Deviation between self-reported and measured occupational physical activity levels in office employees: effects of age and body composition

Objectives Whether occupational physical activity (PA) will be assessed via questionnaires or accelerometry depends on available resources. Although self-reported data collection seems feasible and inexpensive, obtained information could be biased by demographic determinants. Thus, we aimed at comparing self-reported and objectively measured occupational sitting, standing, and walking times adjusted for socio-demographic variables. Methods Thirty-eight office employees (eight males, 30 females, age 40.8 +/- A 11.4 years, BMI 23.9 +/- A 4.2 kg/m(2)) supplied with height-adjustable working desks were asked to report sitting, standing, and walking times using the Occupational Sitting and Physical Activity Questionnaire during one working week. The ActiGraph wGT3X-BT was used to objectively measure occupational PA during the same week. Subjectively and objectively measured data were compared computing the intra-class correlation coefficients, paired t tests and Bland-Altman plots. Furthermore, repeated-measurement ANOVAs for measurement (subjective vs. objective) and socio-demographic variables were calculated. Results Self-reported data yielded a significant underestimation of standing time (13.3 vs. 17.9 %) and an overestimation of walking time (12.7 vs. 5.0 %). Significant interaction effects of age and measurement of standing time (F = 6.0, p = .02, eta (p) (2) = .14) and BMI group and measurement of walking time were found (F = 3.7, p = .04, eta (p) (2) = .17). Older employees (> 39 years) underestimated their standing time, while underweight workers (BMI < 20 kg/m(2)) overestimated their walking time. Conclusions Self-reported PA data differ from objective data. Demographic variables (age, BMI) affect the amount of self-reported misjudging of PA. In order to improve the validity of self-reported data, a correction formula for the economic assessment of PA by subjective measures is needed, considering age and BMI.