JMIR Publications

ABSTRACT

Background:

Accelerometers, often in conjunction with heart rate sensors, are extensively used to track physical activity (PA) in research. Research grade instruments are often expensive, have limited battery capacity, limited storage, and high participant burden. Consumer-based activity trackers are equipped with similar technology and are designed for long-time wearing, and can therefore potentially be used in research.

Objective:

We aimed to assess the criterion validity of the Polar M430 sport watch, when compared with two research grade instruments (ActiGraph and Actiheart), worn on four different locations using one- and three-axis accelerometers.

Methods:

Fifty participants wore two ActiGraphs (wrist and hip), two Actihearts (upper and lower chest position), and one Polar M430 sport watch, for one full day. We compared reported minutes in sedentary behaviour, light, moderate, vigorous, and moderate-to-vigorous PA, steps, activity energy expenditure, and total energy expenditure between devices. We used Pearson correlations, intra-class-correlations, mean absolute percentage errors, and Bland-Altman plots to assess criterion validity.

Results:

Pearson correlations between the Polar M430 and all research grade instruments were moderate or stronger for vigorous PA (r=0.59-0.76), moderate-to-vigorous PA (r=0.51-0.75), steps (r=0.85-0.87), total energy expenditure (r=0.88-0.94), and activity energy expenditure (r=0.74-0.79). Bland-Altman plots showed higher agreement for higher intensities of PA. Mean absolute percentage error (MAPE) was high for most outcomes. Only total energy expenditure measured by the hip-worn ActiGraph and both Actihearts had acceptable or close to acceptable errors with a MAPE of 6.94% (ActiGraph, one axis), 8.26% (ActiGraph, three axis), 14.54% (Actiheart, upper position), and 14.37% (Actiheart, lower position). The wrist-worn ActiGraph had a MAPE of 15.94% for measuring steps. All other outcomes had a MAPE of 24% or higher. For most outcomes, the Polar M430 was strongest correlated with the hip-worn triaxial ActiGraph, with a moderate or strong Pearson correlation for sedentary behaviour (r=0.52), light PA (r=0.7), moderate PA (r=0.57), vigorous PA (r=0.76), and moderate-to-vigorous (r=0.75) PA. In addition, correlations were strong or very strong for activity energy expenditure (r=0.75), steps (r=0.85), and total energy expenditure (r=0.91).

Conclusions:

The Polar M430 can potentially be used as an addition to established research grade instruments to collect some PA variables over a prolonged period. However, due to the high MAPE of most outcomes, only total energy expenditure can be trusted to provide close to valid results. Depending on variable, the Polar M430 over- or under-reports most metrics, and may therefore be better suited to report changes in PA over time for some outcomes, rather than as an accurate instrument for PA status in a population.