Dose-response associations between accelerometry measured physical activity and sedentary time and all cause mortality: systematic review and harmonised meta-analysisBMJ 2019; 366 doi: https://doi.org/10.1136/bmj.l4570 (Published 21 August 2019) Cite this as: BMJ 2019;366:l4570
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Re: Dose-response associations between accelerometry measured physical activity and sedentary time and all cause mortality: systematic review and harmonised meta-analysis
We would like to commend Ekelund et al. on their recent systematic review and meta-analysis (1). This study presents evidence to support the dose response relationship between total physical activity, sedentary time and mortality in middle aged and older adults. A key strength of this study is the use of accelerometer measured physical activity and sedentary time. However, we would like to highlight what we believe is an important limitation of this study.
The authors have applied the same accelerometer thresholds for physical activity intensities (sedentary, light, low-light, high-light, moderate-to-vigorous and vigorous) to the whole study population. These thresholds have been derived from calibration studies based on small numbers of healthy young adults, often based on laboratory activity such as treadmill walking and not free-living activity. This limits applicability to the wider population including older adults and chronic disease, likely underestimating absolute physical activity levels in a proportion of the population (2). We recently highlighted this limitation in another study, where the same threshold for moderate-to-vigorous physical activity was applied to individuals with and without chronic disease.(3,4) Defining physical activity intensity for the whole population with a single accelerometer threshold, makes the strong (and we believe inappropriate) assumption that energy expenditure for a given physical activity is the same for every individual, and does not take into account an individual’s exercise capacity, age, or disease state, all of which have been shown to influence energy expenditure and accelerometer counts (5,6). The likely consequence is the misclassification of physical activity. Specifically, accelerometer values labelled as light intensity in low fit populations are more likely to represent moderate intensity preventing any firm conclusions about the benefits of light intensity physical activity.
The authors also failed to arrive at firm conclusions on the effect of bout duration for a given volume of activity. Again, we suggest this may reflect a limitation of current methods for determining the duration of a bout of activity. In the method adopted in the study a 20-minute continuous bout of physical activity at 1951 cpm would be not be classed as a bout of MVPA whereas 8 minutes (allowing for a 1-2 minute interruption) at 1952 cpm would be, even though the former would accumulate a greater volume.
Consensus regarding accelerometer data processing and improved methods for estimating the intensity of physical activity are required. We agree with future research recommendations made by Ekelund et al., for further calibration studies with use of tri-axial accelerometers and the need to consider individuals’ exercise capacity in the understanding and interpretation of physical activity behaviours, not only in the elderly but also chronic disease populations. This will help to better understand the relationships between physical activity and sedentary time with mortality and other health outcomes. This is an important consideration, as current physical activity guidelines consistently recommend the same levels of physical activity for adults irrespective of age and chronic disease, a recommendation which may not be appropriate.
1. Ekelund U, Tarp J, Steene-Johannessen J, Hansen BH, Jefferis B, Fagerland MW, et al. Dose-response associations between accelerometry measured physical activity and sedentary time and all cause mortality: systematic review and harmonised meta-analysis. BMJ [Internet]. 2019 Aug 21;l4570. Available from: http://www.bmj.com/lookup/doi/10.1136/bmj.l4570
2. Rejeski WK, Marsh AP, Brubaker PH, et al. Analysis and interpretation of accelerometry data in older adults: the LIFE study. J Gerontol A Biol Sci Med Sci 2016;71(4):521-8.
3. Dibben GO, Taylor RS, Dalal HM, Hillsdon M. One size does not fit all – application of accelerometer thresholds in chronic disease. International Journal of Epidemiology 2019; 48(4): 1380.
4. Barker J, Smith Byrne K, Doherty A, et al. Physical activity of UK adults with chronic disease: cross-sectional analysis of accelerometer-measured physical activity in 96706 UK Biobank participants. International Journal of Epidemiology 2019; 1-8.
5. Evenson KR, Wen F, Herring AH, et al. Calibrating physical activity intensity for hip-worn accelerometry in women age 60 to 91 years: The Women’s Health Initiative OPACH Calibration Study. Prev Med Rep2015;2:750-6. doi:10.1016/j.pmedr.2015.08.021 pmid:26527313.
6. Hall KS, Howe CA, Rana SR, et al. METs and accelerometry of walking in older adults: standard versus measured energy cost. Med Sci Sports Exerc 2013; 45(3):574-82.
Competing interests: No competing interests