In all 3 examinations, the self-reported information on level of OPA was obtained by asking the question: “Please describe your level of OPA within the past year” with the following response categories: “(1) predominantly sedentary; (2) sitting or standing, some walking; (3) walking, some handling of material; (4) heavy manual work.” If answering 3 or 4, an additional question regarding heavy occupational lifting was applied. The question was: “Do you lift heavy burdens?” with the response categories: “(1) yes and (2) no.” Participants will be classified as exposed to heavy occupational lifting by answering “yes” to the question concerning heavy burdens, and those participants answering 1, 2, and 3 or 4 in combination with not lifting heavy burdens will be classified as the reference group.