Study design and population
This study was based on data from the Danish National Health Interview Surveys which were carried out in 1987, 1994, and 2000 by the National Institute of Public Health, University of Southern Denmark. The surveys consist of national representative random samples of the Danish population aged 16 or above selected from the Danish Civil Registration System. Data were collected by personal interviews and self-administrated questionnaires. Response rates were 80% in 1987, 78% in 1994, and 74% in 2000 [10]. If a person participated in more than one survey, only data from the first survey was included. In total, 22,672 men and women participated in the surveys. Unemployed participants (n = 9,746) and participants with known ischemic heart disease (IHD) prior to the interview (n = 170) were excluded. Further 143 participants were excluded due to unknown vital status. Thus, the final study population consisted of 6,692 working men and 5,921 working women aged 16–85 years.
Assessments
Data from the population-based surveys were linked to nation-wide registers by using the unique personal identification number (CPR-number). IHD incidence was based on information on both fatal and non-fatal cases. Information on cause-specific mortality was obtained from the Danish Register of Causes of Death updated until 2008-12-31 and information on disease hospitalizations was obtained from the National Patient Register updated until 2010-05-04. IHD was defined by ICD-8 codes 410–414 and ICD-10 codes I20-I25. Data on all-cause mortality were obtained from the Danish Civil Registration System updated until 2010-05-04.
Occupational heavy lifting was estimated from the question: “Are you exposed to lifting or carrying heavy burdens (minimum 10 kg) at work more than 2 days a week?” The answer options were: “yes”, “no”, or “don’t know”.
Occupational physical activity in general was estimated from the self-administrated questionnaire by the question: “Which description most precisely covers your level of physical activity at work?” Groups were defined according to the following responses: 1) Mainly sedentary work; 2) Work that require quite a bit of standing or walking activities; 3) Standing and walking most of the time with quite a bit of carrying or lifting heavy burdens; 4) Work that requires vigorous or strenuous physical activity. Due to few observations in the extremes when stratified by occupational heavy lifting, occupational physical activity level was dichotomised as follows: 1) Low physical activity level combining group 1 and 2 and high physical activity level combining group 3 and 4.
Participants were asked to state their typical level of physical activity in leisure time during the last 12 months in one of four predefined categories: 1) vigorous physical activity (strenuous activities usually involving competition or endurance training performed regularly or several times a week); 2) moderate physical activity (exercise, endurance training or heavy gardening for at least four hours a week); 3) low physical activity (walking, bicycling or other light activities for a minimum of four hours a week); 4) sedentary activities (reading, TV-watching or other sedentary activities). Level of physical activity in leisure time was dichotomised combining low and sedentary physical activity into “low physical activity level” and vigorous and moderate physical activity into “high physical activity level”. Unfortunately, many participants in the 1987-survey were not given the question regarding physical activity in leisure time and for this reason 1,946 participants were not included in the analysis of physical activity in leisure time.
Additional variables included in the analyses were age at the time of survey, education (< 10 years, 10–12 years, and >12 years), smoking (never-smoker, ex-smoker, daily smoker (1–15 cigarettes/day), and heavy smoker (>15 cigarettes/day)), alcohol consumption (<1 drinks/day, 1–2 drinks/day, 3–4 drinks/day, and >4 drinks/day), body mass index (BMI) (<25 kg/m2, 25–30 kg/m2, and > 30 kg/m2), hypertension (yes, no), and self-perceived stress (often, sometimes/no).
Statistical analyses
Initial descriptive analyses involved incidence rates for IHD and all-cause mortality during follow-up and simple frequency distributions of potential confounding variables by each category of occupational heavy lifting, occupational physical activity level, and physical activity level in leisure time. The association between occupational heavy lifting, occupational physical activity and physical activity in leisure time, respectively and the incidence of IHD as well as all-cause mortality was analyzed using the Cox proportional hazards model adjusting for potential confounding factors (age, education, physical activity, smoking habits, alcohol consumption, and self-perceived stress). Further adjustment for occupational status (body mass index, self-employed, salaried worker and skilled/unskilled worker) did not alter the estimates and was therefore omitted from the analysis. Age of the participants was applied as the underlying time in the statistical model. Evaluation of the assumption about proportional hazards was done by visual inspections of log-log plots and tested using Schoenfeld residuals. A combined measure of occupational heavy lifting and general occupational physical activity as well as a combined measure of occupational heavy lifting and physical activity in leisure time were evaluated in relation to the risk of IHD and all-cause mortality. All analyses were carried out for men and women, separately. The number of participants in the different tables may vary due to differences in the number of missing observations in the separate analyses. In all tests, P values were 2 sided and statistical significance was defined as p < 0.05. Analyses were performed using Stata 11.0 (StataCorp LP, Texas, USA).