Doctors can only diagnose diseases that they know . . . This is why new diseases, after their first description, are seen everywhere – Aids, for example. If the physician is unaware that a certain constellation of symptoms and signs indicates a particular disease, there is no possibility of a diagnosis being reached.
(Skrabanek and McCormick, 1990)
Most repetitive tasks require a combination of both static and rhythmic muscle activity. In manual work, postural stabilisation of the hands and arms is essential for carrying out all but the grossest movements in a purposeful way. This stabilisation is provided by muscles farther up the kinetic chain, muscles that cross the elbow and shoulder joints and have their origins in the cervical spine and thoracic regions.
If task demands are excessive, pain may be experienced in the muscles providing the stabilisation or in the muscles and joints of the effectors, or in both. Over time, a medical condition may develop.
Introduction to work-related musculoskeletal disorders
The relationship between task demands, ergonomics and musculoskeletal disorders is of a probabilistic nature and is confounded by the fact the disorders can arise as a result of many activities of daily life, both at work and elsewhere. The disorders may present as co-conditions of other diseases. Disorders such as cervical spondylosis, carpal tunnel syndrome and tennis elbow are very common, particularly in older people. The usual pattern is for symptoms to appear before objective signs of de- generation or disease. By the age of 50 years, 50% of studied populations have had neck, shoulder or arm pain (Lawrence, 1969). This number increases steadily with advancing age.
There appear to be two main camps in the debate about the work-relatedness of musculoskeletal disorders. There are those who believe that if a person experiences pain at work it must be the work that caused it and that the pain itself is evidence of an underlying medical condition, caused by the work. People in this camp draw little distinction between the words ‘pain’, ‘disorder’ and ‘injury’ and use terms such as RSI (repetitive strain injury) and CTD (cumulative trauma disorder) as collective nouns to describe any kind of musculoskeletal problem, irrespective of whether it falls into a medical diagnostic category or not. Those in the second camp believe that there is a lack of evidence that work causes musculoskeletal diseases, apart from
Some terms used in epidemiology
Prevalence: The number of cases of a disorder at any particular time Incidence: The number of new cases over a time period
Odds ratio: A statistic that describes the likelihood of a disorder developing given exposure to a risk factor. In the table below, there are some imaginary data on the prevalence of shoulder problems in a group of workers. The occupational exposure is having to load objects on to and off high shelves. The health outcome is the presence of a shoulder condition.
Shoulder problems
Yes No
Exposed to Row total
loading high Yes 20 20 40
shelves? No 5 80 85
Total 25 100 125
The data can be set out, as above, in a 2 × 2 table. The total number of workers surveyed is 125. Of these, 25 have shoulder problems and 40 carry out work involving high shelves. At first glance, it appears that people who work with high shelves are four times as likely to have shoulder problems than those who do not (of the 25 with shoulder problems, 20 work with high shelves and 5 do not). However, this ignores the null cases – people in the sample who do not have any shoulder problems irrespective of whether they work with high shelves or not. Clearly, in any assessment of risk, we need to take these individuals into account as well.
The odds ratio calculation does this elegantly, as follows:
Let A = Those with shoulder problems and exposed to high shelves = 20 Let B = Those without shoulder problems and exposed to high shelves = 20 Let C = Those with shoulder problems and not exposed to high shelves = 5 Let D = Those without shoulder problems and not exposed to high shelves = 80 The odds ratio (OR) = (A × D)/(B ×C) = (20 × 80)/(5 × 20) = 1600/100 = 16
The overall prevalence rate for the shoulder disorders is 25/125 or 20%, so there is a high rate of shoulder disorders in the sample. The prevalance rate for those exposed to the risk factor is 20/5 or 4. We can conclude that there is a very greatly increased risk of developing the shoulder disorders given exposure to the risk factor: people are 16 times more likely to develop the problem.
Note that the size of the odds ratio in this example is very strongly influenced by the large number of people who do not have shoulder problems and are not exposed to the risk factor (cell D). Thus we can say that the shoulder problem is very specific to the exposure (it is rare in those not exposed). For many musculoskeletal conditions, the odds ratios are much smaller. One of the reasons for this is the lack of specificity – many non-work-related factors can cause the condition as well (cell C is larger and cell D smaller).
minor aches and pains that are reversible. Different jobs simply disclose differences in the health of people, which is why some complain and others do not. People in this camp avoid the use of superordinate categories in favour of distinct medical entities or confine the discussion to symptoms. For them, the observation that carpenters get more arm pain than controls is is merely a fact of life, rather than evidence of a disorder. Only if there is evidence of tissue pathology or overt disability (such as pain divorced from activity) will there be any suggestion of a disorder.
The entire field is marked by a lack of scientific rigour and poor terminology. In the present discussion, the term ‘work-related musculoskeletal disorders’ (WMSDs) will be used (after Kuorinka and Forcier, 1995). The classification scheme of Shilling and Anderson (1986) will be used to frame the discussion of the work-relatedness:
Category 1. The work exposure is a necessary cause of the disorder (as in occupa- tional diseases such as silicosis or lead poisoning).
Category 2. The work exposure is a contributory causal factor but not a necessary one.
Category 3. The work exposure provokes reaction by a latent weakness or aggra- vates an existing disease.
Category 4. The work exposes the worker to potential dangers that may increase the likelihood of a disease developing (such as alcoholism in liquor industry workers).
The main work-related musculoskeletal disorders are reviewed below. The func- tion of the ergonomist is not to attempt diagnosis of these conditions but to take any reported symptoms, such as pain, into account when evaluating workplaces.
Irrespective of whether there is an underlying condition, the goal should be to design or redesign the workplace to enable people to work more comfortably.
Work-related musculoskeletal disorders ( WMSDs)
Musculoskeletal conditions that are considered to be work-related are summarised in Table 5.1, using the categorisation scheme for diagnosis of the International Classifi- cation of Diseases.
In the past, conditions such as pre-patellar bursitis were named after occupations in which they were common – ‘housemaid’s knee’, for example (see Pheasant, 1991).
Quinter (1989) reviewed the history of occupational arm pain. Diffuse pains in the hand and wrist that later spread up the arms as the condition became chronic were known in the nineteenth century as ‘occupational neuroses’ and were found among writers and telegraphers. Both of these groups had jobs that demanded a combina- tion of static fixation of the limbs and fine repetitive actions requiring a high degree of motor control. There was an absence of muscle wastage and no loss of sensation in the affected limb. This led to different theories of causation, with some arguing that the cause was peripheral nerve damage and others that the fault lay in the central nervous system and was a fault of the high-level mechanisms responsible for coordin- ating the automatic performance of movement. Others preferred the purely psycho- logical explanation that the problem was caused by a tendency to overreact emotionally to the normal challenges of life!
Musicians and sportsmen suffer a variety of musculoskeletal problems ranging from vague symptoms of discomfort to clear-cut conditions such as ganglia. Instrumentalists
Table 5.1 ICDa diagnoses for work-related musculoskeletal disorders Nerve root and plexus disorder
• Brachial plexus lesions
• Unspecified nerve root and plexus disorder
Mononeuritis of the upper limb and mononeuritis multiplex
• Carpal tunnel syndrome (median nerve entrapment)
• Lesions of the ulnar nerve (cubital tunnel syndrome)
• Lesions of the radial nerve
• Mononeuritis of the upper limbs (unspecified) Other peripheral vascular disease
• Raynaud’s syndrome
• Raynaud’s phenomonen (hand–arm vibration syndrome) Arterial embolism and thrombosis
• Arteries of the upper extremities (ulnar artery thrombosis) Other disorders of the cervical region
• Cervicobrachial syndrome (diffuse)
• Unspecified musculoskeletal disorders and symptoms referable to the neck Peripheral enthesopathies and allied syndromes
• Disorders of the bursae and tendons in the shoulder region (rotator cuff syndrome, suprasinatus syndrome, bicipital tenosynovitis)
• Enthesopathy of the elbow region (medial and lateral epicondylitis)
(Enthesopathies are disorders of the peripheral ligamentous or muscular attachments.) Other disorders of the synovium, tendon and bursae
• Synovitis and tenosynovitis
• Trigger finger (acquired)
• Radial styloid tenosynovitis (deQuervain’s)
• Other tenosynovitis of the hand and wrist
• Specific bursitides
• Unspecified disorder of the synovium, tendon and bursa Disorders of muscle, ligament and fascia
• Unspecified disorder of muscle, ligament and fascia Other disorders of soft tissues
• Myalgia and myositis (fibromyositis)
• Other musculoskeletal symptoms referable to the limbs (cramping, swelling)
• Other, unspecified, disorders of soft tissue
aInternational Classification of Diseases.
suffer the highest prevalence of such disorders (Brandfonbrener, 1990). The main problem areas are the small joints of the fingers and the wrist and the risk factors are thought to be the number of repetitive movements, the playing posture, the requirement to support the instrument while playing it and while carrying it, and the resistance of the keys or strings. Personal factors such as hypermobility of the finger joints (joint laxity) are involved and appear to be gentically determined.
Usually benign, they may even have been a factor in the individual’s early success.
Brandfonbrener reports the results of survey of musculoskeletal conditions in 2212 orchestral musicians. Keyboard players predominated, followed by string players.
Both groups are required to perform a high number of repetitive movements in
their work. Wind and brass musicians, owing to the physical effort of playing their instruments, cannot endure such long practice hours and rest more frequently, reduc- ing the likelihood of an ‘overuse’ injury.
Most conditions are localised and of a peripheral nature. The exception is focal motor dystonia, normally pain-free but consisting of cramps that interfere with coor- dination or cause involuntary movement. No problems are experienced until the musician attempts to play. The problem is thougt to originate in the central nervous system, possibly the basal ganglia.
Palmer et al. (2001) analysed data from 12 262 people registered with 163 general practitioners in the UK. Data on pain and sensory symptoms such as numbness and tingling in different body sights were captured. Significant associations were found between symptoms in the shoulders, wrist and hands and older age, smoking, headaches and tiredness/feeling stressed. Controlling for these factors, a significant association remained between the symptoms and keyboard use of > 4 hours per average working day. Keyboard users were approximately 1.2 to 1.4 times as likely to experience these symptoms than were non-users.
LeClerc et al. (2001) carried out a survey of carpal tunnel syndrome, lateral epicondylitis and wrist tendinitis in 598 workers in a variety of industries. The preva- lence of these conditions at the start of the study was quite high – 21.9%, 12.2% and 11.2%, respectively. The findings illustrate very well the multifactorial nature of these conditions and the influence of male/female differences. For carpal tunnel syn- drome, the main associated factors for men were tightening with force, holding in position and not pressing with the hand. For women, increase in body mass index and low job satisfaction were the main factors. For lateral epicondylitis, age and the job demand of ‘turn and screw’ were associated as were the number of other dis- orders and the presence of depressive symptoms. Wrist tendinitis was associated with the presence of somatic problems at the beginning of the study (e.g. problems with sleep, headaches, personal worries), lack of social support, increased body mass and having to hit repetitively. Younger workers were at increased risk, suggesting a
‘survivor effect’ in which only those without symptoms stay in the job.
A multifactorial problem
Clearly, WMSDs are the result of the interplay of many different variables. Although workplace ergonomic factors play a role, wider work organisation issues, social as- pects of work and the health of the workers themselves are important. It is incorrect to talk about the ‘prevention’ of these disorders through ergonomic redesign of workspaces at the level of the ergonomic factors identified, since the evidence that ergonomic exposures are the cause is lacking. The current state of knowledge is of a web of factors that are associated with musculoskeletal outcomes. The outcomes themselves are often defined subjectively, inferred from questionnaire responses, and little precise information is captured on the magnitude of the ergonomic exposures, making it difficult to estimate dose–response relationships.
Factors associated with adverse outcomes
The main occupational factors associated with musculoskeletal conditions at work are
• Force
• Posture
• Repetition
• Duration
Bernard reviewed the literature available in 1997 (National Institute of Safety and Health, 1997) and concluded that there was evidence that most of the conditions were associated with one or more of the above factors. In several cases, exposure to more than one factor caused a large increase in the prevalence of the disorder. Bernard concluded that there was evidence that many of the conditions were caused by the work exposures based on the following criteria:
• Temporality: Prospective studies that show that the exposure precedes the out- come in time (cross-sectional studies cannot demonstrate cause and effect for this reason).
• Strength of association: The larger the odds ratio, the stronger the association between the outcome and the exposure and the less likely it is that the findings are due to confounding factors.
• Consistency: Where several studies are done, the same associations keep emerging.
• Specificity: The outcome depends on exposure to specific factors.
Models of the development of WMSDs
Armstrong et al. (1993) have developed a model of musculoskeletal disorders that emphasises exposure, dose, capacity and response. These are summarised in Table 5.2. Exposure refers to work demands such as posture, force and repetition rate that have an effect (the dose) on the internal body parts. Metabolic changes in the muscles, stretching of tendons or ligaments, compression of the articular sur- faces of joints are examples of what is meant by a dose. The dose may produce a response such as a change in the shape of a tissue, the death of cells or accumulation of waste products in the tissues. These primary responses can be accompanied by secondary responses such as pain or a loss of coordination. As can be seen, a response (such as pain), can be a dose that causes another response (e.g. increased muscle contraction).
Capacity refers to the individual worker’s ability to cope with the various doses to which his musculoskeletal system is exposed. An individual’s capacity is not fixed.
According to the model, it may change over time as the person ages or the develop- ment of skill may improve the ability to generate large forces with less effort.
Training can increase strength or endurance, whereas the development of scar tissue to replace injured muscle tissue may impair strength or endurance. Armstrong et al.
point out that muscles can adapt to work demands faster than tendons and that this may lead to reduced (relative) tendon capacity. We might speculate that one of the dangers faced by bodybuilders and others who use illegal anabolic steroids to produce rapid increases in muscle bulk is injury to the tendons because tendon strength does not have time to ‘catch up’ with the increased muscle strength.
Some common musculoskeletal conditions are described in the following sections.
Their work-relatedness is summarised following the conclusions of Bernard (1997) and subsequent literature. Ergonomic interventions are described.
Table 5.2 Key elements of Armstrong et al.’s (1993) model of the development of work- related upper body musculoskeletal disorders
Element Examples
Exposure Physical factors
• workplace layout
• tool design
• size, shape, weight of work
• objects Work organisation
• cycle times
• paced/unpaced work
• spacing of rest periods Psychosocial factors
• job dissatisfaction
• quality of supervision
• future uncertainty
Dose Mechanical factors
• tissue forces
• tissue deformations Physiological factors
• consumption of substrates
• production of metabolites
• ion displacements Psychological factors
• anxiety Primary responses Physiological
• change in substrate levels
• change in metabolite levels
• accumulation of waste products
• change in pH Physical
• change in muscle temperature
• tissue deformation
• increase in pressure Secondary responses Physical
• change in strength
• change in mobility Psychological
• discomfort
Capacity Mechanical
• soft-tissue strength
• bone density/strength Physiological
• aerobic capacity
• anaerobic capacity
• homeostatic control Psychological
• self-esteem
• tolerance of discomfort
• tolerance of stress
Injuries to the upper body at work
The most clear-cut work-related upper body injuries occur as a result of accidents at work and many of them occur when hand tools are being used. Aghazedeh and Mital (1987) carried out a questionnaire survey to determine the frequency, severity and cost of hand-tool-related injuries in US industry and to identify the main problem areas. The hand-powered tools most commonly involved in injury were knives, hammers, wrenches, shovels, and ropes and chains. The powered tools most com- monly involved were saws, drills, grinders, hammers, and welding tools.
Of the main incidents that precipitated an injury, the majority involved the tool striking the user. This was the case with both powered and non-powered tools.
However, a significant minority of injuries were caused by overexertion (approxim- ately 25–30%). The upper extremities were the body area most commonly injured and the most common injuries were cuts and lacerations, followed by strains and sprains. A strain may be defined as overexercise or overexertion of some part of the musculature whereas a sprain is a joint injury in which some of the fibres of a supporting ligament are ruptured although the ligament itself remains intact.
Many powered tools can cause strains or sprains because of the reaction force they exert on the user, particularly if these forces are unexpected or occur suddenly as a result of irregularities at the interface between the tool and the workpiece. Percussive tools, such as paving breakers, exert a reaction force that has to be opposed by the user. In practice, if the tool is well-designed and used on a flat surface, the weight of the tool will dampen much of this force. Rotary powered tools such as drills, sanders and screwdrivers can exert a reaction torque on the user that may force the wrist into ulnar or radial deviation causing a strain or sprain. The design of handles for holding powered tools in place has received the attention of ergonomists, as is described below. It should not be forgotten, however, that additional handles may need to be fitted to provide the user with sufficient mechanical advantage to overcome the reac- tion torque of the tool or to carry it. Bone (1983) reports that in the USA General Motors specifies a maximum allowable torque for freely hand-held powered tools and fits torque-arresting arms or slip clutches to more powerful rotary tools. Such modifications reduce the risk of injury and may enhance the usability of the tool.
It appears that there are several different classes of hand-tool-related injury and that several different approaches for prevention may be needed. The most common injury would seem to be of a catastrophic nature in which the tool itself suddenly strikes the user, causing a laceration, bruise or sprain. A second, more pernicious, type of injury involves sprains or strains that appear to result from the handling of the tool itself over longer periods of time. A third type of injury occurs to the skin in the form of blisters due to pressure ‘hot-spots’ caused by poor handle design.
Attempts to prevent the first type of injury might emphasise training workers safe tool-handling techniques and to think ahead – to recognise potentially dangerous situations and to prepare the workplace to minimise the likelihood of unforeseen events. Attempts to prevent the second type of injury might concentrate on the redesign of the tool itself and training workers to recognise the onset of fatigue and avoid stressful work postures. Handle redesign can prevent the third type of injury, as can increasing the task variety of the job.
Prevention usually requires a multilayered approach involving training, safety propa- ganda and workspace design. Of particular relevance to the present discussion is the