Study design and population
The present 1-year prospective cohort study investigates work-related symptoms among warehouse workers from up to five different retail chains industry warehouses in Denmark. Enrolled study participants will in February-March 2020 receive a baseline questionnaire by e-mail that will address various aspects of physical-, psychological and social work environment and health. The questionnaire will also comprise an invitation to participate in a text message survey. Participants recruited for the text message survey will receive a SMS text message before and after every workday for 3 full weeks (21 days) during March-April 2020 to rate the magnitude of pain in their low back, and to score their current level of perceived physical fatigue and mental stress. During the same 3-week study period, section leaders at the warehouses will provide company records about the workload of each participating warehouse worker (goods handled by each worker, weight of the goods) along with a working schedule for each worker. Additionally, during the same period, pressure pain threshold (PPT) will be measured in about 50 workers along with an optional assessment of maximal isometric trunk extensor strength (MVC) and systemic resting blood pressure. One year after responding to the baseline questionnaire, the participant will be receiving a follow-up questionnaire in February-March 2021 that will evaluate the physical-, psychological and social work factors and health aspects assessed 1 year previously. Fig. 1 illustrates a time line of the study.
Inclusion criteria for the warehouse workers recruited for this study are: working ≥30 h per week in a registered retail industry warehouse, ability to read and understand Danish or English, ≥18 years of age.
According to Danish legislation, scientific questionnaire studies are not required to attain approval by official Danish ethical or scientific committees nor to obtain informed consent from study participants. Nevertheless, all questionnaires and text messages will be handled anonymously. A data manager at the research centre will store the data on a secure drive and de-identify the data before the researcher get access to these. All physiological measurements (PPT, MVC, systemic blood pressure) have been approved by The Danish National Committee on Biomedical Research Ethics (journal number: H-3-2010-062), and the project is registered at the Danish Data Protection Agency and ClinicalTrials.gov (NCT04240184). Prior to giving their informed consent, all participants participating in the physiological measurements will be informed orally and in writing about the objectives and content of the study, as well as of their rights and potential risks of the study in accordance with the Helsinki Declaration.
Baseline and follow-up questionnaires
The baseline and follow-up questionnaires contain questions about basic characteristics, general health and physical-, psychological and social work environmental factors. Questions about the physical work environment are based on previous investigations from our lab and the Work Environment & Health study [14, 21]. Questions pertaining the psychosocial working environment are selected from The Danish Psychosocial Work Environment Questionnaire (DPQ)  while questions about fear avoidance beliefs are selected from the Fear-Avoidance Beliefs Questionnaire . The questionnaires will be sent by e-mail from a web-based survey platform (SurveyXact) while containing a web-link directed to the electronic questionnaire.
Text message survey
All study participants will receive and reply to twice-daily SMS text messages before and after the workday for 21 consecutive days (also on days off work) . The messages will be sent from a web-based survey platform (SurveyXact) containing a web-link that directs the participant to a 3-question survey about pain in low back, perceived physical fatigue and mental stress. To reply, participants choose a number between 0 and 10, where 0 is best and 10 is worst.
During the 3-week observation period, warehouse section leaders will provide company records for all participants in the text message survey. These records will provide information on the specific goods handled (type and weight) manually by each participant during each single workday (workload), supplemented by a specific working schedule for each participant . Based on these data the total weight lifted by each participant per workday for all 3 weeks will be calculated. Additionally, the repetitive lifting pattern of the different goods can be estimated from the amounts of each type of good and information about the total work duration, altogether allowing to quantify the duration, frequency and intensity of the physical work performed by each participant.
Pain pressure threshold
During the 3-week observation period, PPT in the low-back muscles (m. erector spinae longissimus) is measured using an electronic pressure algometer (Somedic Productions AB, Sollentuna, Sweden, Europe) on approximately 50 participants for 2 days per week, i.e. comprising 6 measurement days in total. Control measurements will be conducted in a lower limb muscle (m. tibialis anterior) that is not directly affected by the manual lifting. The first PPT measurement is performed by the beginning of a working day preceded by a day off from work, while another PPT measurement is conducted during the working week just prior to initiating the fourth consecutive working day. PPT measurements are performed with a circular probe with a contact area of 1 cm2 at 3 contact sites on the m. erector spinae longissimus muscles on each side of the spine, and each contact area is measured 3 times with an interval time of 1½ minute [24, 25]. The test leader presses the algometer against the back muscles and slowly increases the pressure. Participants are informed to press a button on a pinch handle mounted to the algometer when the pressure changes from the feeling of pressure to the feeling of pain [24, 25]. The display of the pressure algometer will not be visual to the participants during the measurements. The PPT is expressed as the average value of the 3 measurements. All PPT measurements will be performed by the same test leader. Previous studies have found PPT both valid and a test-retest reliability level of satisfactory to good [26,27,28,29,30,31].
Maximal isometric muscle strength
Participants in which PPT is determined will also be offered (encouraged) to have maximal voluntary contraction strength of the trunk extensors (MVC) assessed. Subsequent to the recording of PPT, MVC measurements are conducted at a working day preceded by a day off from work. Systemic blood pressure is measured prior to the MVC test and participants with blood pressure exceeding 160/100 mmHg will be excluded from testing [24, 32,33,34]. MVC measurements are performed using a standardized MVC procedure in a standing test-position in a custom-built device . With a strap fixed around the shoulders, the back slightly flexed and leaning towards a pillow at the height of the anterior iliac spine, the participant will be informed to extend the back . A warm-up trial consisting of 3 submaximal contraction efforts will be performed followed by 3 maximal MVC efforts separated by 1 min rest interval. Participants will be informed to slowly build up the contraction force to reach their maximum capacity after 2–3 s, and to continue the contraction until the test leader tells them to stop (approximately 3 s). Verbal encouragement will be provided throughout the test. The maximal isometric muscle strength will be expressed as the peak force produced during the 3 MVC trials.
The baseline questionnaire will be sent out to ~ 800 warehouse workers (see Fig. 2 for flow chart illustration). Based on a previous study performed by our lab , we expect a participation and response rate of > 50% in the SMS text message survey, i.e. N ≈ 400. The section leaders at the warehouses have informed of a yearly worker turnover rate of ~ 14% in the different retail chains. Thus, when taken other factors into account, a drop-out rate of 20–30% is expected at the 1-year follow up, i.e. N ≈ 300. In 2017, Andersen & co-workers found significant day-to-day differences in LBP among 95 supermarket workers . The sample size of the present study is therefore sufficient to investigate associations between risk factors at work and LBP.
The statistical analyses are based on methods previously published by our lab . In brief, the statistical analysis will be based on linear mixed models with repeated measures (Proc Mixed, SAS version 9.4, SAS Institute, Cary, NC). LBP is the main outcome variable of the study and will be analysed as a continuous variable. PPT is a supplementary outcome and likewise treated as a continuous variable. Warehouses are entered as a random factor to account for clustering between work sites. Participant is entered as repeated factor using an autoregressive covariance structure. The estimation model is restricted maximum likelihood with degrees of freedom based on Satterthwaite approximation. The explanatory variables (fixed factors) for the first 3 objectives are 1) kg lifted per working day, 2) combination of working days and days off from work, and 3) mean of daily lifting (kg/day) during the 3 baseline weeks in relation to the change in LBP at 1-year follow-up. Analyses will be adjusted for relevant confounders as age, gender and psychosocial work environmental factors. For the fourth research question, the estimates of psychosocial variables will be drawn out of analyses 1–3, and we will also test whether the psychosocial factors interacts with the physical workload in relation to the change in LBP. Results are reported as least square mean differences with 95% confidence intervals (95% CI). An alpha-level below 0.05 is considered statistically significant.