MacArthur SES & Health Network
MacArthur SES & Health Network


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Muscle Tension

Summary prepared by Ulf Lundberg, Department of Psychology, Stockholm University, for the Allostatic Load notebook. Last revised October, 2008.

Chapter Contents

  1. Background
  2. Multifactorial Aetiology
  3. Muscular Tension
  4. Empirical Studies Relevant to Allostatic Load
  5. Gender Differences
  6. Gaps of Knowledge
  7. Research Needs
  8. Psychological and Psychosocial Factors
  9. Conclusions
  10. References

Background

Although people do not die from pain in the neck, shoulders and lower back, musculoskeletal disorders comprise one of the most common and costly health problems in Europe and North America today. According to a survey conducted by the European Commission in 2006, about a third of all workers suffer from backache and muscle pain. The figures are highest in the new European Union member states, such as the Baltic states and countries that earlier belonged to the former East European block, which have undergone rapid and extensive social, economic and political changes. The number of musculoskeletal disorders in the US increased fourfold between 1987 and 1992 and, today, total costs (absenteeism, early retirement, medical treatment and rehabilitation) are estimated to exceed 3% of the gross national product in Europe, North America, Australia and New Zeeland. For example, in Canada costs were estimated at 3.4% of the gross domestic product in 1998 (Coyte et al., 1998). About 30% of these costs can be attributed to work-related factors.

Musculoskeletal disorders differ from many other major health problems, such as cardiovascular disease and cancer, in that symptoms often appear very early in life and after a relatively short exposure to adverse environmental conditions. In repetitive work, pain syndromes are often reported already after 6-12 months on the job (Veiersted et al., 1993).

Despite considerable ergonomic improvements and less heavy lifting in the work environment during recent decades, the number of musculoskeletal disorders has not decreased. Pain syndromes are frequent, not only in physically demanding jobs, but also in work at video display terminals and call centres as well as in light assembly work in the electronic industry, jobs in which workers use only a low percentage of their muscular strength. It is also a great problem among cashiers working at supermarkets. Contrary to expectations, variations in physical capacity, such as muscular strength, aerobic fitness and flexibility, do not seem to predict future musculoskeletal disorders (Battié, 1989; Bigos et al., 1991; Jonsson et al., 1988; Wiker et al., 1990; Veierstedt et al., 1993).

Attempts have been made to establish threshold limit values for static load, e.g., thresholds of 2-5% of maximal voluntary contraction (MVC) have been suggested for the shoulder muscles (Jonsson, 1982). However, Westgaard (1988) has concluded that no safe lower limit may exist. Even very low levels of activation may contribute to the development of chronic pain syndromes.

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Multifactorial Aetiology

It is well documented that physically monotonous or repetitive work is associated with an increase in shoulder/neck pain, but more recent studies also report an association between psychosocial factors at the workplace and musculoskeletal disorders. Time pressure, lack of influence over one's work and constant involvement in repetitive tasks of short duration often characterize jobs associated with a high risk for muscular problems (Bammer, 1990; Bongers et al., 1993; Haldeman, 1991; Johansson, 1994; Moon & Sauter, 1996; Tellnes, 1989; Lundberg, Elfsberg Dohns et al., 1999; Waddell & Burton, 2001).

Like many other important health problems today, musculoskeletal disorders seem to have a multifactorial aetiology, whereby psychosocial factors and the physical environment interact with individual characteristics, behaviour and lifestyle. Thus far it has been difficult to explain the high incidence of musculoskeletal disorders in light physical work. However, it has been suggested that sustained low-level muscle activity may initiate pathogenetic mechanisms resulting in muscle pain and thus form a link to musculoskeletal disorders (Sjøgaard et al., 2000; Forde et al., 2002; Johansson et al., 2003). Several studies support this assumption (Svebak et al., 1993; Tulen et al., 1989; Lundberg et al., 2002; Wærsted, 1997; Johansson, 1994), but the experimental evidence is not conclusive (Wærsted, Bjorklund & Westgaard, 1991; Weber et al., 1980; Westgaard & Bjorklund, 1987).

Muscular Tension

Psychobiological Mechanisms

Complementing traditional theories, which explain musculoskeletal disorders in terms of insufficient blood circulation due to the high intramuscular pressure during contraction (e.g., Maeda, 1977), new theories have been proposed to explain the development of musculoskeletal disorder symptoms in psychologically stressful jobs with a moderate or low physical load (Hägg, 1991; Schleifer & Ley, 1994; Johansson et al., 2003; Knardahl, 2002). See also reviews by Visser & van Dieën (2006) and Forsman & Thorn (2007).

The model proposed by Hägg (1991), "The Cinderella Hypothesis" (referring to Cinderella, who was first to rise and last to go to bed), is based on earlier findings by Henneman et al. (1965), who showed that the motor units in the trapezius muscle are recruited in a fixed order. Small, low-threshold motor units are recruited at low levels of contraction, before larger ones, and are kept activated until complete relaxation of the muscle. Long-lasting activation of these units may cause degenerative processes, damage and pain (Wærsted, 1997; Kadefors et al., 1995; Sjøgaard et al., 2000). Findings from laboratory experiments (e.g., Wærsted et al., 1991, 1996; Lundberg et al., 1994; Lundberg, Forsman et al., 2002; Larsson et al., 1995) show that not only physical demands but also cognitive factors and mental stress may induce muscle tension. This means that ongoing psychological stress may keep low-threshold motor units active more or less continuously. Wærsted et al. (1996) demonstrated continuous activation of low-threshold motor units during a ten-minute exposure to cognitive demands in the laboratory. An experiment (Lundberg, Forsman et al., 2002) using intra-muscular recordings (see below) has demonstrated that the same motor units can be activated by mental as by physical stress. This means that the same muscle fibres may also be active during breaks at work and after work, unless the individual is able to relax mentally. Although these small motor units are assumed to be fatigue-resistant, it is likely that there is an upper limit for continuous activation (Wærsted, 1997).

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According to another theory, proposed by Schleifer (Schleifer & Ley, 1994), stress-induced hyperventilation decreases peak CO2 levels and increases the blood pH level (beyond 7.45 = alkalosis). This contributes to elevated muscular tension and a suppression of parasympathetic activity. The sympathetic dominance may amplify the responses to stress hormones.

Johansson et al. (2003) have suggested that vicious circles may start in muscle spindles during stress and repetitive work, which may contribute to elevated muscle stiffness and dysfunctional coordination, including co-contractions, and a high concentration of inflammatory substances and increased pain sensitivity. The pathological processes may spread from one muscle to another via nerve signals.

Knardahl (2002) has proposed a model according to which "pain originates from the vessel-nerve interactions of the connective tissue of the muscle, rather than from energy crisis of the muscle cells" (p. 68). Different vessel-nociceptor mechanisms known to cause pain, e.g., in migraine, may be involved, such as vasodilation stretching the vessel wall, release of algogenic substances from the nerves and/or the vessels, such as prostaglandins, and inflammatory processes which may sensitize nociceptors.

A possible pathogenic mechanism for muscle pain is that nociceptors are sensitized due to local metabolic changes in fatigued low-threshold (Type I) muscle fibres (Sejersted & Vollestad, 1993). The hypothesis of overload of certain motor units is supported by the observation of an increased number of "ragged red" Type I muscle fibres in the trapezius muscle of workers exposed to monotonous shoulder load (Larsson et al., 1988; Bengtsson and Henriksson, 1989; Lindman et al., 1991; Larsson et al., 1992). If motor units are constantly active, there is no time for the healing of damaged muscle fibres. Another factor of importance in light physical work, for example at the computer, is that there are no adequate signals of fatigue. In contrast to heavy physical work, this means that the worker can continue to work for hours or days without knowing that certain motor units are exhausted.

Measurements of Acute and Chronic Effects

Muscle tension can be measured by the electromyographic (EMG) signal reflecting motor activity. Under static conditions, there is a monotonous relationship between muscle contraction level and the amount of EMG produced by the muscle. EMG acquisition and analysis may relate to both acute and chronic effects. The root mean square (RMS) value of the signal reflects the momentary degree of involvement of the muscle, whereas the spectral composition reflects localized muscle fatigue (Chaffin, 1973). Under prolonged static contraction (Kadefors et al., 1968), there is a general increase in EMG amplitude and a shift in the frequency spectrum from high to low frequencies, due to a decrease in the propagation velocity of the depolarization wave along the muscle fibre as fatigue sets in (Lindström & Petersén, 1983). In fact, the decrease in mean frequency of the spectrum can be plotted as a function of time and the slope of the regression line provides a measure of the rate of fatigue (Lindström et al., 1977). The mean frequency measure may thus serve as an objective indicator of muscle fatigue.

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Today, ambulatory devices for recording and analysing EMG signals are available, which allows the calculation of RMS and mean power frequency at one-second intervals, and the data can be stored on memory cards for off-line analysis. Such a device was first used in a real-life study of female cashiers working at the supermarket (Lundberg et al., 1999).

Surface and Intramuscular Recordings

In field studies, for example at the workplace, recording by means of surface electrodes is often the only choice for practical and ethical reasons. Due to variations in skin resistance, the distance from the surface to the underlying muscles, etc., an absolute EMG measure is not a meaningful value. The bipolar surface EMG electrodes reflect a relatively large volume of the superficial muscle, and the recording has to be standardized against a reference level, such as the maximal voluntary contraction (MVC) or a submaximal reference contraction (RC) comparable between subjects. Thus, a common way to express EMG activity is in terms of percent of MVC or RC. In order to determine MVC, it is important to motivate all subjects to perform a maximum contraction. For certain muscles (e.g., facial muscles), it may not be possible to perform MVC. Fortunately for case-control comparisons, pain syndromes do not seem to influence the MVC value (Roe et al., 1997).

Intramuscular EMG recordings can be performed in the laboratory and reflect the electrical activity in a relatively small volume around the tip of the needle or wire electrode, compared to the surface electrodes. Intramuscular EMG recordings make it possible to measure the activity of muscles located deeper under the skin. New techniques based on fine wire electrodes (Kadefors et al., 1992; Wærsted, 1997) have made it possible to identify and record from single motor units during changes in arm and shoulder position also. The correlation between surface and intramuscular recordings is high (Öberg et al., 1992).

Empirical Studies Relevant to Allostatic Load

A series of real-life studies (review by Lundberg & Johansson, 2000) shows that epinephrine output at work is significantly elevated among both blue and white-collar workers, whereas norepinephrine levels are elevated in blue-collar workers only. This is likely to reflect the fact that epinephrine is influenced mainly by mental stress and norepinephrine by physical demands. In conclusion, blue-collar workers are exposed to a greater total load from mental and physical demands. The influence of the greater mental and physical demands on muscle tension may be related to the higher prevalence of musculoskeletal disorders among blue-collar workers. It is possible that a negative psychosocial work environment not only has additive effects on the musculoskeletal problems caused by physical conditions, but that it may also induce health problems independent of the biomechanical load or, perhaps, even enhance the effects of exposure to adverse physical conditions. Repetitive and monotonous blue-collar jobs are also associated with a slower physiological unwinding after work (Freude et al., 1995; Johansson & Aronsson, 1984; Johansson et al., 1978; Lundberg et al., 1993; Melin & Lundberg, 1997).

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In an interesting prospective study, Veierstedt et al. (1993) measured EMG activity during work and involuntary breaks at work every tenth week from start of employment in 21 female packing workers (mean age = 25 years). Thirteen women contracted clinically diagnosed trapezius myalgia within the first year, half of them already after six months. EMG data show that women developing trapezius myalgia problems had higher muscle activity during breaks at work but not during actual work. It was concluded that "Sustained low-level muscle activity seems to be a risk factor for muscular pain" (Veiersted, 1993, p. 18). Additional information may be obtained through analysis of the short periods of very low muscular electrical activity, i.e., "EMG-gaps." Female workers with a high frequency of EMG gaps seem to have a reduced risk of developing myalgia problems compared to workers with fewer gaps (Veiersted, 1995). Similar findings have been reported by Hägg and Åström (1994), and Sandsjö et al. (2000) found that female cashiers suffering from trapezius myalgia had much less muscle rest during work, compared to cashiers without neck and shoulder pain. In addition, Rissén et al. (2000) found that cashiers reporting distress at work had elevated EMG activity compared to cashiers reporting more positive reactions at work.

There is an ongoing debate as to whether muscle activity is a causal factor in stress-induced muscle pain. Low-level muscle activity is suggested to be an aetiological factor in pain development (Hägg, 1991; Sjøgaard et al., 2000), but it may also be argued that muscle activity is unlikely to be a necessary factor in all forms of muscle pain development (Westgaard, 1999). It is therefore of interest that 24-hour ambulatory recordings show that subjects who are working, but report shoulder and neck pain of at least moderate severity during the past six months, have significantly higher surface EMG activity levels in trapezius during sleep than do subjects who report that they are pain-free (Mork & Westgaard, 2004). This result may be interpreted in support of muscle activity as an aetiological factor in pain development. With regard to the role of sleep as a causal factor in the development of muscle pain, a recent longitudinal study by Canivet et al. (2008) shows that sleeping problems predict musculoskeletal pain in women and men at one-year follow-up after possible confounders are controlled for.

In a laboratory experiment aimed at examining the influence of psychological stress, as well as of standardized physical loads, on muscular tension and psychophysiological stress responses of 62 women (Lundberg et al., 1994), it was found that the stress tests induced a significant increase, not only in perceived stress and blood pressure, but also in the muscular tension (trapezius EMG activity) of the women. EMG activity was found to be correlated with systolic and diastolic blood pressure and norepinephrine levels. A significant increase in EMG activity was also found when one stress test (the Stroop Color Word Test) was combined with the standardized physical load (test contraction). However, it is of particular interest that this increase was significantly larger than that induced by the same stressor separately, which suggests an enhanced effect of psychological stress in, e.g., manual jobs. Significant effects of mental stress on trapezius muscle activity were also found in a more recent experiment with both male and female participants (Lundberg, Forsman et al., 2002), as illustrated in Fig. 1.

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Figure 1 Mean trapezius muscle activity measured by surface EMG

Studies of young male assembly-line workers at a car engine factory with a high prevalence of back pain problems show that psychophysiological arousal was high (Lundberg et al., 1989; Melin et al., 1999), whereas physical load was low (Magnusson et al., 1990).

In a real-life study of 72 female cashiers working at a supermarket (mean age 37 years), it was found that only 22 of the 72 women were free of neck/shoulder symptoms (Lundberg et al., 1999). Psychophysiological arousal measured by self-reports, blood pressure, heart rate, urinary catecholamines and EMG activity of the trapezius muscle increased significantly during work. The increase in EMG activity was most pronounced among cashiers with neck and shoulder symptoms, who also reported more tension during and after work and had higher systolic and diastolic blood pressure.

The upper diagram of Fig. 2 (see below) illustrates a typical EMG pattern from an individual cashier, with a successive increase in EMG activity and a decrease in the amount of rest in the trapezius muscle during the two-hour work period. In contrast, the lower diagram shows the corresponding data for an office worker. The EMG activity and rest periods vary considerably during the work period, with no particular time pattern.

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Figure 2 Shows mean averaged rectified EMG value

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Figure 2. Shows mean averaged rectified EMG value (mean ARV) and the percent of time when ARV is below the rest threshold (RestTime) for successive six-minute periods during two hours of work in a female cashier (upper diagram) and an office worker (lower diagram).

An important implication of these findings is that stressful jobs may induce elevated risk for musculoskeletal disorders through the additional muscular tension caused by psychological factors and by unfavourable work-rest patterns. Psychosocial and psychological factors may prevent the individual from deactivating physiological systems after an acute stress period. Muscle pain associated with psychological factors in the workplace could be explained by a blocking of pauses in muscle activity unrelated to the actual biomechanical work being performed (Elert et al., 1992). This will reduce restitution and contribute to a sustained activity in low-threshold motor units. In the modern work environment, it is possible that lack of relaxation is an even more important health problem than is the absolute level of contraction or the frequency of muscular activation. As mental stress, such as anxiety and distress, is often more long-lasting than physical stress, psychosocial stress is likely to play an important role in neck and shoulder pain for those performing light physical work by keeping low-threshold motor units active for long periods of time.

Gender Differences

Women generally report more MSD than men do, even in the same type of job (Hooftman et al., 2005; Karlqvist et al., 2002; Leijon et al., 2005). With regard to fibromyalgia, this diagnosis is seven times more common among women than men (Goldenberg, 1999).

There are probably several possible explanations for women's higher frequency of reported MSD. One could be the fact that women more often than men perform repetitive tasks, which pose a higher risk of MSD. It could also be that the work environment is less suitable to women than to men, for example, that chairs and tables are too high for women who are shorter, that tools are too large for women's hands, or that the equipment is too heavy. Another possible explanation is that women have fewer opportunities to unwind and relax in non-work conditions, due to a greater unpaid workload (household, children) or emotionally demanding jobs with children or chronically ill, elderly and psychiatric patients.

Gender differences in behaviour and perception can also be of importance. For example, women may be more likely to report symptoms and seek medical treatment and are known to have lower pain thresholds (Hellström & Lundberg, 2000). The gender differences in MSD are particularly pronounced among white-collar workers, and since these workers usually have very low physical demands at work, this difference cannot be explained by sex differences in physical strength.

In a laboratory study involving both men and women (Krantz et al., 2004), it was found that mental stress induced a significant increase in EMG activity in the trapezius, as well as increased blood pressure and heart rate. However, an interesting sex difference appeared. In women, increases in systolic (r=0.67) and diastolic (r=0.60) blood pressure and heart rate (r=0.64) were significantly correlated to EMG activity, but this was not the case in men (r=-0.08, 0.04 and 0.11, respectively). Although this experiment was based on a small sample, it indicates a stronger relationship between stress and muscle tension in women than in men and may be relevant to the sex differences in MSD.

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Gaps of Knowledge

Although there is a relatively consistent pattern of findings indicating an important role of psychosocial factors and muscle tension in the development of MSD, there is still a series of questions to be addressed in order to learn more about the psychobiological mechanisms involved.

The Cinderella Hypothesis was first based on findings from a static muscle (Henneman et al., 1962), but Kadefors et al. (1995) have been able to demonstrate that the same low-threshold motor units are active over a wide range of shoulder and arm movements. Furthermore, it has been shown that the same motor units can be activated by mental and physical stress. However, it has to be tested whether the low-threshold motor units remain active long enough to cause metabolic disturbances. It is possible, for example, that after sustained activation a low-threshold motor unit is derecruited and substituted by another unit that was not previously active and, thus, that the sustained muscle tension is produced by successive motor unit rotation (Westgaard & De Luca, 1997). However, there were no such indications during the ten-minute stress period used by Wærsted et al. (1996).

With regard to the histological changes of the muscle fibres indicating degenerative processes, an increased number of "ragged red fibres" have been reported in myalgic muscles (Larsson et al., 1988). However, this is not conclusive evidence since such changes have also been reported in normal muscles (Lindman, 1992).

Great differences in muscle pain symptoms exist between individuals within the same work environment. Possible explanations for this could be differences in age, gender and certain personality characteristics. For example, beliefs regarding internal versus external locus of control may influence the attribution of health problems to various environmental factors. Type A behaviour has been related to not only cardiovascular illness but also musculoskeletal disorders (Flodmark & Aase, 1992).

Cognitive factors, such as classical conditioning, may be of importance in the development of symptoms. Melin et al. (1996) have shown that peripheral blood flow can be conditioned to visual stimuli, and it is known that side effects of chemotherapy, such as nausea and vomiting, may become conditioned to the hospital building, food consumed at the clinic or the nurse providing the medication (Hursti et al., 1994). Conditioned or anticipatory influences have also been reported on the immune system (Bovbjerg, 1990; Lekander et al., 1995). With regard to musculoskeletal disorders, it is possible that muscle tension and symptoms become conditioned to the workplace as such. Symptoms may then be elicited by visual and auditory stimuli in the work environment and, consequently, ergonomic and organizational improvements at work may not be enough to extinguish the conditioned muscle response. This could explain why some large-scale rehabilitation (Ekberg, 1994) and intervention (Lagerström & Hagberg, 1997) programs have not reduced symptoms markedly. Only workers who move to a different workplace or job seem to be able to significantly improve their musculoskeletal health (Ekberg, 1994).

Cognitive demands may significantly increase trapezius muscle tension (e.g., Wærsted, 1997), but considerable interindividual differences have been found. Subgroups of "responders" and "nonresponders" in terms of EMG activity have been identified (Wærsted et al., 1991), but it is not known whether "responders" are more likely to develop myalgia problems than "non-responders" are. Kirschbaum et al. (1995) have found that some individuals do not habituate to stress but continue to increase their cortisol secretion when exposed to the same stressor repeatedly. The reason for these interindividual differences is not known.

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Research Needs

In conclusion, priority should be given to the following research questions:

  • Do mental and physical demands have synergistic or additive effects on muscle tension or can physical demands under certain conditions mask the influence of, for example, cognitive factors?
  • What is the role of frequent or intense muscle activation versus lack of relaxation ("EMG gaps") in the risk of developing muscular disorders?
  • How can interindividual differences in "vulnerability" be explained? Is susceptibility to musculoskeletal disorders associated with differences in personality, coping behaviour or biochemical parameters?
  • Can muscle tension be conditioned to environmental stimuli, for example, at the workplace?
  • Is muscle tension part of a generalized psychophysiological stress response or specifically related to certain environmental conditions?

Psychological and Psychosocial Factors

Due to the cross-sectional nature of most epidemiological studies, a reliable causal relationship cannot be demonstrated between psychosocial stress exposure at work and musculoskeletal disorders. Nevertheless, the association means that workers with muscle pain also report a stressful work situation, i.e., many workers, particularly in low status jobs, suffer from a "double burden" (Johansson Hanse, 1997). Psychological and psychosocial factors, muscle pain and illness behaviour may form a vicious circle, with increasing sensitization and pain symptoms. The individual may adapt to the sick role and, eventually, become chronically ill. Possible neural mechanisms behind these feedback loops have been proposed (Ursin, 1997).

Musculoskeletal disorders are often diagnosed on the basis of subjective statements from the patients, as conclusive clinical and laboratory findings explaining their symptoms are usually lacking (Tellnes, 1989). As a consequence, patients with pain syndromes may feel that their condition is not taken seriously by medical professionals or other people in their environment, and is not accepted as a somatic disease. They may get the impression that their symptoms are attributed to "only" psychic disturbances and that they are more or less "imagining" being in pain. As, in addition, medical treatment and rehabilitation of these patients has not been very successful, their life situation is characterized by chronic stress and may create feelings of lack of control, helplessness, distress and depression, i.e., a "defeat reaction" with activation of the hypothalamic pituitary adrenocortical axis (Henry, 1992; Björntorp, 1997).

Negative psychosocial conditions may also affect various lifestyle factors and health-related behaviours contributing to musculoskeletal risk. Low socioeconomic status is also likely to reduce the individual's ability to cope with various environmental demands and his/her motivation to seek proper medical treatment and, thus, increase the risk that transient symptoms develop into chronic illness. The role of sleep in MSD has been emphasized above. However, sleep and pain may cause a vicious circle, with sleeping problems contributing to muscle pain and pain contributing to sleeping problems (Moffit et al., 1991; Ohayon, 2005; Zee & Turek, 2006).

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Conclusions

There is evidence that both the temporal pattern and the level of upper trapezius activity are relevant risk indicators for neck/shoulder disorders. Quantification should therefore focus on four factors: (1) the level of work load, (2) the repetitiveness of the work cycles, (3) the duration of the work (Winkel and Westgaard, 1992) and (4) the possibilities for rest and recovery. Any factor that provokes sustained muscle activity may increase the risk of contracting work-related muscle pain. Thus, it is relevant to explore the extent to which mental aspects of a work task contribute to sustained muscle activity, in sedentary work with low physical demands as well as in more physically demanding jobs.

Several studies indicate that lack of relaxation during and after work may be of significant importance in the development of musculoskeletal disorders. Psychosocial and psychological factors are likely to prevent relaxation of the muscles when the acute exposure has terminated. This points to the significance of rest, recovery and restitution as the most important factors in preventing MSD in modern society.

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