Musculoskeletal disorders represent the most common occupational diseases in the European Union . Besides the direct effect on employee health and work disability, work-related musculoskeletal disorders impose a major socioeconomic burden due to extensive use of health care services, sickness absence, disability pension and loss of productivity [2–6]. The prevalence of work-related musculoskeletal disorders, especially in the shoulder, neck and upper extremity is higher in occupations involving a high rate of repetitive movements compared with less repetitive job settings [7–9]. In 2005 about 23% of European workers reported that their work negatively affected their health in the form of significant pain in the shoulder, neck, and/or upper/lower limbs . In a Danish survey approximately one third of the general workforce reported moderate to severe neck/shoulder pain . Thus worksite interventions to effectively prevent and rehabilitate musculoskeletal pain in these anatomical regions seem highly needed.
Slaughtering and meat processing operations involves a high degree of repetitive and forceful upper limb movements and implies an elevated risk of work-related musculoskeletal disorders [10, 11]. The rate of nonfatal occupational injuries and illnesses for workers engaged in animal slaughtering is more than twice as high as the US-national average, and the number of cases with days away from work, job transfer, or restriction are almost three times the national average . Especially the prevalence of musculoskeletal disorders in the shoulder, arm and hand is high among slaughterhouse workers, allegedly due to high loading intensities and cyclic repetitive actions of these body regions during work [7–9, 13]. The increased prevalence of musculoskeletal disorders is associated with several work-related risk factors including highly repetitive and forceful exertion, lack of sufficient recovery and awkward postures [7, 14]. Furthermore, for hygienic reasons slaughterhouse temperature is often low, leading to increasing cutting resistance of the meat and thereby increased risk of developing pain in the arm, shoulder and hand [15–17]. Temporary work disability is a common consequence of the above scenario, manifested by pain in the arm, shoulder and hand, with psychosocial factors related to the job and work environment playing an additional role in the development of work-related musculoskeletal pain [7, 18, 19]. Monotonous work, limited job control, poor self-efficacy and low social support at work have been associated with various musculoskeletal disorders .
The recent implementation of mechanically based production systems in the meat cutting industry seems to reduce the variation in biomechanical exposure. The preferable long work-cycles combined with a variety of movements is currently being replaced by machine directed line-production systems with shorter work-cycles and a higher degree of repetitive and monotonous work tasks . However, not all work tasks can be automatized, leaving a great deal of cutting and lifting tasks to be carried out manually, often using piece-rate salary systems . Common for all these manual operations is the repetitive and forceful arm, shoulder and hand motions and inclined/reclined body postures, which has been associated with nonspecific muscle-tendon pain and related to musculoskeletal disorders such as shoulder tendinitis, epicondylitis, hand and wrist tendinitis and carpal tunnel syndrome (CTS) [7, 13, 22]. Also, not only the dominant side is prone for developing musculoskeletal disorders in slaughterhouse workers. Falck and Aarnio  reported an elevated flexor carpi radialis muscle activity in the left (assisting) vs. right (knife handling) side, and a clustering of left-sided CTS has previously been observed among slaughter workers . Thus, motions such as tearing and holding with the assisting (non-dominant) hand may increase exposure to the non-dominant side as well.
Only few quantitative studies have assessed musculoskeletal disorders in slaughterhouse workers, while mainly focusing on ergonomic exposure evaluated by biomechanical analysis and risk factors associated with specific meat cutting operations. Madeleine & Madsen  reported a more stable motor strategy, including shorter work cycle duration, smaller range of motion and less movement variability and higher complexity during a de-boning task in experienced workers compared with less experienced workers. Further, pain and discomfort in the shoulder/neck region have been associated with changes in motor activity patterns, in particular characterized by decreased motor variability [25, 26]. Muscle pain seems to influence shoulder and neck posture, while also leading to altered spinal loading patterns during specific lifting tasks [27, 28].
Sharpness of blades used in meat cutting operations has been extensively studied, demonstrating associations with reduced force exposure in the upper extremity [29, 30]. MvGorry and co-workers  showed that tool and workstation modifications scaled to the individual meat cutter could minimize wrist deviation, which is ergonomically desirable, and improve upper extremity posture.
Lowering the physical exposure through participatory ergonomic interventions may represent a strategy to reduce musculoskeletal loading intensity and/or rehabilitate musculoskeletal pain. A review by Rvilis et al.  found partial to moderate evidence that participatory ergonomic interventions are effective in improving different health outcomes. The main reason for not finding full evidential support was due to the low number of methodologically sound studies available in the literature. In opposition, pooled data obtained in a subgroup of employees with musculoskeletal disorders indicated that workplace interventions may be effective in reducing sickness absence, but not effective in improving general health outcomes . Despite lack of scientific evidence, ergonomic training and education seems to be the general worksite approach on the prevention and treatment of musculoskeletal disorders.
An alternative strategy to reduce or prevent work-related musculoskeletal pain may be achieved by increasing the workers physical capacity through strength training interventions. Previous studies from our research group have shown promising and effective reductions in neck/shoulder/arm pain in response to 10–20 weeks of strength training using kettlebells [34, 35], elastic rubber bands [36, 37] or free weight exercises [19, 38, 39] in office workers and laboratory technicians. However, office workers and laboratory technicians have vastly different working conditions than slaughterhouse workers, and our previous findings may therefore not be directly transferable to other occupational groups. Nevertheless, from a theoretical point of view, increasing physical capacity by means of on-site progressive strength training of the shoulder, arm and hand muscles may provide an alternative way of reducing chronic pain and work disability in slaughterhouse workers. On the other hand, slaughterhouse workers are exposed to highly repetitive high-force work tasks that may hinder adequate recovery between subsequent strength training sessions. Therefore, relevant grounds exist to investigate whether strength training is a relevant and feasible intervention modality compared with a more traditional participatory ergonomics approach in slaughterhouse workers.
The aim of this study was to investigate the effect of two contrasting interventions, i.e. load reduction (participatory ergonomic intervention) versus increased physical capacity (strength training) on musculoskeletal pain and work disability in slaughterhouse workers with chronic musculoskeletal pain of the shoulder, elbow/forearm, and/or hand/wrist.