Aetiology and Epidemiology of work-related Musculoskeletal Diseases

Which diseases and disorders rank among the work-related musculoskeletal diseases?

Work-related musculoskeletal diseases (MSDs) include degenerative diseases of the spine and the joints, as well as of their muscular and other structures. MSDs are closely connected with inappropriate physical strains, which are still found in many occupations today.

Work-related diseases and disorders of the musculoskeletal system are primarily degenerative diseases, that is, signs of wear and tear. They include, for example:

degenerative diseases of the cervical and lumbar spine,
degenerative diseases of the joints, such as osteoarthritis of the wrists, the elbow joints, the knee and the hip joints,
specific and unspecific reactions of muscles and other structures to static or repeated long-term biomechanical demands or physical workload. These include diseases of the tendons, ligaments, menisci and bursa.

These diseases and biomechanical strains can lead to pain, discomforts and functional restrictions in all regions of the body:

in the upper extremities: hands, wrists, lower arm, elbow, shoulder;
in the spine: cervical, thoracic and lumbar spine;
in the lower extremities: knee, hip joint, ankles, feet, lower legs.

In the context of discomforts and pain in the musculoskeletal system, those affected frequently refer to the following unspecific symptoms or pain syndromes:

pains in the lower back (low back pain),
cervical syndromes: pain in the neck,
RSI (repetitive strain injury syndrome): pain in the upper extremities following frequently repeated (powerful) movements; also referred to as cumulative trauma disorder (CTD).

Damage to the musculoskeletal system also includes direct nerve and vascular disorders through mechanical influences:

nerve compression syndromes, for example, the carpal tunnel syndrome,
hypothenar hammer syndrome: damage to blood vessels of the hand through the impact of heavy forces, for example through hammering,
vibration-related vasospastic syndromes: "Raynaud syndrome", which occurs in connection with the use of vibrating hand-held tools such as chainsaws.

Work-related MSDs are frequently the result of heavy physical work or inappropriate physical strain. However, it should not be forgotten that heavy physical work can also lead to inappropriate strains, physical stress and diseases in other organ systems. These include the consequences of static pressure loads in body cavities, organs and compartments, such as hernias or consequences for the cardiovascular system, for example hypertension and varicose veins.

It should not be forgotten that there is an increased risk of accidents with heavy physical work. For example, injuries during load handling, or when pushing and pulling, joint injuries, muscle injuries, tendon injuries, damage to nerves, or the consequences of fractures are all possible. Thereby, amputations also rank among the spectre of diseases.

Risk factors in the workplace

The spectrum of factors in the workplace environment that lead directly to disorders and diseases in the musculoskeletal system is wide. It comprises both biomechanical workloads through physical demands during work and the effect of physical and chemical factors as well as modifications through factors of work organisation and of the psychosocial situation or the employee's position in the company.

Inappropriate biomechanical workload to the muscles, ligaments, tendons, tendon insertions, joints and joint structures is particularly important in the aetiology. However, it should be stressed that it is not simply high or extreme physical strains that bring about the corresponding inappropriate workload to the musculoskeletal system. An imbalance between reduced resilience, e.g. through a lack of training, through underload or through constitutional and dispositional conditions, and normal physical workload, can also lead to corresponding inappropriate physical workload.

Work organisation and social structures are also relevant

It is also known that not only adverse ergonomic work conditions but also factors such as work organisation, the arrangement of work contents, social structures, management behaviour, opportunities for development in the company and resulting psychosocial stresses can at least influence the advance (progression) of disorders in the musculoskeletal system.

Along with work-related factors, it is also necessary to take account of individual factors in the emergence and development of disorders and/or of mainly degenerative diseases in the musculoskeletal system. Especially with lower back pain it is known that age, anthropometrics and a lack of fitness play their respective parts. There is no clear evidence on the factors of gender, consumption of alcohol and smoking. Genetic factors play a part in particular with differences in the increase in age-related degenerative changes.

Various factors interact with one another

Experts assess individual psychosocial factors, such as depression, fear, feelings of stress, personality factors and coping strategies, differently. If a person belongs to a specific occupational group, e.g. they are a blue-collar worker, the risk of lower back pain is regarded as higher. Whereby these groups usually also have to cope with high physical demands and exposure in their work for longer periods of time.

Here, physical work factors and psychosocial and personal factors mutually interact. The greater and more intensive physical demands or the impact of physical factors, the less significant are psychosocial and personal factors in the development of MSDs. For example, if a person has to lift and carry heavy loads, or if an employee is exposed intensively to whole-body vibrations, the lumbar spine is heavily stressed. In comparison, psychosocial and individual factors play a greater part if physical demands/exposures tend to be minor, e.g. with office work.

For years, many publications have been available on the evidence of occupational risk factors as well as disorders and diseases of the musculoskeletal system.

The socio-political significance of prevention

MSDs are one of the most important causes of work absence, outpatient and inpatient hospital treatment, rehabilitation and early retirement. Over one in four of all working days lost due to work absence and cases of work absence are results of these groups of diseases, as well as about half of all rehabilitation cases and over a quarter of all early retirements. According to an estimate for the year 2005, in Germany diseases of the musculoskeletal system and the connective tissues (Chapter M ICD 10) were responsible for approx. 98 million working days lost due to incapacity, loss of production in the amount of € 8.8 billion, or 0.4% of the gross national income, and a loss of gross value added of € 15.5 billion, or 0.7% of the gross national income.

Not every disease of the musculoskeletal system that is defined in accordance with the International Classification of Diseases (ICD 10) is encountered frequently. Measured against the frequency of cases of working days lost due to work absence, some diseases tend to be rare. With both men and women, the disease that is most frequently found is the relatively unspecific diagnosis group M54 "Dorsalgia". Here there are over 100 cases of work absence per 1000 employed insured persons. Other frequent reasons for work absence are other diseases of the spine, diseases of the tendons and the tendon sheath, with men shoulder lesions and interior damage to the knee joints; here there are over 10 cases of work absence per 1000 employed insured persons. Other disk damage, osteoarthritis of the knee or hip and other forms of osteoarthritis only occasionally lead to cases of work absence (over 1 case per 1000 employed insured persons). Rarely found reasons for work absence are diseases such as fibromatoses or polyarthritis, rhizarthrosis (osteoarthritis of the carpometacarpal joint of the thumb) among others with less than 1 case of work absence per 1000 employed insured persons.

MSDs have a particular effect on work.

Craft occupations in particular as well as work in industry and the services sector are accompanied by a much higher risk of work absence through diseases of the musculoskeletal system.

MSDs are socio-economically significant because they usually lead to functional impairment of the musculoskeletal system and therefore of the capabilities that a person can deploy at the workplace. Occupational activities always presuppose specific motor skills. This involves physical strength, motility, posture, speed, coordination and dexterity as well as stamina. Diseases of the musculoskeletal system regularly mean that motor demands such as the application of force, or taking up specific postures and carrying out specific movements can only be done with discomfort, if at all. Examples of this are lifting a load with lower back pain, or grasping a tool with carpal tunnel syndrome.

How do pain and damage to the musculoskeletal system develop through occupational risk factors?

In the pathogenetic model for the development of disorders and diseases of the musculoskeletal system, biomechanical strain plays an important part - but not the only one. If thrust forces have to be applied in combination with specific postures during an activity - e.g. high compression forces in the disk of the lumbar spine when lifting loads - great physical forces are generated in the musculoskeletal apparatus. The tensile, compression and shearing forces that occur here can change and traumatise the structure of the musculoskeletal system both acutely and in the long term. This biomechanical explanatory approach applies as well among others to the development of bursitis following external compressive strains, nerve damage through external pressure, inflammatory reactions of the tendons and tendon insertions after high and repetitive strains or articular effusions.

The development of muscle pain is more complex. High mechanical strain on isometric or concentric contractions primarily triggers peripheral muscle fatigue. This type of strain is connected with the release of radicals and mechanically influenced membrane damage. This leads to the release of Ca²⁺ through extension-activated canals. An ATP deficiency or membrane damage is activated by proteases (proteolytic enzymes). With intensive dynamic (concentric) contractions, the consequence of high metabolic strain is lactic acid acidosis through a local O₂ deficiency. In addition, extended physical exertion leads to exhaustion of the energy supplies in the muscles through depletion of an energy-rich compound in the muscle cells (ATP, glycogen, creatine phosphate). This releases radicals and inflammatory cytokines and interferes with the ion pumps. This in turn reduces the relaxing speed of the muscles. Contraction remains and spasms occur. Those affected experience the process as "muscle soreness" or "muscle pain".

The muscles adapt to the strains

In the long term, the muscles adapt to the strain. This can be seen in fibre transformation (change to the protein isoforms and thus to the fibre types), hypertrophy (enlargement), splicing, neogenesis, atrophy (degrading of proteins), apoptosis (cell death of unwanted cells) and possibly in necrosis as well, that is, the unplanned, complete loss of fibres. This process is controlled both mechanically through the external cytoskeleton and channel proteins, which induce gene transcription, as well as autocrinously (mechano-growth factor, cytokines, interleukin-6, stress proteins). In addition, body hormones such as insulin and thyroxine, neurotransmitters such as acetylcholine and noradrenaline, testosterone and glucocorticoids influence muscle adaptation.

For this reason, there are different hypotheses as to how muscle pains occur: the muscle spasm hypothesis assumes that an alpha-gamma coupling triggers a tone increase in the muscles. The compression of muscle vessels associated with this is claimed to cause local ischaemia and oedema. Nocireceptors are stimulated, neuralgetic and vasoneuroactive substances are released.

Hypothesis sees muscle pain as the body's reaction to stress

The microtrauma hypothesis explains the development of muscle pain through the traumatisation of capillaries, muscle fibres, fascias and tendons on eccentric contractions. Nocireceptors are stimulated directly in a mechanical way. This process triggers pain cascades and actuates regeneration processes (prostaglandins, vasoneuroactive substances, bradykinin, 5-hydroxytryptamine, neuropeptides).

In addition, the sympaticus hypothesis brings muscle pain in conjunction with the body's stress reaction. Activation of the sympathetic nerve system in the stress reaction is coupled with the release of catecholamine. A sympathetic-afferent coupling to nocireceptors or after sensitisation or traumatisation is also postulated. A sympathetic innervation of the intrafusal muscle spindle is claimed to lead to a tone increase in the muscles and thus to muscle pain.

Finally, psycho-physical explanation models are included for the development of disorders of the musculoskeletal system through a neuroplastic damage model. Pain on movement is learned, or connected with the movement, so that a movement without actual damage is considered to be painful. This applies, for example, for lower arm complaints with highly repetitive work.

The most important occupational effects

The German Working group 1.7 "Belastungen des Muskel- und Skelettsystems" (Strain in the musculoskeletal system) has summarised which occupational demands and activities are associated with special strains for the musculoskeletal system, and published them in DGUV-I 240-460 (previously BGI 504.46) "Handlungsanleitung für die spezielle arbeitsmedizinische Vorsorge nach dem Berufsgenossenschaftlichen Grundsatz G 46 'Belastungen des Muskel- und Skelettsystems'" (Guidelines for special occupational medicine treatment in accordance with principle G 46 'Strains in the musculoskeletal system' of the employers' occupational liability scheme). The working group is part of the occupational medicine committee at the Statutory Social accident Insurance (DGUV) (AG "Gefährdungsbeurteilung" - "Risk assessment" working group).

The definition of the categories of physical workloads was updated in the currently ongoing research project F 2333 (MEGAPHYS - "Multilevel risk assessment of physical exposures", a cooperation project between DGUV, BAuA and research partners). According to this, relevant activities and exposures that put strains in the musculoskeletal system are:

Manual handling of (heavy) loads

lifting, holding carrying (manual load handling)
pulling, pushing loads

Work in constrained positions (forced postures)

squatting, kneeing, lying
working with the hands above shoulder height
working in (extremely) stooped positions
(uninterrupted long lasting) sitting
(long lasting) standing

Manual work (working mainly using hands/arms)

repetitive manual work with high handling frequencies and/or high forces and/or in extreme joint position
using the hand/arm system as a tool (knocking, hammering, turning, pressing)

Using entire physical strength

work with increased physical exertion and/or exertion of force, e.g. exertion of force/pressure when operating tools

Physical workload through bodily movement

walking, running during work
work in places that are difficult to access (climbing, scrambling)

Effect of mechanical vibrations

effects of hand-arm vibrations
effects of whole body vibrations

In the ongoing cooperation project between the BAuA and the DGUV (MEGAPHYS, project F 2333 of the BAuA) the above-mentioned workload categories were re-classified in the following way (Ditchen et al 2015):

lifting, holding carrying
pulling and pushing
manual work processes
whole body forces
(strained) postures
forced postures of the body
(mixed work)

Information on the risk assessment of physical workload situations can be found at another place on the BAuA website, or in the Risk assessment portal.

Frequency of occupational risk factors for the musculoskeletal system in the world of work

In the last decades, physical exposure at work has on the whole been reduced. In spite of this, employees in service occupations, in agriculture and forestry and craft-based trades, plant operators and unskilled workers still state very often that they have to stand or walk during their work, move loads, carry out repetitive hand-arm movements, move people or are exposed to mechanical vibrations. This involves in particular work in the service sector. In addition, an increasing tendency to unsuitable postures and insufficient physical activity in sedentary occupations, e.g. in clerical occupations, can be seen.