Reactivity to accelerometer measurement of children and adolescents.
OBJECTIVE
Awareness of being monitored can influence participants' habitual physical activity (PA) behavior. This reactivity effect may threaten the validity of PA assessment. Reports on reactivity when measuring the PA of children and adolescents have been inconsistent. The aim of this study was to investigate whether PA outcomes measured by accelerometer devices differ from measurement day to measurement day and whether the day of the week and the day on which measurement started influence these differences.


METHODS
Accelerometer data (counts per minute [cpm]) of children and adolescents (n = 2081) pooled from eight studies in Switzerland with at least 10 h of daily valid recording were investigated for effects of measurement day, day of the week, and start day using mixed linear regression.


RESULTS
The first measurement day was the most active day. Counts per minute were significantly higher than on the second to the sixth day, but not on the seventh day. Differences in the age-adjusted means between the first and consecutive days ranged from 23 to 45 cpm (3.6%-7.1%). In preschoolchildren, the differences almost reached 10%. The start day significantly influenced PA outcome measures.


CONCLUSIONS
Reactivity to accelerometer measurement of PA is likely to be present to an extent of approximately 5% on the first day and may introduce a relevant bias to accelerometer-based studies. In preschoolchildren, the effects are larger than those in elementary and secondary schoolchildren. As the day of the week and the start day significantly influence PA estimates, researchers should plan for at least one familiarization day in school-age children and randomly assign start days.
1Swiss Federal Institute of Sport Magglingen SFISM, Magglingen, SWITZERLAND; 2Department of Epidemiology and Public Health, Swiss TPH, Basel, SWITZERLAND; 3University of Basel, Basel, SWITZERLAND; 4Institute for Pre-Primary/Early Education, School for Teacher Education, University of Applied Sciences and Arts Northwestern Switzerland, Brugg, SWITZERLAND; 5Interdisciplinary Center for General Ecology (IKAÖ), University of Bern, SWITZERLAND; 6Division of Endocrinology, Diabetes and Metabolism, University of Lausanne, Lausanne, SWITZERLAND; and 7Institute of Social and Preventive Medicine, University of Zurich, SWITZERLAND.
Alain
A
Dössegger
Nicole
N
Ruch
Gerda
G
Jimmy
Charlotte
C
Braun-Fahrländer
Urs
U
Mäder
Johanna
J
Hänggi
Heidi
H
Hofmann
Jardena J
JJ
Puder
Susi
S
Kriemler
Bettina
B
Bringolf-Isler
D056044:Actigraphy / Q000295:instrumentation*; D000293:Adolescent; D001364:Awareness; D002648:Child; D002675:Child, Preschool; D015444:Exercise / Q000502:physiology* / Q000523:psychology*; D005260:Female; D015438:Health Behavior; D006801:Humans; D008297:Male; D015203:Reproducibility of Results