The Copenhagen Male Study was established in 1970–1971 as a prospective cohort study of physical fitness and cardio-vascular disease in employees in 14 private and public companies in Copenhagen. All men aged 40–59 years were invited; 5,249 men, 87% of potential participants, took part in the examinations at baseline [8, 9]. In the present study, only men without a history of back disorders were included.
Baseline data were obtained by interview based on a precedent structured questionnaire, and a clinical examination including measurement of height and weight , and indirect measurement of aerobic capacity (physical fitness) was performed with a bicycle ergometer . Thirty-five men with orthopaedic problems unable to perform the bicycle test were excluded from the study. Information on working conditions, lifestyle, and general health was obtained from the questionnaire. Variables that could represent potential risk factors for lumbar intervertebral disc herniation were chosen [12–17]. Details on the questionnaire have previously been published .
Physical fitness Heart rate was measured during submaximal bicycle work in steady state with the aid of a stopwatch and stethoscope. The loads used were 100, 150 and 200 W. One, two or in a few cases three different loads were used. The loads chosen in each case were determined from the weight and age of the person or heart rate during the first minute of the test, and the estimation of VO2Max was accomplished with the aid of Åstrand’s nomogram . The correlation between directly and indirectly measured physical fitness is high. The method used has previously been described in detail .
Physical occupational workload was assessed from one question addressing ergonomic load to the back: “Is your work a) sedentary, b) slightly physical without lifting, c) physical with some lifting, or d) hard physical with heavy lifting, shovelling or the like”? The two first categories were condensed in the analyses. Additionally, a more general question about physical workload was asked: “Do you perform strenuous work (regularly resulting in sweating)?” Answer options were: “often”, “occasionally” and “seldom or never”.
Physical activity in leisure time was classified into one of four different groups: Predominantly sedentary, slightly active, fairly active or very active. For analytical purposes answers in the two middle groups were condensed.
Mental stress at work and during leisure time was evaluated separately by asking: “Do you feel under mental stress when performing your job/during leisure time?” Answer options were: “Seldom” and “Regularly”. In addition it was asked whether sedatives were taken regularly, occasionally or never.
Lifestyle factors included current and previous tobacco and alcohol consumption.
Health related factors included questions on present or previous episodes of back pain or back injury. Answer options were yes and no.
Anthropometrics included height and weight based on measurements and body mass index (BMI) calculated as kg * m–2. In separate analyses of height and weight the study population was divided into thirds based on tertile values. BMI was divided into three groups: Normal weight or lean: BMI < 25, overweight: BMI 25–29.9, and obese: ≥ 30.
Social class was grouped in five categories based on educational level, job type and position in terms of number of subordinates [8, 19]. Typical jobs in the cohort were as follows. Social class I: head of department, officer, civil engineer. Social class II: head clerk, engineer, resident architect. Social class III: engine driver, train guard. Social class IV: skilled workers. Social class V: unskilled worker, driver.
End-point was hospitalization due to herniated lumbar disc disease identified in the National Hospital Register between 1977 and 2003. Code 725.11 from the International Classification of Diseases (ICD), 8th Revision was applied from 1977 to 1994, and code M51.1 from ICD 10th Revision from 1994 to 2003.