One size does not fit all - translating absolute accelerometry to relative individual physical activity intensity for health
Authors: Fridolfsson J , Arvidsson D , Ekblom-Bak E , Ekblom Ö , Bergstrom G , Borjesson M 
Published: May 2023

Abstract

Funding Acknowledgements
Type of funding sources: Foundation. Main funding source(s): The main funding body of The Swedish CArdioPulmonary bioImage Study (SCAPIS) is theSwedish Heart-Lung Foundation. The study is also funded by the Knut and Alice Wallenberg Foundation, the Swedish Research Council and VINNOVA (Sweden’s Innovation agency) the University of Gothenburg and Sahlgrenska University Hospital, Karolinska Institutet and Stockholm county council, Linköping University and University Hospital, Lund University and Skåne University Hospital, Umeå University and University Hospital, Uppsala University and University Hospital.

Introduction
Physical activity intensity can be expressed in either absolute (e.g. brisk walking or metabolic equivalents) or relative terms (e.g. proportion of maximal oxygen consumption or perceived exertion) (1). Although intervention studies typically use relative intensity for exercise prescription, large scale observational studies measuring physical activity with accelerometers always use absolute intensity. The association between relative physical activity intensity and cardiometabolic risk factors has not been studied using accelerometry previously.

Purpose
To compare absolute and relative measures of physical activity intensity in terms of physical activity level and associations with cardiovascular risk factors among individuals with different fitness level.

Methods
A subsample of the Swedish CArdioPulmonary bioImage Study (SCAPIS), in total 4234 men and women aged 50-64, was analysed (2). Physical activity was measured by accelerometers and the raw data processed with the 10 Hz frequency extended method (FEM) to get a more accurate measure of physical activity intensity compared to previous methods (3). Maximal oxygen consumption (fitness) was estimated by a submaximal ergometer test. Waist to hip ratio, systolic blood pressure, glycated haemoglobin (HBA1c) and high-density lipoprotein to total cholesterol ratio were combined into a composite cardiometabolic risk factor score. Partial least squares regression was used to investigate the associations of absolute and relative physical activity intensity with fitness as well as the composite score. The sample was divided into tertiles of fitness for stratified analyses.

Results
Overall, there was an association between physical activity at absolute moderate intensity and above, and the health outcomes. Yet, the main associations were found in the absolute moderate intensity range for the low fitness group and in the absolute vigorous intensity range for the high fitness group. When considering relative intensity however, all the main associations started in the upper part of the moderate intensity range and peaked in the vigorous intensity range (Figure 1). In addition, when comparing absolute and relative cut-offs for moderate intensity, absolute moderate intensity was too low for 95% of individuals in the sample (Figure 2). When using absolute intensity, 99% of individuals reached the general guidelines of 150 minutes of moderate-to-vigorous physical activity per week, while only 21% reached the guidelines based on relative intensity.

Conclusions
Health benefits of absolute intensity are misleading for most individuals in this sample and absolute measures of physical activity overestimate time spent at moderate intensity and above. Relative intensity should be used when interpreting accelerometer measured physical activity and when communicating health promoting physical activity.