Study population and design
Data from the FEPA (Flemish Employees’ Physical Activity) study were used, this study has been described in detail elsewhere (23). In short, participants were recruited by means of convenience sampling across seven different companies within the service and manufacturing sector. The included companies approved that the data collection could be carried out at the workplace during working hours. Inclusion criteria for the workers were non-pregnancy, no exclusive night workers, employment rate of at least 50%, and a sufficient knowledge of the Dutch language. In total 1135 eligible workers were contacted and invited to participate in the study. Of this sample, 430 workers (response rate of 38%) were willing to participate voluntarily and signed an informed consent. Data were collected from February 2017 until June 2018. Eventually, 29 participants did not fill in the questionnaire and/or did not participate in the objective ambulatory registration due to drop-out or technical issues, which leads to a final sample of 401 participants. The final sample included both workers engaged in physically demanding work (i.e. employees working in health care, manufacturing, food and plastic sector) and workers primarily engaged in sedentary work (i.e. administrative professions). A more detailed overview of the flow of the recruitment of the study population is shown in an additional figure [see Additional file 1]. The FEPA study was approved by the Research Ethical Committee of Ghent University Hospital (number 2017/0129).
Procedure
After receiving a study invitation letter, interested participants were asked to complete a written informed consent form, to fill in a self-reported questionnaire including the OSPAQ (paper or online version) and to hand it over at the clinical screening appointment. A trained researcher conducted the clinical screening at the workplace of the participants during working hours and attached two accelerometers that were worn for at least 2 consecutive working days, 24-hours/day. More specific, the participants were asked to wear the devices on typical/normal working days. During this recording time, the participants were asked to fill in a paper-based diary to report their daily routines (time intervals of work, leisure time, bedtime, and non-wear time). Additionally, they had to report daily the time at which they performed the reference measurement (15 seconds of standing still in an upright and neutral position). This measurement was needed to calibrate the accelerometers. After the measurement period, the participants returned their equipment and diary to the researcher.
Measurements
Questionnaire
The questionnaire included information about participants’ socio-demographic situation (e.g. age, sex, education level), working conditions (e.g. specific type of profession, seniority, work schedule, working hours), and different activities during working hours. SB and PA during working hours were captured by the Occupational Sitting and Physical Activity Questionnaire (OSPAQ), an instrument to subjectively assess the percentage of the occupational time spent sitting, standing, walking, and the time spent doing heavy labour (16, 19). Participants were asked to indicate the percentage of time spent on those activities during working hours on a typical workday (i.e. “How would you describe a typical working day in your current job? This only includes your workday. Commuting and everything outside the work setting is not included.”). So when accumulating all percentages a total percentage of 100 was obtained: for example, 55% occupational sitting, 25% walking, 15% standing and 5% performing heavy labour.
Clinical screening
Body mass index (BMI) calculations were based on measurements of height and weight by using the Seca 704 column scale (SECA Medical Measuring Systems and Scales, Birmingham, UK, scales 701/704). A tapeline was used to measure the waist circumference.
Objective ambulatory registrations
The eligible participants were asked to wear two Axivity AX3 accelerometers to measure SB and PA, one placed on the middle of the back and one on the right thigh. Fixation of the accelerometers on the skin was realised with Opsite Flexifit wound foil to make it waterproof. For the data analysis, Acti4 software was used to determine the type and duration of each activity (The National Research Centre for the Working Environment, Copenhagen, Denmark and Federal Institute for Occupational Safety and Health, Berlin, Germany). In order to obtain data of OPA, the Acti4 software (24) divided the accelerometer data into intervals (i.e. work, leisure time and sleep intervals) based on the information found in the paper-based diaries. Intervals were identified as non-wear time if (a) the Acti4 software showed no movement for more than 90 minutes, (b) the participants reported ‘non-wear periods’ in their diary, or (c) artefacts or missing data were detected by the Acti4 software. A work interval was considered to be valid if it comprises more than 4 h/day of accelerometer wear-time or more than 75% of the individual’s average reported wear-time across days. Data on a daily basis were only included in the data pool if a minimum of 10 hours of accelerometer data was available. Only participants with at least 1 valid workday accelerometer data were included for further analyses.
Data and statistical analyses
The percentage of time spent in occupational sitting, standing, and walking was estimated based on accelerometer data by dividing the total measured hours of each activity during working hours by the total amount of time spent at work. The different occupational activities were then expressed as the percentage of total time at work. The total amount of occupational heavy labour was calculated by the sum of cycling, fast walking (>1.67 steps/sec), walking on stairs and running during working hours. The sum of the previous activities was then divided by the total measured time of working hours. Heavy labour is then also expressed as the percentage of total time at work.
Statistical analyses were conducted in SPSS Statistics, Version 25.0 (SPSS Inc., Chicago, Illinois) and the level of significance was set at p<0.05 (5%). Descriptive statistics were computed for all variables and shown as mean (Standard Deviation (SD)) and as percentage of occupational time. Differences between objective and subjective measurements were calculated by using paired samples t-tests. To compare the items of the OSPAQ questionnaire with the objectively measured amount of occupational sitting, standing, walking, and heavy labour, intra-class correlation coefficients (ICC) were calculated. ICCs were interpreted as weak (<0.50), low (0.50-0.59), moderate (0.60-0.69), or strong (≥0.70) (19). Spearman correlations were used to verify the criterion validity (concurrent validity) of the OSPAQ occupational sitting, standing, walking, and heavy labour items by comparing the questionnaire items with accelerometer-measured values. The strength of the Spearman correlation was interpreted as weak (<0.30), low (0.30-0.49), moderate (0.50-0.69), strong (0.70-0.89), or very strong (>0.90) (25). Subgroup analyses were done for type of profession (sedentary professions versus physically active professions).