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Physical activity is increasingly assessed using wrist-worn accelerometry.1 The primary unit of measurement is acceleration which lacks an obvious concrete meaning in the clinical and public health settings. If the scientific community agreed on a minimum clinically important difference (MCID) that would greatly help users interpret accelerometry data in a more meaningful way. Here we present converging evidence to inform estimation of the MCID in physical activity for inactive adults, expressed as average acceleration measured from wrist-worn accelerometers.
What is average acceleration?
Contemporary accelerometers measure acceleration at high frequency in three axes. Typically, the Euclidean norm is calculated and 1g subtracted to correct for gravity (Euclidean Norm Minus One (ENMO)). Average acceleration across the day is expressed in milli-gravitational units (mg) and reflects overall daily activity, as does steps/day. Multiple days of measurement are usually summarised as daily average acceleration. Reflecting the value of capturing all physical activity across the whole day—rather than moderate-to-vigorous activity alone—average daily acceleration is an appropriate summary variable of physical activity status in most cases. Average daily acceleration is the recommended variable for physical activity analysis in UK Biobank, where >100 000 people wore a wrist-worn accelerometer for 7 days.1
What is the MCID in average acceleration (ENMO, mg) measured at the wrist?
We used three approaches to provide converging evidence for a preliminary estimate of the MCID in mg for inactive people (figure 1).
Contributors Conception/design: TY and AR. Data analysis/acquisition/interpretation: AR, TY, PD, CR and CE. Drafting/revision critically for important content: All authors. Final approval: All authors.
Funding University of Leicester authors are supported by the NIHR Leicester Biomedical Research Centre, and the NIHR Applied Research Collaborations – East Midlands (NIHR ARC – EM). The SMArT Work trial was funded by the Department of Health Policy Research Programme (project No PR-R5-0213-25004) and the SMART Work and Life trial is funded by the National Institute for Health Research Public Health Research programme (project number 16/41/04). The views expressed are those of the authors and not necessarily those of the NHS, NIHR or Department of Health. PCD is supported by a National Health and Medical Research Council of Australia Fellowship (#1142685) and the UK Medical Research Council (#MC_UU_12015/3).
Competing interests This research was supported by the NIHR Leicester Biomedical Research Centre.
Patient consent for publication Not required.
Ethics approval This is a secondary data analysis. All participants gave written informed consent prior to data collection.UK Biobank has full ethical approval from the NHS National Research Ethics Service (16/NW/0274).
Provenance and peer review Not commissioned; internally peer reviewed.
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