Wrist-worn accelerometers: recommending ~1.0 mg as the minimum clinically important difference (MCID) in daily average acceleration for inactive adults

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. Contemporary accelerometers measure acceleration at high frequency in three axes. Typically, the Euclidean norm is calculated and 1 g subtracted to correct for gravity (Euclidean Norm Minus One (ENMO)). Average acceleration across the day is expressed in milli-gravitational units (m g ) 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 We used three approaches to provide converging evidence for a preliminary estimate of the MCID in m g for inactive people (figure 1). Figure 1 Converging evidence for a preliminary estimation of the MCID in m g for inactive people. (1) An increase or difference of 500 steps per day, is associated with a 2% to 9% decreased risk of cardiovascular morbidity and all-cause mortality.3–5 (1a) A change in 500 steps per …