Effect of epoch length on intensity classification and on accuracy of measurement under controlled conditions on treadmill: Towards a better understanding of accelerometer measurement

Abstract

Purpose: The aim of this study was to analyze the effect of epoch length on intensity classification during continuous and intermittent activities.

Methods: Ten active students exercised under controlled conditions on a treadmill for four 5-min bouts by combining two effort intensities (running and walking) and two physical activity (PA) patterns (continuous or intermittent). The testing session was designed to generate a known level of moderate to vigorous PA (MVPA) for each condition. These PA levels were used as criterion measures to compare with the accelerometer measures. Data obtained from the accelerometer were reintegrated into 1-sec, 10-sec, 30-sec and 60-sec epochs. Equivalence testing was used to examine measurement agreements between MVPA values obtained with the different epochs and the reference values. Mean absolute percent errors (MAPE) were also calculated to provide an indicator of overall measurement error.

Results: During the intermittent conditions, only the value obtained with the 1-sec epoch was significantly equivalent to the reference value. With longer epochs the difference increased for both intermittent conditions but in an opposite way: with longer epochs, MVPA decreased during walking but increased during running. Regarding the measurement accuracy, the pattern of variations according to the epoch length selected during the intermittent conditions was identical between walking and running: MAPE increased with the increase in epoch length. MAPE remained low only for the 1-sec epoch (7.6% and 2.7% for walking and running, respectively), increased at 31.3% and 34% for the 10-sec epoch and until near 100% with the 30- and 60-sec epoch lengths.

Conclusion: This study highlighted the misclassification of exercise intensity based on accelerometer measurement and described for the first time the extent and the direction of this misclassification. Moreover, we can confirm that the shorter epochs are more accurate to measure the real exercise intensity during intermittent PA whatever the intensity.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1. Results from 95% equivalence testing for agreement in time MVPA according to the different epoch lengths selected.

Upper and lower panels represent data from the continuous and intermittent conditions, respectively. Dark lines represent the reference values (i.e., 5-min for continuous conditions and 2.5-min for intermittent conditions) and the dotted lines indicate the equivalence zone (±10% of the reference value); Grey and dark bars represent the 90% confidence interval for the mean MVPA value obtained with the corresponding epoch. * Within the equivalence zone. a Significantly different from the 10-sec epoch with p<0,001. b Significantly different from the 30-sec epoch with p<0,001. c Significantly different from the 60-sec epoch with p<0,001.

Fig 2. Mean absolute percentage error (±SD) between measurements with the different epochs and the reference value (i.e., MVPA = 5-min for continuous conditions and MVPA = 2.5-min for intermittent conditions).

a Significantly different from the 10-sec epoch with p<0,001. b Significantly different from the 30-sec epoch with p<0,001. c Significantly different from the 60-sec epoch with p<0,001.