John D. and
Catherine T. MacArthur
Research Network on Socioeconomic Status and Health
| MUSCLE TENSION
Summary prepared by Ulf Lundberg, Department of Psychology, Stockholm University for the Allostatic Load notebook. Last revised September, 2003. Chapter Contents:
Although people don't die from pain in the neck, shoulders and lower back, the 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 1999, about a third of all workers suffer from backache. The number of musculoskeletal disorders in the US increased fourfold between 1987 and 1992 and, today, the costs are estimated to exceed 65 billion dollars annually. A recent investigation in the Nordic countries shows that the costs associated with musculoskeletal diseases (absenteeism, early retirement, medical treatment and rehabilitation) account for between three and five percent of the gross national product. About 30 percent 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 of the work environment during the last 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 in light assembly work in the electronic industry. 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 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. 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 work place 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). Like many other important health problems today, musculoskeletal disorders seem to have a multifactorial etiology, where psychosocial factors and the physical environment interact with individual characteristics, behavior and life style. So 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 a pathogenetic mechanism resulting in muscle pain and thus form a link to musculoskeletal disorders(Sjøgaard et al., 2000; Forde et al., 2002). Several studies support this assumption (Svebak et al., 1993; Tulen et al., 1989; Lundberg et al., 1994; Wærsted, 1997; Johansson, 1994), but the experimental evidence are not conclusive (Wærsted, Bjorklund & Westgaard, 1991; Weber et al., 1980; Westgaard & Bjorklund, 1987). 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 & Sojka, 1991; Knardahl, 2002). The model proposed by Hägg (1991),"The Cinderella Hypothesis" (referring to Cinderella who was first to raise and last to go to bed), is based on earlier findings by Henneman et al. (1965), who showed that the motor units of a given muscle are recruited in a fixed order. Small, low threshold motor units are recruited at low levels of contraction, before larger ones, and kept activated until complete relaxation of the muscle. Long-lasting activation of these units may cause degenerative processes, damage and pain (Wærsted, 1997; Kadefors, 1995). Recent 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 10 min exposure to cognitive demands in the laboratory. A recent 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 motor units may be active also 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, there is likely to be an upper limit for continuous activation (Wærsted, 1997). 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 catecholamines. Johansson and Sojka (1991) have suggested that vicious circles may start in muscle spindles during stress and repetitive work, which may contribute to elevated muscle stiffness, dysfunctional coordination and a high concentration of inflammatory substances and increased pain sensitivity. 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). 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 relation 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 fiber 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. Recently, an ambulatory device for recording and analysing EMG signals has been developed (Kadefors et al., 1992). This device allows calculation of RMS and mean power frequency at 1 sec intervals and the data is stored on a memory card for off-line analysis. This 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 work place, 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, it may not be possible to perform MVC (e.g., facial muscles). 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. Recently, new techniques based on fine wire electrodes (Kadefors et al., 1995; Wærsted, 1997) have made it possible to identify and record from single motor units also during changes in arm and shoulder position. The correlation between surface and intramuscular recordings is high (Öberg et al., 1992). Empirical studies relevant for allostatic load A series of real life studies (Lundberg et al., 1989, 1997; Frankenhaeuser et al., 1989; Melin & Lundberg, 1997) 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 it may also induce health problems independently of the biomechanical load or, maybe, 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 et al., 1997). In an interesting prospective study, Veierstedt et al. (1993) measured EMG activity during work and involuntary breaks at work every 10th week from start of employment in 21 female packing workers (mean age = 25 years). 13 women contracted clinically diagnosed trapezius myalgia within the first year, half of them already after six months. EMG data shows 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 from 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. (2002) 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. (2002) found that cashiers reporting distress at work had elevated EMG activity compared to cashiers reporting more positive reactions at work. 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 was also found in a more recent experiment with both male and female participants (Lundberg, Forsman et al., 2002) as illustrated in Fig. 1.
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 recent real-life study of 72 female cashiers working in the 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 (Fig. 2).
Figure 2. Psychophysiological responses of female cashiers with and without trapezius muscle pain during 2-hour periods of rest and work at the work place and at home after work. The upper diagram of Fig. 3 (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 2-hour work period. In contrast, the lower diagram shows the corresponding data for an office worker. The EMG activity and the rest periods vary considerably during the work period with no particular time pattern.
Figure 3. Shows mean averaged rectified EMG value (mean ARV) and the percent of time when ARV is below the rest threshold (RestTime) for successive 6 min periods during 2-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 by the additional
muscular tension caused by psychological factors and by unfavorable work-rest patterns.
Psychosocial and psychological factors may prevent the individual from deactivating
physiological systems after an acute stress period. Muscle pain associated to
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 the absolute level of contraction
or the frequency of muscular activation. Although there is a relatively consistent pattern of findings indicating an important role of psychosocial factors and muscle tension in the development of musculoskeletal disorders, 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, recently, 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, for example, possible that after sustained activation, a low threshold motor unit is derecruited and substituted by another unit, 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 10 min stress period used by Wærsted et al. (1996).With regard to the histological changes of the muscle fibers indicating degenerative processes, an increased number of "ragged red fibers" 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 behavior has not only been related to cardiovascular illness but also to musculoskeletal disorders (Flodmark & Aase, 1992 ). Cognitive factors, such as classical conditioning, may be of importance for the development of symptoms. Melin et al. (1996) have shown that peripherical 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, to food consumed at the clinic or to 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 work place 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, 199 4) and intervention programs (Lagerström & Hagberg, 1997) have not reduced symptoms markedly. Only workers who move to a different work place or job seem to be able to significantly improve their musculoskeletal health (Ekberg et al., 1995).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 if "responders" are more likely to develop myalgia problems than "non-responders". 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. Research needs In conclusion, priority should be given to the following research questions:
Psychological and
psychosocial factors Due to the cross-sectional nature of most epidemiological studies, a reliable causal relation 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, are suffering from a "double burden" (Johansson Hanse, 1997). Psychological and psychosocial factors, muscle pain and illness behavior may form a viscious circle with increasing sensitization and pain symptoms. The individual may adapt to the sick-role and, eventually, will become chronically ill. Possible neural mechanisms behind these feed-back 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 usually are lacking (Tellnes, 1989). As a consequence, patients with pain syndromes may feel that their condition is not taken seriously by the medical professionals, or by other people in their environment, and not accepted as a somatic disease. They may get the impression that their symptoms are attributed to psychic disturbances "only" 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 HPA axis (Henry, 1992; Björntorp, 1997). Negative psychosocial conditions may also affect various life style factors and health related behaviors 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. 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 three factors: (1) the level of work load, (2) the repetitiveness of the work cycles and (3) the duration of the work (Winkel and Westgaard, 1992). Any factor that provokes sustained muscle activity may increase the risk for contracting work-related muscle pain. Thus, it is relevant to explore to what extent mental aspects of a work task contribute to sustained muscle activity, in sedentary work with low physical demands as well as in physically more demanding jobs. 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