The study described herein is a prospective cohort study among professional computer users, with an observation period of 10 months. At baseline a self-administered questionnaire was used to assess work-related exposures, individual factors, and symptoms from the neck and upper extremities.
In addition, observations of working technique were performed by ergonomists using an ergonomic checklist designed for the assessment of computer work exposures [26
The study was approved by the local ethics committee at the Karolinska Institutet and the regional ethics committee at Gothenburg University.
The participants were recruited by ergonomists employed by different Occupational Health Care Centers. The initial study population included 1529 participants. The baseline questionnaire was answered by 1283 subjects. The study group consisted of 853 participants from 46 different work places, representing a great variety of professionals (librarians, engineers, graphic designers, receptionists, secretaries, journalists, researchers, insurance officers and call center personnel) from both the private and the public sector (382 men and 471 women) who, at baseline, had been free from neck and upper extremity symptoms during the preceding month (Figure ). The mean age of the men was 42 years and 44 years for the women. The self-reported time spent on computer work was, on average, 3.7 hours/day for the men and 3.8 hours/day for the women (Table ). During the remaining working time the participants, depending of their professions, performed ordinary office work including reading, writing, sorting, calculating, making telephone calls, attending meetings etc. Thus, there was a great variability in self-reported time with computer work between participants.
Chart showing the study population, participants at baseline, and number of cases affecting the investigated body regions for men and women.
Baseline characteristics of participants, presented as mean values with range within brackets
Incidence data were collected by means of 10 postal monthly questionnaires, asking for information on the occurrence of neck and upper extremity symptoms. The questions referred to the time period since completion of the preceding questionnaire, which was approximately one month but could have been longer due to vacations or other reasons for absence. If a follow-up questionnaire was not returned before the next one was available, the time frame used for reporting symptoms covered the whole period since the previous questionnaire was answered, i.e. approximately two months. If two consecutive questionnaires were missing, the calculated person-time connected to that participant was closed when the last questionnaire was answered.
Assessment of symptoms
The monthly questionnaires asked if the participants had experienced symptoms of pain or ache during the preceding month, in any of the following body regions: neck and right and left scapular areas, shoulder joint/upper arms, elbow/forearms, wrists, and hands/fingers (see Figure ). If they reported any symptoms the duration (number of days) in the respective regions were requested.
Showing the body regions considered for rating of perceived exertion in the present study.
Ratings of perceived exertion
Perceived exertion after a typical work day was rated, in the baseline questionnaire, on a modified Borg RPE scale ranging from 0 (very, very light) to 14 (very, very strenuous) for each of the 11 body regions under examination (Figure ) [27
Ratings of comfort
Perceived comfort was rated, in the baseline questionnaire, on a 9-point scale ranging from -4 (very, very poor) to +4 (very, very good) in relation to the chair, computer screen, keyboard, and computer mouse [28
Observation of working technique
Working technique was observed at baseline by ergonomists (n = 32) according to the checklist. They were trained in using the checklist accurately and in a standardized manner during seminars before the "real life" observations were made. The participants were observed at their work places during ordinary computer work (observation time ranging from 10-15 minutes for each subject). The observations were characterized on the basis of eight variables: 1) use of forearm support during keyboard work; 2) use of forearm support during computer mouse work; 3) elevation of the shoulders during keyboard work; 4) elevation of the shoulders during computer mouse work; 5) sitting in a tense position when using the keyboard; 6) sitting in a tense position when using the computer mouse; 7) range of movement when performing computer mouse work; 8) speed and/or jerkiness of the computer mouse movements.
Data treatment and analysis
Symptoms from the 11 different body regions were combined into symptoms from three major regions: a) the neck and the scapular region (hereafter called the neck region); b) shoulder and upper arms (hereafter called the shoulder region); c) elbows/forearms, wrists and hands/fingers (hereafter called the arm/hand region). A "case" was defined as a participant who reported maximum 2 days with symptoms during the preceding month in all body regions ("symptom free") at baseline and who later, at any follow up occasion, reported symptoms lasting ≥ 3 days during the preceding month in the specific body region, i.e. a, b or c. The incidence rate was calculated as the frequency of "new" cases divided by the total person-time-at risk. Subjects contributed with person-time corresponding to the period between the dates of the baseline questionnaire and the date when they became a case or the date when they completed their last questionnaire (non-cases).
To evaluate perceived exertion, a sum score was calculated for the neck, shoulder, and arm/hand region, respectively. The sum score for each body region was then divided by the number of areas included for each body region in order to obtain a mean value. Subjects were classified into three groups, with 0-4 (less than relatively light exertion) as the reference group, 5-7 (relatively light - somewhat strenuous) as the medium exertion group, and ≥8 (strenuous or very strenuous) as the high exertion group.
For comfort, a sum score for comfort was calculated for the included items and divided by the number of items included in the score in order to obtain a mean value. Subjects were then classified into three groups, where -4 to -1 was classified as poor comfort, 0 to +2 as acceptable or medium comfort and ≥ 3 as good comfort (the reference group).
The working technique scores for each of the eight variables included were combined into an overall score ranging from 1-22 [25
]. Subjects scoring ≥14 were classified as having a good working technique, those scoring 12-13 as having an acceptable working technique, and those scoring < 12 as having a poor working technique [29
Incidence rate ratios (relative risks, RR) with 95% confidence intervals (95% CI) for symptoms in the neck, shoulder, and arm/hand region were calculated using Cox proportional hazard models in the software JMP version 5.0.1 and Proc Phreg (SAS v.9.0) and adjusted for age, sex and time spent undertaking computer work. The rationale behind controlling for the computer use time was partly the great variability in the amount of time spent undertaking computer work in the study population and partly the assumption that computer time might co-vary with both the perceived exertion and comfort and musculoskeletal symptoms and thus a potential confounder as described by Rothman [30