The accelerometer has an advantage over the pedometer by its ability to distinguish between walking and running on level terrain. Also, many of the newer accelerometer can separate human movement from movement outside of the human range such as mechanical vibration which may result from riding in a car. Currently, accelerometers are considered to be one of the current standards in assessing free living physical activity levels and are often used to validate the much simpler, less expensive pedometer and physical activity questionnaires.
Accelerometers operate by measuring acceleration along a given axis, using any of a number of technologies including piezo–electric, micro–mechanical springs, and changes in capacitance (Welk, 2002)1. Multiple axis measurements can also be bundled into a single monitor, allowing movement to in multiple planes of movement to be captured. The major function of accelerometers is that the sensor converts movements into electrical signals (counts) that are proportional to the muscular force producing motion (Melanson and Freedson, 1996)2. These counts are summed over a specified person of time (epoch) and stored. The counts recorded by accelerometers can be compared to laboratory establish cut points to relate to MET values. Many accelerometers have the storage capacity to assess physical activity over 21 day periods using a 60 second epoch.
Accelerometers have been utilized in a variety of research settings and proven to be both reliability and valid assessment of physical activity in various populations ranging from children to older adults. However, it is important to note that each population has its own challenges and can require a range 3–12 days of monitoring for accurate assessment. To obtain consistent measures of physical activity patterns in adults, Matthews et al3 found at least seven days of monitoring is required. Trost et al.4 determined 4–5 days of accelerometer monitoring in children and 8–9 days in adolescents were necessary to achieve a between day intra–class correlation (ICC) reliability level of 0.80.
While accelerometer has been shown to be valid at measuring walking and running activities, there are limitations worth nothing. Accelerometer equations have been developed for specific activities (e.g. walking, running, rest) and do not accurately estimate other activities (e.g. stationary biking, elliptical trainer). The consensus appears to be that for activities entirely composed of flat-ground ambulation and rest, accelerometers can provide objective measures of activity. Accelerometers also have the inability to capture upper extremity movement such may lead to underestimating of household activities such as washing dishes. Underestimation may also result when performing weight bearing activities.