BELSTRESS II was the second phase of an epidemiological cohort study on work-related stress and health and was conducted in 2002-’03 . A total number of 2.821 men and women between 40 and 64 years old from nine companies or public administrations in Belgium volunteered to participate in the study by means of self-administered questionnaires and a bioclinical examination. The response rate of the study was 68.5%.
For a supplementary study with ambulatory measurements of BP and physical activity, four of the nine organizations were involved. This selection of organizations, which included two public administrations, one company from the secondary sector and one organization from the service sector, was based on practical reasons regarding feasibility of the fieldwork . A subsample of participants was randomly selected within the four organizations. People taking medication for elevated BP were excluded from this study, as well as participants with a previous hospitalization for coronary heart disease. This study design was developed in order to obtain a population free from established CVD, in which relations can be investigated between work-related factors and BP patterns. The population of this study included 182 middle-aged workers (109 men and 73 women), which represented 66% of the invited subjects. Informed consent was obtained from all participants. Valid ambulatory measures of physical activity were available in 151 participants. The study was approved by the ethics committees of the University Hospital of Ghent and the Faculty of Medicine of the Free University of Brussels.
Questionnaire and anthropometric measurements
The questionnaire contained information on gender, age, educational level, occupational group and smoking status. Primary school level was defined as low education, secondary school level as medium education and high school or university as high education. Occupations were defined according to the International Standard Classification of Occupations and grouped into white-collar and blue-collar workers .
Participants were asked to indicate their usual level of leisure time physical activity on a set of four response possibilities: no weekly activity; only light physical activity during most weeks; heavy physical activity (i.e. resulting in sweating and elevated pulse rate) during 20 min or more once or twice per week; heavy physical activity during 20 min or more three times or more per week. High physical activity was attributed to persons who engage in heavy physical activity during 20 min or more at least three times per week, corresponding to the upper category.
The usual level of occupational physical activity was measured with a scale including five items from the Job Content Questionnaire . The scale is composed of three items assessing physical exertion (high physical effort, lifting heavy loads, rapid physical activity) and two items assessing isometric loads (awkward body positions, awkward positions of head or arms), and has been shown an internationally validated tool to measure physical job demands . The items were scored on a four-point Likert scale from ‘totally disagree’ to ‘totally agree’. For each of the five items, participants who (totally) agreed were assigned high levels of physical activity for that dimension. A summary scale of occupational physical activity was composed of the sum score of the five items and ranged from 5 to 20. This variable was highly skewed to the right and was therefore categorized. A minimum score of 13 was classified as a high level of overall occupational physical activity which generally corresponds to an exposure to three up to five of the individual items.
Stress at work was measured by items from the Job Content Questionnaire, based on the Job-Demand-Control model [23, 24]. The psychological job demand scale was composed of the sum score of five items. Job control was composed of the sum score of two subscales: ‘skill discretion’ or the level of skill and creativity required on the job (six items) and ‘decision authority’ or the possibilities for workers to make decisions about their work (three items). Job strain was defined as the ratio of job demands over job control.
Participants were medically examined by trained members of the research team, following standardized procedures as described in a manual of operations. Height and body weight were assessed using calibrated devices while participants wore only light clothing and no shoes. Height was recorded to the nearest cm, body weight to the nearest 200 g. Body mass index (BMI) was calculated as body weight (in kg) divided by the square of the height (in m). Overweight was defined as a BMI of 25 or higher; those with a BMI of 30 or higher were classified as obese.
Ambulatory 24-h monitoring of blood pressure and physical activity
At the start of a regular working day, a trained member from the research team initiated the 24-h monitoring procedure at the workplace. For 24 h, participants wore an ambulatory BP monitor (Model 90121, SpaceLabs Medical, Inc., Redmond, WA, USA). The monitor was programmed to measure the arterial BP every half hour during the day (from 6 AM until 10 PM) and every hour at night (from 10 PM until 6 AM). During the day, every measurement was preceded by a warning tone. Participants were asked to keep their arm motionless and in a vertical position beside the body every time they heard the tone. In case a measurement failed due to excessive motion of the body, a new reading automatically followed a few minutes later. Before the automatic measurements started, two subsequent test readings were manually initiated to make participants familiar with the process.
In order to measure the physical activity during the 24-h monitoring, an activity monitor (Model 7164, Computer Science and Applications, Inc., Shalimar, FL, USA) was attached to the waist. This single-channel Actigraph continuously records accelerations of the body and has been proven a valid tool in assessing physical activity .
Moreover, participants were asked to perform their regular activities at work and at home during the monitoring period, and not to detach the devices until the next day. They were also asked to register their 24-h schedule (time spent at work, at home, and sleeping) in a diary.
Based on the information from the diaries, the average systolic ambulatory BP of every participant was calculated for the periods at work, at home, and during sleep. The average sleep BP could not be calculated for three participants because there were no valid readings at night. The average number of readings was 16 (± 3 s.d.) for the work period, 14 (± 3 s.d.) for the home readings, and 7 (± 2 s.d.) for the readings during sleep.
The activity monitor was programmed to register an activity count on every minute during the 24- h monitoring. Valid data could be processed in 151 participants; 31 were excluded due to technical software problems or because the device had been detached. The activity count data were processed separately for the time at work and leisure awake time. The average number of counts was 494 (± 75 s.d.) for the work period and 295 (± 205 s.d.) for the awake leisure time period. In order to assess the level of physical activity during work and leisure time in each participant, the proportion of work and leisure time spent in physical activity of moderate or vigorous intensity (≥ 2020 counts) was calculated for all participants. This cut-off was based on intensity thresholds for adults resulting from calibration studies that relate accelerometer counts to measured activity energy expenditure . On group level, because this variable was highly skewed to the right, the upper quartile was classified as a high proportion of work and leisure time activity counts of moderate or vigorous intensity, as opposed to the lowest three quartile groups.
Descriptive statistics were reported through numbers and proportions. Crude mean systolic ambulatory BP values were compared between occupational and leisure time physical activity groups, based on both questionnaire and monitoring data, with Independent Sample t-tests. Analyses of Covariance were used to assess multivariate associations adjusting for relevant confounders (gender, age, BMI, smoking, job strain and usual level of leisure time or occupational physical activity respectively). The use of linear regression analysis was not warranted since the physical activity variables were highly skewed and the condition of normally distributed residuals was not met.
All analyses were conducted using IBM SPSS Statistics software version 19 (IBM SPSS, Inc., Chicago, IL, USA).