(Un)aware of change? Objective and subjective reactivity to the sensor‐based measurement of physical activity and sedentary behaviour

$\textbf{Objectives}$
Despite being the gold standard for assessing physical activity (PA) under free-living conditions, accelerometer-based assessments are prone to measurement reactivity, that is, changes in people's cognitions, emotions or behaviour due to being measured. Little is known about whether participants are aware of these changes. This preregistered secondary analysis assesses objective and subjective reactivity to the sensor-based measurement of PA (steps, light PA [LPA], moderate-to-vigorous PA [MVPA]) and sedentary behaviour (SB) in adults with and without attention-deficit/hyperactivity disorder (ADHD), depression and insulin resistance.

$\textbf{Design}$
Intensive longitudinal design.

$\textbf{Methods}$
Two existing datasets were used, comprising 354 adults (64% female; Mage = 35.63, SDage = 13.98; MBMI = 26.27, SDBMI = 5.85) who wore an accelerometer for 7 days and self-reported whether they changed their PA due to the activity tracker (i.e. subjective measurement reactivity). Objective measurement reactivity was defined as decreases in PA and increases in SB.

$\textbf{Results}$
Multilevel models found small, but significant decreases of PA over the study period and from the first study day(s) to the remaining study days. ... mehrYet, over 90% of participants indicated to have (rather) not changed their PA. SB did not systematically change. Subjective measurement reactivity and clinical diagnoses were not related to measurement reactivity.

$\textbf{Conclusions}$
Steps, LPA and MVPA, but not SB, systematically changed in line with common definitions of measurement reactivity. Yet, most participants were unaware of these changes, potentially because changes were small. Measurement reactivity may be predominantly driven by unconscious mechanisms and appears universal, with no differences found by diagnoses.