Every step you take

Sir,

The article by Ekelund et al provides interesting insights into the relationship between physical activity, sedentary behaviour, and mortality.(1) One of the strengths of their research is the use of objective measures of physical activity and sedentary behaviour, in contrast to self-reported physical activity.(1,2) Their findings indicate a non-linear dose-response between all levels of physical activity and mortality, with low light intensity physical activity showing a consistent decline over time. In contrast, the reduction in mortality for all other levels of physical activity declines up to a threshold, then either plateaus, or increases. The reverse is true of sedentary behaviour, with greater sedentary behaviour associated with greater risk of mortality. These findings support guidance on physical activity issued by the World Health Organization (WHO).(3,4)

However, there are some concerns with respect to the research.

First, over 70% of subjects were female. Globally, women are less physically active than men across ages.(4) Therefore, the physical activity levels obtained from subjects are less likely to be representative of the general population, even in the USA and Scandinavian countries (where most of the studies were conducted).

Next, the subjects were on average 62.6 years old, by which time most individuals experience considerable musculoskeletal morbidity. This would adversely affect physical activity levels, and potentially increase sedentary behaviour. The authors state that what is classified as low-intensity physical activity might actually constitute moderate-vigorous physical activity for some subjects- perhaps an acknowledgement of their decreased physical ability to engage in physical activity. This also explains the paucity of readings that fell in the moderate-vigorous physical activity range.

Nevertheless, the findings bolster efforts to encourage any physical activity and decrease sedentary behaviour. However, similar research on younger individuals, and more representative samples is required. It would have been interesting to examine if the data supported findings from another study where one hour of physical activity mitigated the effects of sedentary behaviour.(5)

Given the propensity of physically inactive individuals to defer physical activity, it is heartening to note that this research provides evidence of all-cause mortality declining with increased duration of even low light-intensity physical activity. Such activity can be performed by most individuals without difficulty, and should be encouraged by clinicians and public health professionals alike. Moreover, the simplest of these activities - walking - does not require any equipment, and can be performed safely both outdoors and indoors. Since the evidence does not support a minimum duration of 10 minutes for physical activity, individuals could engage in brief activities multiple times in a day.(1) Slogans such as ‘Move for Life’, ‘Walk for 30’ and ‘Sitting is the new smoking’ could help reinforce research evidence in the general public.(3)

References:
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. Roopesh Johnson L, Prabha CC, Prakash D, Vasantha DG, Mohandas G, Reghunath K, et al. Pattern of And Risk Factors For Physical Activity Among Medical Students In A Private Institution. Natl J Res Community Med [Internet]. 2018 Jul 31;7(2):223. Available from: http://commedjournal.in/article/Volume7Issue3Jul-Sep2018/fulltext/16NJRC...
3. Johnson LR. Current physical activity guidance is misinterpreted. BMJ [Internet]. 2016 Sep 14;354(September):i4811. Available from: http://www.bmj.com/lookup/doi/10.1136/bmj.i4811
4. World Health Organization. WHO Fact sheet on Physical activity [Internet]. 2018 [cited 2019 Aug 28]. Available from: http://www.who.int/en/news-room/fact-sheets/detail/physical-activity
5. Ekelund U, Steene-Johannessen J, Brown WJ, Fagerland MW, Owen N, Powell KE, et al. Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality? A harmonised meta-analysis of data from more than 1 million men and women. Lancet [Internet]. 2016 Sep;388(10051):1302–10. Available from: http://dx.doi.org/10.1016/S0140-6736(16)30370-1