured in 1 to 3 orthogonal planes: anterior-posterior, medial-lateral, and vertical (Troiano, 2005). Movement in multiple planes can be captured when multiple axis measurements are bundled into a single monitor (PARCPH, 2015). The sensor converts movements into electrical signals – counts – that are proportional to the muscular force producing motion (PARCPH, 2015). These counts are summed over a specified portion of time, epochs, and are then be compared to MET values (PARCPH, 2015). Some accelerometers have the storage capacity to assess physical activity over a 21 day period by simply using a 60 second epoch (PARCPH, 2015). Using accelerometers is commonly related to intensity of physical activity, and they can be placed on the wrist, ankle, hip, or lower back (Troiano, 2005). For locomotion, the hip or back is preferred because it is closest to an individual’s center of mass (Troiano, 2005). Using multiple sensors captures the movement of extremities, but the marginal improvement in accuracy is not always worth the practical costs (Troiano, 2005).

Practical Applications

For Children

In recent years, accelerometers have been widely used to characterize physical activity behavior and sedentary behaviour in children (Freedson et al, 2005). These objective methods of assessing physical activity are recommended as an alternative to self-report because they are not subject to the sources of error associated to recall (Freedson et al, 2005). Accelerometers have also been used in studies to monitor change in physical activity while undergoing childhood obesity treatment interventions (Robertson et al, 2011). Children were asked to wear an Actigraph accelerometer for 7-days, which measured a wide range of their physical activities including cycling, running, and trampolining (Robertson et al, 2011). Accelerometers are able to measure locomotor activity, the primary source of physical activity (Troiano, 2005). They have been utilized in various research settings and have proven to be reliable and valid assessments of physical activity in children (PARCPH, 2015).

Accelerometers have become part of the quantified self movement: the increasing use of technology to collect data about oneself (Rettner, 2013). Technologies such as smartphone apps, GPS devices, and physical activity trackers with accelerometers have allowed individuals to track every aspect of their daily lives. This includes their total activity, the number of steps they take, the food they eat, the amount of sleep they get, their heart rate and even their mood (Rettner, 2013). People are then able to learn more about themselves and therefore take action to become healthier and improve their lives (Rettner, 2013). A specific example of a quantified self-tracking tool that uses an accelerometer is the Fitbit, which can track steps taken, distance traveled, calories burned, and hours and quality of sleep (Rettner, 2013). Devices that are similar to the Fitbit include the Jawbone Up, Withings Pulse, and the Nike+ FuelBand (Rettner, 2013). The improved technology in fitness trackers has changed the way researchers study exercise by allowing them to gather much more detailed information about how people move throughout the day (Rettner, 2012). The advances in accelerometers has provided researchers the opportunity to examine not only exercise, but also sitting, standing and walking, to then get a better idea of how these activities affect health (Rettner, 2012).

Benefits

Accelerometers are superior to pedometers because of their ability to distinguish between walking and running on level terrain (PARCPH, 2015). Newer accelerometers are able to separate human movement from movement outside of the human range such as mechanical vibration that results from riding in a car (PARCPH, 2015). They can be used when we are assessing activities that are routine and occur throughout the day (Troiano, 2005). By using this technology to track these behaviours that can be highly subjective, we improve the reliability and validity of our findings (Troiano, 2005). Acclerometers provide objective data and remove the cognitive aspect of data collection (Troiano, 2005). Research using accelerometers has escalated since the mid 1990s because they can measure the intensity and frequency of physical activity (Robertson et al, 2011). Advances in accelerometer technology has increased the information they can collect, with some able to capture data 80 times a second, in three directions (Rettner, 2012).

Limitations

Accelerometers have been proven to precisely measure physical activity, but there are still some limitations that should be noted (PARCPH, 2015). Accelerometers do not provide the context of an activity; they primarily measure locomotive activity, rather than total activity or energy expenditure (Troiano, 2005). Their equations were developed for specific activities such as walking and running, but they do not accurately measure other activities like stationary biking or elliptical training (PARCPH, 2015). Thus, accelerometers provide objective measure of activity for flat-ground ambulation and rest (PARCPH, 2015). Underestimation is common, as the devices are prone to miss upper or lower extremity movement, depending on where the accelerometer is placed on the body. The devices also fail to distinguish between weight-loaded activities and non-loaded activities. As a result, accelerometers are most commonly used for walking and running activities to achieve the highest level of accuracy (Troiano, 2005).

References

Adriana, N. (2010). An accelerometer and its many different uses. http://ezinearticles.com/?An-Accelerometer-And-Its-Many-Different-Uses&id=5531500

Evenson, K. R., & Terry Jr., J. W. (2009). Assessment of differing definitions of accelerometer nonwear time. Research Quarterly For Exercise & Sport, 80(2), 355-362.

Freedson, P., Pober, D., Janz, K.F. (2005) Calibration of accelerometer output for children. Journal of the American College of Sports Medicine. Vol. 37, No. 1 I (Suppl), pp. S523-S530, 2005.

PARCPH - Physical Activity Resource Center for Public Health. (2015). Accelerometers. Retrieved from http://www.parcph.org/accDef.aspx

Rettner, R. (2012). Fitness & big data: how wearable tech is changing exercise research? Retrieved from http://www.livescience.com/45634-accelerometers-exercise-research.html