**Daniel Arvidsson 1,\*, Jonatan Fridolfsson 1, Christoph Buck 2, Örjan Ekblom 3, Elin Ekblom-Bak 3, Lauren Lissner 4, Monica Hunsberger <sup>4</sup> and Mats Börjesson 1,5,6**


Received: 30 June 2019; Accepted: 30 July 2019; Published: 1 August 2019

**Abstract:** Accelerometer calibration for physical activity (PA) intensity is commonly performed using Metabolic Equivalent of Task (MET) as criterion. However, MET is not an age-equivalent measure of PA intensity, which limits the use of MET-calibrated accelerometers for age-related PA investigations. We investigated calibration using VO2net (VO2gross <sup>−</sup> VO2stand; mL·min−1·kg<sup>−</sup>1) as criterion compared to MET (VO2gross/VO2rest) and the effect on assessment of free-living PA in children, adolescents and adults. Oxygen consumption and hip/thigh accelerometer data were collected during rest, stand and treadmill walk and run. Equivalent speed (Speedeq) was used as indicator of the absolute speed (Speedabs) performed with the same effort in individuals of different body size/age. The results showed that VO2net was higher in younger age-groups for Speedabs, but was similar in the three age-groups for Speedeq. MET was lower in younger age-groups for both Speedabs and Speedeq. The same VO2net-values respective MET-values were applied to all age-groups to develop accelerometer PA intensity cut-points. Free-living moderate-and-vigorous PA was 216, 115, 74 and 71 min/d in children, adolescents, younger and older adults with VO2net-calibration, but 140, 83, 74 and 41 min/d with MET-calibration, respectively. In conclusion, VO2net calibration of accelerometers may provide age-equivalent measures of PA intensity/effort for more accurate age-related investigations of PA in epidemiological research.

**Keywords:** physical activity; accelerometer; VO2; calibration; MET; VO2net; speed; equivalent speed; free-living; children; adolescents; adults
