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Protecting Workers' Health Series No. 5
Preventing musculoskeletal disorders in the workplace:
Previous page | 1,2,3

Part 1: Objectives

Work-related musculoskeletal disorders - a definition

The term musculoskeletal disorders denotes health problems of the locomotor apparatus, i.e. of muscles, tendons, the skeleton, cartilage, ligaments and nerves. Musculoskeletal disorders include all forms of ill-health ranging from light, transitory disorders to irreversible, disabling injuries. This booklet focuses on musculoskeletal disorders, which are induced or aggravated by work and the circumstances of its performance. Such work-related musculoskeletal disorders are supposed to be caused or intensified by work, though often activities such as housework or sports may also be involved.

Characteristic health problems

Health problems occur, in particular, if the mechanical workload is higher than the load-bearing capacity of the components of the musculoskeletal system. Injuries of muscles and tendons (e.g. strains, ruptures), ligaments (e.g. strains, ruptures), and bones (e.g. fractures, unnoticed microfractures, degenerative changes) are typical consequences. In addition, irritations at the insertion points of muscles and tendons and of tendon sheaths, as well as functional restrictions and early degeneration of bones and cartilages (e.g. menisci, vertebrae, intervertebral discs, articulations) may occur.

There are two fundamental types of injuries, one is acute and painful, the other chronic and lingering. The first type is caused by a strong and short-term heavy load, leading to a sudden failure in structure and function (e.g. tearing of a muscle due to a heavy lift, or a bone fracture due to a plunge, or blocking of a vertebral joint due to a vehement movement). The second results from a permanent overload, leading to continuously increasing pain and dysfunction (e.g. wear and tear of ligaments, tendovaginitis, muscle spasm and hardening). Chronic injuries resulting from long-term loading may be disregarded and ignored by the worker because the injury may seemingly heal quickly and it may not result in an actual significant impairment.

The number of such injuries is substantial. In industrialized countries, about one-third of all health-related absences from work are due to musculoskeletal disorders. Back injuries (e.g. lower back pain, ischiadics, disc degeneration, herniation) have the highest proportion (approximately 60%). The second position is taken by injuries of the neck and the upper extremities (e.g. pain syndromes of the neck, shoulders, arms, "tennis elbow", tendinitis and tendovaginitis, carpal tunnel syndrome, syndromes related to cumulative traumata, the so-called cumulative trauma disorders (CTDs) or repetitive strain injuries (RSIs)), followed by injuries of knees (for example, degeneration of menisci, arthrosis) and hips (e.g. arthrosis). It is generally accepted that working conditions and workload are important factors for the development and continuance of these disorders.

Basic risk factors for the development of musculoskeletal disorders

Mechanical overload, repetition frequency, exposure time, posture and accidents

Work-related musculoskeletal disorders are supposed to be causally linked to physical load resulting from occupational activities. Disorders or injuries affecting muscles, tendons, joints, ligaments and bones are mainly caused by mechanical overload of the respective biological structures. Potential overload of tissues results from high intensity forces or torques acting on and inside the body. Examples of occupational activities coinciding with high mechanical requirements are handling of objects, as in transportation jobs, or the application of pushing or pulling forces to tools or machines. The detrimental effect of mechanical overload depends mainly on the magnitude of the force. Furthermore, the duration of exposure is an important factor in the development of musculoskeletal disorders. It is mainly determined by the number of repetitions per unit time (e.g. per day) as well as by the total exposure time (e.g. hrs per day or days per month). Regarding the character of exposure, occasional vocational loading events can be discriminated from long-lasting activities occurring over many years during the entire occupational life. Short-term loadings may primarily lead to acute health disturbances whereas long-lasting exposure may, in a final stage, cause chronic disorders. The risk for the musculoskeletal system depends to a great extent on the posture of the operator. Especially, twisting or bending the trunk can result in an increased risk for the development of diseases at the lower back. Postural demands play an important role, particularly, when working in confined spaces.

Besides such types of occupational loading resulting from usual work-site conditions, musculoskeletal disorders can also be caused by unique, unforeseen and unplanned situations, e.g. by accidents. The origin of disorders due to accidents is characterized by a sudden overstrain of the organs of locomotion.

Total mechanical loading

Musculoskeletal disorders fig.5

The total load affecting the musculoskeletal system depends on the level of the different load factors, mentioned before, such as

  • the level and direction of forces,
  • the duration of exposure,
  • the number of times an exertion is performed per unit of time,
  • postural demands

Risk qualities

According to the factors mentioned before, different risk categories can be derived using different combinations or qualities thereof, such as

  • high intensity forces,
  • long exposure duration,
  • highly repetitive exertions,
  • strong postural demands,
  • strong or long-lasting muscular strain,
  • disadvantageous environmental or psychosocial conditions.

Factors contributing to the development of musculo-skeletal disorders

In the following, musculoskeletal load is characterized with respect to the main influences, such as the level of force, repetition and duration of execution, postural and muscular effort as well as environmental and psychosocial factors.

1. Exertion of high-intensity forces may result in acute overloading of the loaded tissues. High-intensity forces are active within the body tissues particularly during lifting or carrying heavy objects. Furthermore, pushing, pulling, holding or supporting an object or a living being are a matter of high-intensity forces.

2. Handling loads over long periods of time may lead to musculoskeletal failures if the work is continued for a considerable part of the working day and is performed for several month or years. An example is performing manual materials- handling activities over many years, which may result in degenerative diseases, especially of the lumbar spine. A cumulative dose can be regarded as an adequate measure for the quantification of such types of loadings. Relevant factors for the description of the dose are duration, frequency and load level of the performed activities.

Musculoskeletal disorders fig.6

3. Musculoskeletal disorders may also result from frequently- repeated manipulation of objects, even if the weight of the objects handled or the forces produced are low. Such jobs (e.g. assembling small work pieces for a long time, long-time typing, supermarket checkout work) may induce disadvantages for the musculature, even if the forces applied to the handled objects are low. Under such conditions, the same parts and fibers of a muscle are activated for long periods of time or with a high frequency and may be subject to overload. Early fatigue, pain, and possible injuries are the consequences.

Musculoskeletal disorders fig.7

4. In a well-designed workstation, work can be performed most of the time in an upright posture with the shoulders upraised and the arms close to the trunk. Working with a heavily bent, extended or twisted trunk can result in an overload of spinal structures and increased activity of entire muscles. If the trunk is simultaneously bent and twisted the risk of spinal injury is considerably increased. If movements or postures with the hands above shoulder height, below knee level or outstretched are performed over prolonged periods or recurrently, working conditions should be changed. Working in a kneeling, crouching or squatting position augments the risk of overloading musculoskeletal elements. Also, long-time sitting in a fixed posture isaccompanied by long-lasting muscular activity which may lead to an overload within muscular structures. Such working positions should be avoided and the time for working in such positions should be kept to a minimum, if such work is not completely avoidable

5. Static muscular load is found under conditions where muscles are tensed over long periods of time in order to keep a certain body posture (e.g. during work with the hands overhead in drilling holes into the ceiling, holding the arms abducted in hair dressing, holding the arms in typing position above the keyboard, working in a confined space). A characteristic of static muscular load is that a muscle or a group of muscles is contracted without the movement of the corresponding joints. If the muscle has no opportunity to relax during such a task, muscular fatigue may occur even at low-force levels, and the function of muscles may be impaired and it may hurt. In addition, static load leads to a deficiency in blood circulation in muscles. Under normal conditions, the permanent change between contraction and relaxation acts as a circulation-supporting pump. Continuous contraction restricts the flow of blood from and to the contracted muscle. Swelling of legs, for example, is an indicator of such a lack in blood circulation.

Musculoskeletal disorders fig.8

Musculoskeletal disorders fig.9

6. Muscular inactivity represents an additional factor for the development of musculo-skeletal disorders. Muscles need activation to maintain their functional capacity, and the same is true of tendons and bones. If activation is lacking, a deconditioning will develop, which leads to functional and structural deficits. As a result, a muscle is no longer able to stabilize joints and ligamental structures adequately. Joint instabilities and failures, incoordination connected with pain, movement abnormalities and overloading of joints may be the consequences.

Musculoskeletal disorders fig.10

Musculoskeletal disorders fig. 11

7. Monotonous repetitive manipulations with or without an object over long periods of time may lead to musculoskeletal failures. Repetitive work occurs when the same body parts are repeatedly activated and there is no possibility of at least a short period of relaxation, or a variation in movement is not possible. Relevant determining factors are the duration of the working cycles, their frequency and the load-level of the performed activity. Examples of repetitive work are keyboard use while typing, data entry, clicking or drawing a computer mouse, meat cutting, etc. Unspecific complaints due to repetitive movements of the upper extremities are often summarized in the term "repetitive strain injury - RSI".

8. Strain on the locomotor system may also occur due to application of vibration. Vibration may result from hand-held tools (e.g. rock drill) and, therefore, exert vibration strain on the hand-arm system. Hand-arm vibration may result in the dysfunction of nerves, reduced blood circulation, especially in the fingers (white finger syndrome) and degenerative disorders of bones and joints of the arms. Another risk concerns whole body vibration generated by vibrating vehicles and platforms such as earth-moving machines, low-lift platform trucks or tractors and trucks driving off-road. The vibration is transferred to the driver via the seat. Whole-body vibration can cause degenerative disorders, especially in the area of the lumbar spine. The effect of vibration may be intensified if the vehicle is driven in a twisted body posture. A vibration-attenuating driving seat may help to reduce the effect of vibration.

9. Physical environmental factors such as unsuitable climatic conditions can interact with mechanical load and aggravate the risk of musculoskeletal disorders. In particular, the risk of vibration-induced disorders of the hands is considerably enhanced if low temperature coincides with the use of hand-held vibrating tools. Another example of environ-mental factors influencing the musculoskeletal strain is the lighting conditions: If lighting and visual conditions are deficient, muscles are strained more intensively, particularly in the shoulder and neck region.

Musculoskeletal disorders fig.12

10. Besides the mechanically-induced strain affecting the locomotor organs directly, additional factors can contribute to the beginning or aggravation of musculoskeletal disorders. Psychosocial factors can intensify the influence of mechanical strain or may induce musculoskeletal disorders by themselves due to increasing muscle tension and by affecting motor co-ordination. Furthermore, psychosocial influences such as time pressure, low job decision latitude or insufficient social support can augment the influence of physical strain.

A summary of the main factors contributing to the risk of developing work-related musculoskeletal disorders is provided in the table.

Factor Possible result or consequence Example Good practice example or solution
Exertion of high- intensity forces Acute overloading of the tissues Lifting, carrying, pushing, pulling heavy objects Avoid manual handling of heavy  objects
Handling heavy loads over long periods of time Degenerative diseases especially of the lumbar spine Manual materials- handling Reduce mass of objects or number of handlings per day
Frequently repeated manipulation Fatigue and overload of  muscular structures Assembly work  long time typing, check-out work Reduce repetition frequency
Working in unfavorable posture Overload of skeletal and muscular elements Working with heavily bent or twisted trunk, or hands and arms above shoulders Working with an upright trunk and the arms close to the body
Static muscular load Long-lasting muscular activity and possible overload Working overhead, working in a confined space Repeated change between activation and relaxation of muscles
Muscular inactivity Loss of functional capacity of muscles, tendons and bones Long-term sitting with low muscular demands Repeated standing up, stretching of muscles, remedial
Monotonous repetitive manipulations Unspecific com- plaints in the upper extremities (RSI) Repeated activation of the same muscles without relaxation Repeated interruption of activity and pauses alternating tasks
Application of vibration Dysfunction of nerves reduced blood flow, degenerative disorders Use of vibrating hand-tools, sitting  on vibrating vehicles Use of vibration-attenuating tools and seats
Physical environmental factors Interaction with  mechanical load and aggravation of risks Use of hand-held tools at low temperatures Use gloves and heated tools at low temperatures
Psychosocial  factors Augmentation of physical strain, increase in absence from work High time pressure, low job decision latitude, low social support Job rotation, job enrichment,  reduction of negative social factors

Factors to be considered in prevention

Musculoskeletal disorders figl 13

The ideal balance

With regard to maintenance and promotion of health, a weighed balance between activity and rest is necessary. Rest pauses are a prerequisite for recovery from load-induced strain and for preventing accumulation of fatigue. Movement should be preferred to static holding, the aim should be a combination of active periods with loading and inactive periods of relaxation. The individual "favourable load" can vary from subject to subject depending on functional abilities and individual resources. Overload as well as inactivity should be avoided. Appropriate load effects training of muscles leading to adaptation and thus an increase in the capacity of muscles, tendons and bones. This is essential for health and well-being.

CAVEAT: This general view, however, needs refinement in special cases, since parts of the musculoskeletal system may not adapt to loads in the same way. For example, repetitive lifting of heavy loads probably does increase muscle capacity, but probably does not increase the capacity of the spinal discs to withstand mechanical loading. As a consequence, strength training could mislead individuals to believe they could safely lift greater loads and thus risk back problems. Jobs should, therefore, be so designed that most people are able to carry them out, rather than only a few strong individuals.

The principle of ergonomics

A risk for disorders of the musculoskeletal system appears if the load and the functional capacity of the working person are not balanced. The basic principle of ergonomics is to create an appropriate balance between the requirements of the work and the capacity of the working person, by either adapting the work to the person by design of the respective work, or by developing the stable objects is indispensable. Only sufficiently stable and steady scaffolds should be used and these should be fixed to the building. Furthermore, securing the position of the worker by roping to the climbing aid (ladder; scaffold) or building is an important measure for preventing plunges.

The primary aim should be the adaptation of working conditions to the capacity of the working persons, whereby it is of particular importance to consider the dependency of individual capacity on age and gender. The adaptation of human beings to work should not be preferred, as here the work cannot be performed by all persons, but only by selected and specifically trained groups. Additionally, the possibility of developing human abilities during work execution should not be misused as a pretence for maintaining poorly designed conditions of work or work environment.

Work performance strategies

A risk factor for overloading the musculoskeletal system results from the method of performing the work by the worker. There are risky and less risky strategies to execute the task. An example is lifting heavy objects having the centre of gravity near the body. To fulfil this demand, heavy objects should be lifted, whenever possible, by bending the knees instead of bending the back. Further measures to reduce overloading risk are avoiding twisted and laterally bent postures, working continuously at a moderate pace and not during short time-periods with high time-pressure. The worker must be informed about those possibilities and should be motivated to use them.

Avoid accidents and injuries

The avoidance of accidents is another important field for the prevention of musculoskeletal disorders. Hazardous situations, in particular for plunges, can occur during work at greater height, for example on a ladder, a scaffold or a construction site. The risk for plunges can be reduced by securing the standing position and by stabilizing the equipment on which the worker climbs. In particular, the use of steady ladders and fixing the ladders to the floor or on stable objects is indispensable. Only sufficiently stable and steady scaffolds should be used and these should be fixed to the building. Furthermore, securing the position of the worker by roping to the climbing aid (ladder; scaffold) or building is an important measure for preventing plunges.

Protecting Workers' Health Series No. 5
Preventing musculoskeletal disorders in the workplace:
1,2,3 | Next page

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