Physical Risk Factors Associated with the Work-Related Neck / Cervical Musculoskeletal Disorders : A Review

Work-related musculoskeletal disorders (MSD) of the neck or cervical spine result in longer sick leaves, substantial levels of human suffering, and high costs for society. Epidemiological studies clearly indicate strong associations between MSD of the neck and the work activities requiring forceful arm exertions and heavy lifting. However, most of the existing studies that evaluate the neck or cervical spine disorders focus mainly on exertions demanding sustained neck postures of repetitive arm exertions. The purpose of this study is to conduct a systematic review of existing epidemiological and experimental studies that evaluates neck or cervical spine disorders and identify the gaps in the literature for future research.


Work-related Musculoskeletal Disorders of the Neck or Cervical Spine
Among work-related musculoskeletal disorders (MSD), incidents of neck pain as a cause of absenteeism are not as frequent as low-back pain, yet they contribute significantly to the morbidity in many working populations (Hales &Bernard, 1996).Work-related MSD of the neck encompass a wide range of inflammatory and degenerative diseases and disorders (Buckle & Devereux, 2002).Many types of tissue in the cervical region can be sites of pain, including the neck muscles, intervertebral discs, the posterior longitudinal ligament, and the facet joints (Cailliet, 1991).The neck disorders associated with musculoskeletal pain (i.e., inflammatory types) include tension neck syndrome and trapezius myalgia and are common among the occupations requiring prolonged and repetitive submaximal exertions by the neck muscles, e.g., VDT workers, sewing machine operators, and dentists.On the other hand, the disorders associated with degenerative disc diseases mainly include disorders of the cervical spine and are associated with occupations involved in physically demanding work activities (Hagberg & Wegman, 1987), e.g., health care, construction work, farm work, and manual material handling.Despite of having a high incidence rate among various occupational groups, cervical spine disorders currently remain largely unstudied, with only inferential hypotheses for their etiologies.NIOSH (1997) performed a critical review of available epidemiologic studies that associate the MSD of the upper extremity and the lower back with exposure to physical factors at work.Over 40 epidemiologic studies were examined to understand the causal relationship between physical workplace factors and the neck/shoulder MSD.Substantial evidence for forceful exertions leading to the occurrence of neck/shoulder MSD was observed in the epidemiologic literature.The "forceful work" for the neck/shoulder was defined as work activities which involve forceful arm or hand movements that apply loads to the neck/shoulder area.Ariens et al. (2000) reviewed 22 cross-sectional studies, two prospective cohort studies, and one case-referent study to identify physical risk factors for neck pain.The authors reported moderate evidence for a correlation between the workrelated forceful arm exertions and neck-shoulder pain, as well as a correlation between heavy lifting and neck-shoulder pain.Malchaire et al. (2001) reviewed fifty-seven cross-sectional and seven longitudinal studies to determine factors associated with musculoskeletal disorders of the neck and upper limbs.Sufficient evidence was found for a linkage between neck/shoulder disorders and occupational risk factors like repetitiveness, physical workload and static efforts.Walker-Bone and Cooper (2005) presented a review of existing epidemiological studies that evaluate soft tissue musculoskeletal disorders of neck and upper extremity among a wide range of occupations.The study includes epidemiological studies performed during 1998-2001, as listed in the EMBASE and MEDLINE databases.The results of the reviewed studies indicate that neck pain is associated with exposure to sustained abnormal posture (e.g., prolonged sitting, neck/trunk held in prolonged flexion or rotation), forceful and/or repetitive arm exertions, poor workplace support from supervisors/colleagues, and high work demands on the workers.A review presented by Larsson et al. (2007) confirmed that there was a causal relationship between neck and shoulder disorders and highly repetitive work, forceful exertions, high level of static contractions, prolonged static loads, and extreme postures, as well as combination of these factors.
A state-of-the-art review was presented by NRC/IOM (2001) considering the epidemiological evidence as well as basic science including human biomechanics and experimental evidence research and the evidence from effective interventions.The state-of-the-art review scrutinized the available documentation on possible occupational risk factors for work-related low back and upper extremity disorders.From the perspective of observational epidemiology, the review summarized that there was a significant positive association between upper extremity MSD and exposure to repetitive tasks, forceful tasks, the combination of repetition and force, and the combination of repetition and cold.Vibration was also demonstrated as an important role for upper extremity MSD.From the upper limb biomechanical perspective, the review identified relationships between physical work attributes and external loads for force, posture, vibration, and temperature.

Search strategy
A comprehensive search was performed in MEDLINE, CINAHL, Google Scholar, and all EBSCOhost databases to find epidemiological and experimental studies on neck and cervical MSD.In order to follow up on the earlier reviews, the search was conducted for the epidemiological studies within the past 10 years (Jan 2000 -Apr 2010), while the search for the experimental studies was conducted within the past 15 years (Jan 1996 -Apr 2010).
The databases were searched for MSD of neck or cervical spine regions, including the terms like tension neck syndrome, trapezius myalgia.Then the search was restricted to the combination of keywords such as work-related, risk factors, association, relationship, predictor, physical exposure.Specifically, the epidemiological studies were searched with words such as cross-sectional, case-control, prospective, cohort; the experiment studies were searched with words such as muscle, activity, electromyography (EMG), neck movement, posture, and load.The references of all identified relevant studies, including reviews, were examined to identify additional relevant articles.The retrieved articles using these strategies were focused on the association between physical exposures and the neck or cervical spine MSDs in an occupational setting.

Inclusion criteria
Studies were included in the present review if the following conditions were met: (a) The paper was published in English (b) The study was conducted among or oriented towards a working population (c) The exposures assessed were physical risk factors at work (d) The outcome of epidemiological study included the prevalence/incidence of chronic neck and/or shoulder pain, complaints, or disorders (e) In epidemiological studies on upper limb disorders or upper extremities or other symptoms, data on neck and/or shoulder symptoms were presented separately (f) A consistent or significant relationship/association was found between neck and/or shoulder disorders and physical risk factors

Exclusion criteria
The exclusion criteria were as follows: (a) Articles of review (b) Studies focused on back pain and lower extremity symptoms (c) Studies dealt with upper limb disorders as a whole, not presenting the data on neck and/or shoulders separately (d) Studies focused on individual or psychosocial factors in the findings of exposures (e) Studies on acute or traumatic injuries (f) No consistent or significant relationship/association was found between neck and/or shoulder disorders and physical risk factors.

Data extraction
The results of the retrieved studies were presented in the form of tables.The findings of physical risk factors were grouped into five categories, according to the previous research and reviews: (1) repetitive motions, (2) awkward postures, (3) forceful exertions, (4) vibration, and (5) poor work or workstation design.
For each epidemiological study, characteristics extracted were the study(Author-year) and design(cross-sectional, cohort, or case-control design), study population (sample size, response rate, and occupational setting), exposure basis (questionnaire responses, or answers to interview questions, or workplace investigations, or other), outcome measurement (selfreported symptoms, or clinical examination, or other), results (prevalence ratio of neck and/or shoulder MSDs, or relative risk ratio of the exposure), and findings of risk factors for neck/shoulder MSD in the study.
For experimental study, characteristics extracted were the study, subjects (sample size), effects of interest (interested factors of the study), objective measurement in the laboratory-based study (muscles measured using electromyography, or body posture, or heart rate recordings, or others), and findings of risk factors for neck/shoulder MSD in the experiment.

3.1: Epidemiological Studies
A total of 35 epidemiological studies were finally included in this review: 3 studies were carried out among general working population, not related to any specific occupational groups, 6 studies were carried out among health care workers, 2 studies were carried out among dentists, 11 studies were carried out among visual display terminal (VDT) workers, and 13 studies were carried out among industrial and service sector workers.Table 1 presents a summary of the study characteristics.

General working population
The general working population was examined in some studies to understand the overall trends of MSD, including neck or cervical spine disorders.Of the three studies, two were designed as prospective cohort study, one case-control study and one cross-sectional study.Awkward postures involving twisting or bending of the trunk, prolonged neck bending, as well as working with the arms at or above shoulder height were found in all the reviewed studies as a risk factor for neck/shoulders disorders.

Health care workers
Among the various working populations, the health care workers, especially nurses, ambulance attendants, and home care professionals have a high prevalence of work related neck MSD.The jobs of these professionals demand lifting heavy loads, work in awkward postures, and transfer of patients (Marras et al., 1999).A number of studies used survey questionnaires to identify the prevalence of neck/shoulder disorders among these health care professionals.Of the retrieved six studies, five were cross-sectional and one was longitudinal design.The factor of high force exertions in the patient transfer and manual materials handling tasks was identified by all the studies as a significant contributing factor for the development of neck/shoulder MSD.Half of the studies considered awkward posture as a risk factor for the disorders.

Dentists
Work-related musculoskeletal disorders, especially of the neck and upper limbs, are common among the dentists, dental hygienists.Dental work, a vision-demanding precision task, is characterized by work postures involving a prolonged static work load for the neck, shoulders, and arms (Akesson et al. 1997).Various investigators have pointed out that the dentists and dental hygienists adopt strenuous postures with excessive bending and twisting of the neck, forward bending of trunk, and elevation of the shoulders.The results of the two reviewed studies associated such postures with the risk of developing neck MSD.Alexopoulos et al. (2004) also reported exposure to vibration tools as another risk factor for the occurrence of neck/shoulder MSD among the dental professionals.

Visual Display Terminal (VDT) workers
The use of computers at work as well as during leisure time has increased worldwide in recent years.The various risk factors, such as static and constrained head and neck posture, long duration of computer and/or mouse and/or keyboard use, repetitive elevated arm movements in seated posture, and poor placement of screen and/or mouse and/or keyboard, associated with the extensive use of computers, motivated researchers to study the development of musculoskeletal symptoms caused by working with computers.Our search retrieved 11 articles that studied the association of work-related physical risk factors and neck/shoulder MSD among VDT workers, with eight cross-sectional studies and three cohort studies.

Workers from various industrial occupations
Both NIOSH (1997) and NRC/ IOM (2001) reviewed a wide range of working populations in various industrial and service sector setting, including: shoe manufacturing assembly line workers, industrial plant workers, foundry workers and manual laborers, workers in aircraft engine division, workers in spinning industry, shipyard welders and plate workers, construction workers, female workers in fish processing, machine operators and carpenters, chocolate manufacturing workers, and letter carriers.Table 1 shows the 13 newest studies since 2000, covering various manufacturing assembly workers, manual workers, call center operators, farmers, machinists, mechanics, house painters, urban bus drivers, cosmetologists, forest industry workers, and sewing machine operators.
Repetitive motion, awkward posture, high force exertions, and use of vibrating tools were concluded as risk factors for the development of neck/shoulder MSD.While poor task related factors like fewer rest breaks, inadequate thermal comfort, and driver-seat mismatch were also listed as the contributing factors for the development of neck/shoulder MSD.

Summary of Epidemiological Studies
Epidemiological studies show a prevalence of neck/shoulder MSD among a variety of occupational groups, showing a strong association between neck/shoulder MSD and forceful exertions, sustained and constrained working postures over an extended period of time, and repetitive arm or shoulder movements.Based on the epidemiological evidence, several laboratorybased studies experimentally examined the work activities of various occupational groups, to understand the mechanism that could cause the neck/shoulder MSD.In the following section, a number of experimental investigations, focusing on the neck/shoulder MSD for various occupational groups, are presented in detail.

Experimental Studies
This section provides a review of the experimental studies on neck or cervical spine MSD in different occupational settings.The objective of this section is to examine the biomechanical evidence of association between physical occupational demands and the risk of developing neck or cervical spine MSD.A total of 31 experimental studies were finally retrieved, which were carried out among dentists (2 studies), VDT workers (13 studies), and different industrial workers (16 studies).Table 2 shows the summary of the study characteristics.Westgaard et al. (2001) studied shoulder muscle activity as a risk indicator for shoulder and neck pain in female healthcare workers.Authors recorded the EMG activity of the upper trapezius muscles throughout the workday from shopping center workers (n=22) and healthcare workers (n=44).Holte and Westgaard (2002) did a further study among 96 workers from four occupational groups: health care (n=20), retail (n=22), banking (n=26), and university secretaries (n=26).In these two studies the EMG activity level in the trapezius muscles was found to be very low, despite the high prevalence of neck/shoulder MSD among the population studied.Authors of the two studies concluded that it was difficult to argue that the observed trapezius EMG activity represented a risk of shoulder and neck complaints.Alternatively, psychosocial stressors appeared to be causative in the development of shoulder and neck pain for workers in service occupations.

Dentists
Even with the best ergonomic equipment, dentists can find themselves in sustained awkward postures.These postures often consist of forward bending and repeated rotation of the head, neck and trunk to one side.Valachi and Valachi (2003) found that dentists frequently assume forward-head-and-rounded-shoulder posture that increases forces on the upper neck muscles (upper trapezius and levator scapulae) and anterior muscles (scalene, sternocleidomastoid and pectoralis), and these muscles become much weaker over time.
Two studies were retrieved since 1996.They concluded that awkward postures, e.g.prolonged neck flexion and/or forward head bending, and/or working with upper arm abduction, as a risk factor for dentists to develop neck or cervical spine MSD.

VDT workers
In recent years, several studies have been performed to evaluate the effects of different aspects of the computer workstations, together with their effect on the role of the neck/shoulder musculature.
The Table 2 present 13 studies on relationship of VDT work to the increased risk of neck or cervical spine MSD.Half of them found awkward body posture, such as neck flexion, head or trunk inclination and arm movements in seated posture, as a risk factor for neck/shoulder symptoms.Other half studied the impact of poor work or workstation design, e.g., poor placement of screen and/or mouse and/or keyboard, use of bifocal lenses, strenuous work-rest schedules, all of which resulted in the increased risk of neck/shoulder MSD symptoms among VDT workers.

Workers from various industries
Table 2 shows16 studies among workers from various industries.In the selected studies, subjects were instructed to do special industrial work activities: masonry works with handling blocks, static overhead drilling or tapping, repetitive forearm supination, static heavy lifting, meat cutting, prolonged writing, working with hand grip in elevated arm position, light manual precision work, and house painting with different work techniques.
Most of the reviewed studies have reported awkward posture (e.g., overhead exertion, neck flexion and extension, forearm supination and additional shoulder abduction, working with arms at or above shoulder level) as the risk factors for neck/shoulder MSD.

Summary of Experimental Studies
A wide range of occupations and/or work activities have been studied experimentally to understand the causal risk factors associated with the neck/shoulder pathologies.
Mostly EMG of the neck/shoulder region muscles was used to understand the contribution of the neck/shoulder muscles, which then were interpreted to understand the underlying mechanism of neck/shoulder MSD.In the investigations ISSN (Online): 2329-0188 Sun et al.

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52 http://iser.sisengr.orgpresented above, the EMG of the neck/shoulder muscles was found to show a characteristic association with the different work activities; however, work activities studied so far mainly consisted of repetitive arm movements with little or no forceful arm exertions and tasks requiring prolonged static neck postures.

Discussion
In most epidemiological studies reviewed, the exposure assessment was based on the questionnaire responses from subjects, making the studies difficult to interpret.Very few studies (Andersen et al., 2003;Ariens et al., 2001;Rocha et al., 2005;Svendsen et al., 2004;Szeto and Lam, 2007) described the job characteristics, such as working postures, arm and head movements, physical assessment, and workstation observations.Overall, the results of experimental studies indicate that neck and shoulder muscles play a significant role in various tasks in occupational settings, such as VDT workers, dentists, light assembly, and construction workers.Repeated muscle contraction predominantly has been shown to be a cause of neck and shoulder MSD.Based on the review of physical risk factors associated with neck shoulder pain, by Westgaard (1999), muscle activation at or above 8-10% of maximal voluntary contractions (MVC) is associated with MSD .Particularly isometric muscle contraction has been suggested to create intramuscular pressure, which might lead to tears of muscle fibers, inadequate arterial circulation, reduced blood flow, and delayed dissipation of metabolites, which can result in structural tissue deformation and localized muscle fatigue (Cailliet, 1991;Armstrong et al., 1993;Hagberg and Wegman, 1987).
The work-related physical risk factors for neck and shoulder MSD have been documented by the reviewed experimental studies commonly include: (1) repetitive motions, (2) awkward postures, (3) forceful exertions, (4) vibration, and (5) poor work or workstation design.

Repetitive motions
There is evidence for a causal relationship between highly repetitive motion and neck/shoulder MSD from the reviewed epidemiological studies, e.g.Cassou et al. (2002), Alexopoulos et al. (2003), Alipour et al. (2008), andAndersen et al. (2003).Most of the studies defined "repetitive task" as work activities that involve continuous repetitive hand or arm movements.Examples of repetitive tasks are sewing machine work, packing, continuous data entering, shop cashier, deboning ham or poultry, and manual machine feeding (Andersen et al., 2003).Leonard et al. (2010) studied subjects performing prolonged and continuous writing tasks, and found that the mean EMG activity of upper trapezius muscle was significantly higher in subjects with neck pain as compared to the asymptomatic group.

Awkward postures
There is strong evidence that working with sustained awkward postures involving neck/shoulder muscles put individuals at high risk of neck/shoulder disorders.In particular, working with arm at or above shoulder level, shoulder abduction, neck flexion and extension, and forward head posture have been frequently concluded as the important parameters of awkward postures.

Neck flexion and extension
Neck flexion was suggested as the main cause of neck/shoulder disorders.The assumed causative factor is static contraction of the neck and shoulder muscles to counteract the weight of the head (Hagberg, 1984).Thus, the greater the angle of the neck flexion, the greater the load on the muscles and joints (Magnusson and Pope, 1998).Szeto et al. (2002) found that office workers with neck and shoulder pain had generally more head tilt and neck flexion posture than asymptomatic workers.
More flexed neck were found to impose more static load on the upper trapezius muscle as measured by EMG, among computer workers (Delisle et al., 2006), dentists (Finsen et al., 1998), and construction workers doing manual materials handling tasks (Nimbarte et al., 2010).The study performed by Villanueva et al. (1997) also found that there is a significant correlation between neck flexion and neck extensor muscle activity among VDT workers.Among construction workers performing manual material handling tasks, independent of the weight lifted, the sternocleidomastoid muscle was most active at the extended neck posture (Nimbarte et al., 2010).

Forward head posture
The discomfort posture of forward bending of the head has been suggested as a risk factor for developing neck and shoulder MSD among VDT workers (Bauer and Wittig, 1998;Haughie et al., 1995;McLean, 2005;Szeto et al., 2002) and dentists (Akesson et al., 1997).
Dentists and VDT workers often assume prolonged forward head posture, which may shorten the soft tissues, and cause muscle tension, weakness and fatigue.When head position is too far forward, it will make the small spinal muscles to work twice as hard as they normally would (Ming et al., 2004).Because neck muscles partially loose the support of the spine in this position and must do much of the work to keep head in this awkward posture, they become overloaded and may result in various pain conditions in the neck and shoulder area.
When the arms are elevated during work activities, large number of muscles of the shoulder joint are involved.Poppen and Walker (1978) found the glenohumeral joint forces at 90 o of shoulder abduction to be close to body weight.Thus, great muscle forces are necessary to keep the arms elevated, especially in working postures where the arms are at or above shoulder level and the arms are unsupported (Magnusson and Pope, 1998).Hagberg et al. (1984) proposed that working with elevated arms may accelerate degeneration of the rotator cuff tendons through impairment of circulation due to static tension and compression of tendons against underlying bone.

Forceful exertions
There is also evidence for correlation between forceful exertions or strain and the occurrence of neck/shoulder MSD.Most of the epidemiological studies reviewed found high prevalence rate of neck/shoulder MSD among health care workers, who are frequently involved in heavy lifting and patient handling tasks (Aasa et al., 2005;Alexopoulos et al., 2003;Lorusso et al., 2007;Smedley etal., 2003;Smith et al., 2006;Trinkoff et al., 2003).Chee et al. (2004) found that there is a causal relationship between frequent heavy lifting and neck/shoulder MSD among female semiconductor workers.Farmers who are involved in lifting and carrying heavy materials were found to have high prevalence rate of neck and shoulder disorders compared to other farmers (Rosecrance et al., 2006).Nimbarte et al. (2010) found that there was high levels of muscle activation and static load of the sternocleidomastoid and upper trapezius muscles during manual materials handling tasks that involve forceful arm exertions among construction workers.

Vibration
Three reviewed epidemiological studies found some evidence that exposure to vibrating hand tools or machines may increase the risk of neck/shoulder MSD.Alexopoulos et al. (2004) found statistically significant association between exposure to vibrating tools and the complaints of shoulder pain with high odds ratio (OR) of 2.57 (95% CI: 1.01-6.51)among 430 dentists.Palmer el al. (2001b) demonstrated that the relative risk (RR) of developing neck pain among workers who were exposed to more than 2.8 m/s 2 daily were 1.8 times (95% CI: 1.4-2.3)higher than those never exposed to hand-transmitted vibration tools.However, since vibration is maximally transmitted to the fingers and hands, the transmission to the upper limb and neck depends on vibration frequency.Thus the highest relative risk was observed at the elbow with the value of 3.7 (95% CI: 1.8-7.5).Tsigonia et al. (2009) found that handling vibrating tools appeared to have a significant relationship for cosmetologists to care seeking because of neck pain."Use of vibrating tools" may indicate that cosmetologists need to work with their trunk severely flexed, which may lead to awkward and constrained neck posture burdening the neck muscles and vertebrae.

Poor work or workstation design
Poor work or workstation design factors such as long duration of computer and/or mouse and/or keyboard use among VDT workers (Blatter and Bongers, 2002;Brandt et al., 2004;Devereux et al., 2002;Eltayeb et al., 2009;Palmer et al., 2001a), improper placement of monitor/screen and/or mouse and/or keyboard (Cook et al., 2000;Fogleman and Lewis, 2002;Jensen, 2003;Korhonen et al., 2003;Sillanpaa et al., 2003), fewer rest breaks during working time (Alexopoulos et al., 2004;Rocha et al., 2005), poor physical environment (Korhonen et al., 2003;Rocha et al., 2005), and other improper anthropometric workstation design (Fogleman and Lewis, 2002;Szeto and Lam, 2007) were found as risk factors for the developments of neck/shoulder MSD in the epidemiological studies.
Laboratory studies have shown that computer users working with poor VDT workstation, such as workstations without forearm support, improper placement of monitor/screen and/or mouse and/or keyboard may increase muscular activity in the upper trapezius muscle as well as other muscles in the neck-shoulder area, e.g.anterior and medial deltoid, cervical erector spinae, and sternocleidomastoid muscles (Chen and Leung, 2007;Dennerlein and Johnson, 2006;Delisle et al., 2006;Turville et al., 1998).The study of Psihogios et al. (2001) showed that the participant's location preference generally corresponded to the location in which he or she experienced neck discomfort.Balci and Aghazadeh( 1998) found that the 40 o angle monitor caused less neck discomfort than the 15 o angle monitor.Authors also demonstrated that VDT users with bifocal lenses had significantly higher neck discomfort and lower performance than nonbifocal users.Balci and Aghazadeh (2003) found that 15-minute work/micro breaks work-rest schedule resulted in significantly lower discomfort in the neck, as well as lower back and chest region than other schedules for data entry work.Nussbaum et al. (2001)  The results also suggested that emphasis should be placed on the anterior and middle deltoid muscles, given that signs of fatigue were more frequently apparent in either upper trapezius or infraspinatus muscles.

Conclusion
The present review took a comprehensive and interesting approach from both epidemiological and experimental perspectives, developing a systematic list of physical factors that are associated with MSD of the neck region.The present review confirms findings from previous studies about the causal relationship between neck MSD and repetitive movements, awkward working postures, forceful exertions, as well as poor work or workstation design and high dose of vibration.There is need for future research that explores effect of exposures that combine physical, workstation and psychosocial factors on the response of neck musculature to improve our understanding of causation and prevention of work-related neck/cervical MSD.
simulate overhead assembly tasks with two different duty cycles: 20-minute work/40-minute rest (20/40) and 40-minute work/20-minute rest (40/20).The study demonstrated that work-rest schedule is a critical parameter in the work tasks design.

Table 1 . Epidemiological studies on occupational physical risk factors for neck and/or shoulder disorders (35 studies in total) Study and design Study population Exposure basis Outcome measurement Results Risk factors Among general working population (3 studies)
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