Can stress-related shoulder and neck pain develop independently of muscle activity?

Frequency of breaks, amount of muscular rest, and sustained muscle activity related to neck pain in a pooled dataset

BACKGROUND

Neck pain remains a persistent challenge in modern society and is frequently encountered across a wide range of occupations, particularly those involving repetitive and monotonous tasks. It might be expected that patterns of trapezius muscle activity at work, characterized by few breaks and prolonged periods of sustained muscle activity, are linked to neck pain. However, previous cross-sectional studies have generally failed to establish a definitive association. While some longitudinal studies have suggested that extended periods of heightened muscle activity could be a risk factor for neck pain, these findings often relied on limited participant numbers or specific professional groups. This study aimed to investigate the relationship between trapezius muscle activity and neck pain by pooling data from seven Scandinavian research institutes encompassing a diverse range of occupational backgrounds.

METHODS

Electromyographic (EMG) data for the upper trapezius muscle, collected during working hours, were coupled with questionnaire responses pertaining to neck pain, individual characteristics, and potential confounding variables for a total of 731 subjects. Additionally, longitudinal data from 258 subjects were available. The various EMG datasets were consolidated into a standardized format, and efforts were made to harmonize inquiries about neck pain. Regression analyses, adjusting for sex and height, were conducted to explore the associations between muscle activity variables and neck pain. An exposure index was devised to quantify the cumulative neck load experienced during working hours and to differentiate between various occupational categories.

RESULTS

The cross-sectional data displayed a distinct pattern characterized by positive associations for brief periods of sustained muscle activity (SUMA) and negative associations for prolonged SUMA-periods and neck pain. The longitudinal data exhibited a contrasting trend, although it was not as pronounced as the cross-sectional findings. When employing the exposure index, notable differences in cumulative muscle load emerged among occupational groups, and positive associations with longitudinal neck pain were identified.

DISCUSSION

The results suggest that individuals with neck pain experience higher cumulative workloads and extended periods of muscle activity over the long term. In the short term, they appear to compensate by taking frequent short breaks, resulting in a lower cumulative workload. Regardless of their occupation, it is crucial to distribute work breaks throughout the workday to ensure that the cumulative load remains manageable.

Discomfort glare and psychological stress during computer work: subjective responses and associations between neck pain and trapezius muscle blood flow

PURPOSE

Exposure to additional environmental stress during computer work, such as visual and psychological demands, is associated with increased eye and neck discomfort, altered moods, and reduced well-being. The aim of this study is to elucidate further how subjective responses in healthy, young females with normal binocular vision are affected by glare and psychological stress during computer work, and to investigate possible associations between trapezius muscle blood flow and neck pain development.

METHODS

43 females participated in a laboratory experiment with a within-subject design. Four 10-min computer work conditions with exposure to different stressors were performed at an ergonomically optimal workstation, under the following series of conditions: no additional stress, visual stress (induced as direct glare from a large glare source), psychological stress, and combined visual and psychological stress. Before and immediately after each computer work condition, questionnaires regarding different visual and eye symptoms, neck and shoulder symptoms, positive and negative state moods, perceived task difficulty, and perceived ambient lighting were completed. Associations between neck pain and trapezius muscle blood flow were also investigated.

RESULTS

Exposure to direct glare induced greater development of visual/eye symptoms and discomfort, while psychological stress exposure made participants feel more negative and stressed. The perception of work lighting during glare exposure was closely related to perceived stress, and associations between visual discomfort and eyestrain, and neck pain were observed in all conditions. Furthermore, participants with high trapezius muscle blood flow overall reported more neck pain, independent of exposure.

CONCLUSIONS

Exposure to visual and psychological stresses during computer work affects the development of symptoms and negative moods in healthy, young females with normal binocular vision, but in different ways. The results also demonstrate the complex interactions involved in symptom development and lighting appraisal during computer work. When optimizing computer workstations, the complexity of the field must be taken into account, and several factors, including visual conditions, must be considered carefully.

The Effect of Functional Mandibular Shift on the Muscle Spindle Systems in Head-Neck Muscles and the Related Neurotransmitter Histamine

The aim of this study is to explore the effects of abnormal occlusion and functional recovery caused by functional mandible deviation on the head and neck muscles and muscle spindle sensory-motor system by electrophysiological response and endogenous monoamine neurotransmitters' distribution in the nucleus of the spinal tract. Seven-week-old male Wistar rats were randomly divided into 7 groups: normal control group, 2W experimental control group, 2W functional mandible deviation group, 2W functional mandible deviation recovery group, 4W experimental control group, 4W functional mandible deviation group, 4W functional mandible deviation recovery group. Chewing muscles, digastric muscle, splenius, and trapezius muscle spindles electrophysiological response activities at the opening and closing state were recorded. And then the chewing muscles, digastric, splenius, trapezius, and neck trigeminal nucleus were taken for histidine decarboxylase (HDC) detection by high performance liquid chromatography (HPLC), immunofluorescence, and reverse-transcription polymerase chain reaction (RT-PCR). Histamine receptor proteins in the neck nucleus of the spinal tract were also examined by immunofluorescence and RT-PCR. Electromyography activity of chewing muscles, digastric, and splenius muscle was significantly asymmetric; the abnormal muscle electromyography activity was mainly detected at the ipsilateral side. After functional mandibular deviation, muscle sensitivity on the ipsilateral sides of the chewing muscle and splenius decreased, muscle excitement weakened, modulation depth decreased, and the muscle spindle afferent impulses of excitation transmission speed slowed down. Changes for digastric muscle electrical activity were contrary. The functions recovered at different extents after removing the deflector. However, trapezius in all the experimental groups and recovery groups exhibited bilateral symmetry electrophysiological responses, and no significant difference compared with the control group. After functional mandibular deviation, HDC protein and messenger ribonucleic acid (mRNA) levels on the ipsilateral sides of the chewing muscle and splenius increased significantly. HDC level changes for digastric muscle were contrary. After the removal of the mandibular position deflector, HDC protein and mRNA levels decreased on the ipsilateral sides of the chewing muscle and splenius while they increased in the digastric muscle. The difference of histamine decarboxylase content in the bilateral trapezius in each experimental group was small. After functional mandibular deviation, the temporomandibular joint mechanical receptors not only caused the fusimotor fiber hypoallergenic fatigue slow response on the ipsilateral sides of splenius, but also increased the injury neurotransmitter histamine release. The authors' results further support the opinion that the temporomandibular joint receptors may be involved in the mechanical theory of the head and neck muscles nervous system regulation.

Changes in division of labour and tasks within public dentistry: relationship to employees work demands, health and work ability

OBJECTIVE

By 2023, fewer dentists are expected in Sweden, at the same time as the demand for dental care is expected to increase. Older people, in particular, are expected to require more dental health than previous generations. To meet this demand, the public sector dentistry in Sweden is moving towards changes in division of labour among dental professionals, including dentists, dental hygienists and dental nurses. However, the impact of this reallocation on the physical and psychosocial wellbeing of employees is unknown. The aim of this study was to compare workplaces with an equal or larger proportion of dental hygienists than dentists (HDH) with workplaces with a larger proportion of dentists than dental hygienists (HD) on the physical and psychosocial work load, musculoskeletal and psychosomatic disorders and sickness presence.

MATERIAL

A total of 298 persons employed in the Public Dental Service in a Swedish County Council participated in this study.

CONCLUSION

The medium large clinics HDH reported 85% of employee's with considerably more high psychosocial demands compared to employees in medium HD (53%) and large HD (57%). Employees in medium large clinics HDH also reported sleep problems due to work (25%) compared with employees in medium large clinics HD (6%), large clinics HD (11%) and small clinics HDH (3%). Clinic size does not seem to influence the outcome of the HD and HD clinics to any great extent. Of all employees, about 94-100% reported high precision demands and 78-91% poor work postures.

Intrinsic movement variability at work. How long is the path from motor control to design engineering?

For several years, increasing numbers of studies have highlighted the existence of movement variability. Before that, it was neglected in movement analysis and it is still almost completely ignored in workstation design. This article reviews motor control theories and factors influencing movement execution, and indicates how intrinsic movement variability is part of task completion. These background clarifications should help ergonomists and workstation designers to gain a better understanding of these concepts, which can then be used to improve design tools. We also question which techniques--kinematics, kinetics or muscular activity--and descriptors are most appropriate for describing intrinsic movement variability and for integration into design tools. By this way, simulations generated by designers for workstation design should be closer to the real movements performed by workers. This review emphasises the complexity of identifying, describing and processing intrinsic movement variability in occupational activities.

Shoulder muscle loading and task performance for overhead work on ladders versus Mobile Elevated Work Platforms

A high incidence of Musculoskeletal Disorders (MSDs) has been reported in the construction sector. The use of ladders in the workplace has long been identified as a significant risk that can lead to workplace accidents. However, it is unclear if platform types have an effect on the physical risk factors for MSDs in overhead work. The aim of this study is to perform a pilot study on the effects of hand activity on both shoulder muscle loading and task performance while working on ladders versus Mobile Elevated Working Platforms (MEWPs). It is hypothesised that work on ladders would result in greater muscle loading demands, increased levels of discomfort, and reduced performance due to the restrictions on postures that could be adopted. A field study (n = 19) of experienced electricians on a construction site found that workers spent approximately 28% of their working time on ladders versus 6% on MEWPs. However, the durations of individual tasks were higher on MEWPs (153 s) than on ladders (73 s). Additionally, maximum levels of perceived discomfort (on a VAS 0-100) were reported for the shoulders (27), neck (23), and lower regions of the body (22). A simulated study (n = 12) found that task performance and discomfort were not significantly different between platform types (ladder vs. MEWP) when completing either of three tasks: cabling, assembly and drilling. However, platform and task had significant effects (p < 0.05) on median electromyographic (EMG) activity of the anterior deltoid and upper trapezius. EMG amplitudes were higher for the deltoid than the upper trapezius. For the deltoid, the peak amplitudes were, on average, higher for ladder work over MEWP work for the hand intensive cabling (32 vs. 27% Maximal Voluntary Exertion (MVE)) and the assembly task (19 vs. 6% MVE). Conversely, for drilling, the peak EMG amplitudes were marginally lower for ladder compared to the MEWP (3.9 vs. 5.1% MVE). The general implication was that working on the MEWP involved lower shoulder muscle loading for cabling and assembly task. A difference due to platform type was not present for drilling work.

Comparative study of upper limb load assessment and occurrence of musculoskeletal disorders at repetitive task workstations

This study explored the relationship between subjectively assessed complaints of pain in the arm, forearm and hand, and musculoskeletal load caused by repetitive tasks. Workers (n=942) were divided into 22 subgroups, according to the type of their workstations. They answered questions on perceived musculoskeletal pain of upper limbs. Basic and aggregate indices from a questionnaire on the prevalence, intensity and frequency of pain were compared with an upper limb load indicator (repetitive task index, RTI) calculated with the recently developed Upper Limb Risk Assessment (ULRA). There was relatively strong correlation of RTI and general intensity and frequency of pain in the arm, and general intensity and frequency of pain in the arm and forearm or prevalence of pain in the arm. Frequency and intensity of pain in the arm were weakly correlated. An aggregate indicator of evaluation of MSDs, which was calculated on the basis of the prevalence, intensity and frequency of pain, was to a higher degree associated with the musculoskeletal load of a task than basic evaluative parameters. Thus, such an aggregate indicator can be an alternative in comparing subjectively assessed MSDs with task-related musculoskeletal load and in establishing limit levels for that load.

The influence of mental load on muscle tension

We examined the differences in muscle tension and in physiological measures depending on the type of mental task. Fifteen participants performed tests for sustained attention, vigilance and maintaining posture only. We analysed electromyogram (EMG) measures of extensor digitorum (ED), flexor carpi ulnaris (FU), deltoideus (DE) and trapezius (TR), and heart rate (HR) and respiratory frequency (RF). Measures indicated higher values for mental tasks than for maintained posture only with significant differences in all measures. The following relationships were also significant: between DE and physiological measures (HR and RF), between ED and the amplitude of EMG of the other three muscles, between FU and TR and between HR and RF. The results of this study showed that the relationship between mental demands and muscle tension was mostly reflected by tension in the arm and shoulder girdle muscles and, to a lesser degree, in forearm muscles.

PRACTITIONER SUMMARY

We focused on physiological and muscle tension measures differentiating work according to the level of mental demands. Differences in sustained attention, vigilance and maintaining posture only proved that mental demands were mostly reflected by tension in arm and shoulder girdle muscles and, to a lesser degree, in forearm muscles.

Evidence for repetitive load in the trapezius muscle during a tapping task

Many studies describe the trapezius muscle activation pattern during repetitive key-tapping focusing on continuous activation. The objectives of this study were to determine whether the upper trapezius is phasically active during supported key tapping, whether this activity is cross-correlated with forearm muscle activity, and whether trapezius activity depends on key characteristic. Thirteen subjects (29.7 ± 11.4 years) were tested. Surface EMG of the finger's extensor and flexor and of the trapezius muscles, as well as the key on-off signal was recorded while the subject performed a 2-min session of key tapping at 4 Hz. The linear envelopes obtained were cut into single tapping cycles extending from one onset to the next onset signal and subsequently time-normalized. Effect size between mean range and maximal standard deviation was calculated to determine as to whether a burst of trapezius muscle activation was present. Cross-correlation was used to determine the time-lag of the activity bursts between forearm and trapezius muscles. For each person the mean and standard deviation of the cross-correlations coefficient between forearm muscles and trapezius were determined. Results showed a burst of activation in the trapezius muscle during most of the tapping cycles. The calculated effect size was ≥0.5 in 67% of the cases. Cross-correlation factors between forearm and trapezius muscle activity were between 0.75 and 0.98 for both extensor and flexor muscles. The cross-correlated phasic trapezius activity did not depend on key characteristics. Trapezius muscle was dynamically active during key tapping; its activity was clearly correlated with forearm muscles' activity.

On functional motor adaptations: from the quantification of motor strategies to the prevention of musculoskeletal disorders in the neck-shoulder region

BACKGROUND

Occupations characterized by a static low load and by repetitive actions show a high prevalence of work-related musculoskeletal disorders (WMSD) in the neck-shoulder region. Moreover, muscle fatigue and discomfort are reported to play a relevant initiating role in WMSD.

AIMS

To investigate relationships between altered sensory information, i.e. localized muscle fatigue, discomfort and pain and their associations to changes in motor control patterns.

MATERIALS & METHODS

In total 101 subjects participated. Questionnaires, subjective assessments of perceived exertion and pain intensity as well as surface electromyography (SEMG), mechanomyography (MMG), force and kinematics recordings were performed.

RESULTS

Multi-channel SEMG and MMG revealed that the degree of heterogeneity of the trapezius muscle activation increased with fatigue. Further, the spatial organization of trapezius muscle activity changed in a dynamic manner during sustained contraction with acute experimental pain. A graduation of the motor changes in relation to the pain stage (acute, subchronic and chronic) and work experience were also found. The duration of the work task was shorter in presence of acute and chronic pain. Acute pain resulted in decreased activity of the painful muscle while in subchronic and chronic pain, a more static muscle activation was found. Posture and movement changed in the presence of neck-shoulder pain. Larger and smaller sizes of arm and trunk movement variability were respectively found in acute pain and subchronic/chronic pain. The size and structure of kinematics variability decreased also in the region of discomfort. Motor variability was higher in workers with high experience. Moreover, the pattern of activation of the upper trapezius muscle changed when receiving SEMG/MMG biofeedback during computer work.

DISCUSSION

SEMG and MMG changes underlie functional mechanisms for the maintenance of force during fatiguing contraction and acute pain that may lead to the widespread pain seen in WMSD. A lack of harmonious muscle recruitment/derecruitment may play a role in pain transition. Motor behavior changed in shoulder pain conditions underlining that motor variability may play a role in the WMSD development as corroborated by the changes in kinematics variability seen with discomfort. This prognostic hypothesis was further, supported by the increased motor variability among workers with high experience.

CONCLUSION

Quantitative assessments of the functional motor adaptations can be a way to benchmark the pain status and help to indentify signs indicating WMSD development. Motor variability is an important characteristic in ergonomic situations. Future studies will investigate the potential benefit of inducing motor variability in occupational settings.

Functional imaging of sympathetic activation during mental stress

Activation of the sympathetic nervous system (SNS) is essential in adapting to environmental stressors and in maintaining homeostasis. This reaction can also turn into maladaptation, associated with a wide spectrum of stress-related diseases. Up to now, the cortical mechanisms of sympathetic activation in acute mental stress have not been sufficiently characterized. We therefore investigated cerebral activation applying functional magnetic resonance imaging (fMRI) during performance of a mental stress task with graded levels of difficulty, i.e. four versions of a Stroop task (Colour Word Interference Test, CWT) in healthy subjects. To analyze stress-associated sympathetic activation, skin conductance and heart rate were continuously recorded. The results show that sympathetic activation through mental stress is associated with distinct cerebral regions being immediately involved in task performance (visual, motor, and premotor areas). Other activated regions (right insula, dorsolateral superior frontal gyrus, cerebellar regions) are unrelated to task performance. These latter regions have previously been considered to be involved in mediating different stress responses. The results might furthermore serve as a basis for future investigations of the connection between these cortical regions in the generation of stress-related diseases.

Patterns of sympathetic responses induced by different stress tasks

Stress tasks are used to induce sympathetic nervous system (SNS) arousal. However, the efficacy and the patterns of SNS activation have not been systematically compared between different tasks. Therefore, we analyzed SNS activation during the following stress tasks: Presentation of negative, positive, and - as a control - neutral affective pictures, Color-Word interference test (CWT), mental arithmetic under time limit, singing a song aloud, and giving a spontaneous talk. We examined 11 healthy subjects and recorded the following SNS parameters: Activation of emotional sweating by quantitative sudometry, skin vasoconstriction by laser-Doppler flowmetry, heart rate by ECG, blood pressure by determination of pulse wave transit time (PWTT), and electromyographic (EMG) activity of the trapezius muscle. Moreover, subjective stress ratings were acquired for each task using a visual analog scale. All tasks were felt significantly stressful when compared to viewing neutral pictures. However, SNS activation was not reliable: Affective pictures did not induce a significant SNS response; singing, giving a talk and mental arithmetic selectively increased heart rate and emotional sweating. Only the CWT globally activated the SNS. Regarding all tasks, induction of emotional sweating, increase of heart rate and blood pressure significantly correlated with subjective stress ratings, in contrast to EMG and skin vasoconstriction.Our results show that the activation of the SNS widely varies depending on the stress task. Different stress tasks differently activate the SNS, which is an important finding when considering sympathetic reactions - in clinical situations and in research.

A unilateral sympathetic blockade does not affect stress-related pain and muscle activity in patients with chronic musculoskeletal pain

OBJECTIVE

Chronic musculoskeletal pain is often exacerbated by mental and social stress. The association between stress and musculoskeletal pain is potentially mediated by peripheral sympathetic nerves, either directly or indirectly through muscle activity. In the present study we wanted to determine whether sympathetic blockade could affect either the pain or the muscular activity experienced during mental stress in patients with chronic musculoskeletal pain.

METHODS

We performed a unilateral anaesthetic blockade of the lower cervical sympathetic ganglion (ganglion stellatum) in 18 patients with chronic musculoskeletal pain (10 with fibromyalgia and eight with chronic shoulder/neck pain). After the blockade the patients performed a 60-minute stressful task with low-grade mental stress that has induced pain and muscle activity in earlier experiments. Surface electromyography (SEMG) of the forehead, temples, neck, and shoulders, and heart rate and blood pressure were recorded together with ratings of pain.

RESULTS

We did not find any side or sidextime effect for pain or muscular activity in any of the four muscle groups (p>0.12).

CONCLUSION

We investigated the potential involvement of peripheral sympathetic nerves in stress-related musculoskeletal pain. A peripheral sympathetic block did not affect pain and muscle responses to a stressful task. Other explanatory models should be implemented and tested experimentally to further investigate the clinical impression that mental stress exacerbates pain in patients with chronic musculoskeletal pain.

Similarities in stress physiology among patients with chronic pain and headache disorders: evidence for a common pathophysiological mechanism?

One common feature of chronic musculoskeletal pain and headaches are that they are both influenced by stress. Among these, tension-type headache (TTH), fibromyalgia (FMS) and chronic shoulder/neck pain (SNP) appear to have several similarities, both with regard to pathophysiology, clinical features and demographics. The main hypothesis of the present study was that patients with chronic pain (TTH, FMS and SNP) had stress-induced features distinguishing them from migraine patients and healthy controls. We measured pain, blood pressure, heart rate (HR) and skin blood flow (BF) during (1 h) and after (30 min) controlled low-grade cognitive stressor in 22 migraine patients, 18 TTH patients, 23 FMS patients, 29 SNP patients and 44 healthy controls. FMS patients had a lower early HR response to stress than migraine patients, but no differences were found among FMS, TTH and SNP patients. Finger skin BF decreased more in FMS patients compared to migraine patients, both during and after the test. When comparing chronic pain patients (chronic TTH, FMS and SNP) with those with episodic pain (episodic TTH and migraine patients) or little or no pain (healthy controls), different adaptation profiles were found during the test for systolic and diastolic blood pressure, HR and skin BF in the chronic group. In conclusion, these results suggest that TTH, FMS and SNP patients may share common pathophysiological mechanisms regarding the physiological responses to and recovery from low-grade cognitive stress, differentiating them from episodic pain conditions such as migraine

Neck movement and muscle activity characteristics in female office workers with neck pain

STUDY DESIGN

Cross-sectional study.

OBJECTIVE

To explore aspects of cervical musculoskeletal function in female office workers with neck pain.

SUMMARY OF BACKGROUND DATA

Evidence of physical characteristics that differentiate computer workers with and without neck pain is sparse. Patients with chronic neck pain demonstrate reduced motion and altered patterns of muscle control in the cervical flexor and upper trapezius (UT) muscles during specific tasks. Understanding cervical musculoskeletal function in office workers will better direct intervention and prevention strategies.

METHODS

Measures included neck range of motion; superficial neck flexor muscle activity during a clinical test, the craniocervical flexion test; and a motor task, a unilateral muscle coordination task, to assess the activity of both the anterior and posterior neck muscles. Office workers with and without neck pain were formed into 3 groups based on their scores on the Neck Disability Index. Nonworking women without neck pain formed the control group. Surface electromyographic activity was recorded bilaterally from the sternocleidomastoid, anterior scalene (AS), cervical extensor (CE) and UT muscles.

RESULTS

Workers with neck pain had reduced rotation range and increased activity of the superficial cervical flexors during the craniocervical flexion test. During the coordination task, workers with pain demonstrated greater activity in the CE muscles bilaterally. On completion of the task, the UT and dominant CE and AS muscles demonstrated an inability to relax in workers with pain. In general, there was a linear relationship between the workers' self-reported levels of pain and disability and the movement and muscle changes.

CONCLUSION

These results are consistent with those found in other cervical musculoskeletal disorders and may represent an altered muscle recruitment strategy to stabilize the head and neck. An exercise program including motor reeducation may assist in the management of neck pain in office workers.

Prognostic factors for chronic neck pain in persons with minor or moderate injuries in traffic accidents

Prognostic factors for chronic neck pain were identified in a prospective Norwegian insurance cohort comprising 636 persons with minor or moderate traffic injuries. One questionnaire was used at baseline and another at 3 years post-injury. Daily severe or very severe neck pain at three years follow up was defined as chronic neck pain. Eight significant independent prognostic factors for chronic neck pain were identified: (1) rear-end or frontal and rear-end collision, odds ratio (OR): 4.10 (95% confidence interval (CI): 1.72-10.82); (2) neck and/or shoulder pain before the accident, OR: 2.38 (95% CI: 1.07-5.37); (3) post-accident (a) memory and concentration problems, OR: 4.62 (95% CI: 1.99-11.20), (b) bodily tension, OR: 3.43 (95% CI: 1.49-8.42), (c) difficulties to climb stairs, OR: 5.03 (95% CI: 1.77-14.95), (d) difficulties to bend forward, OR: 4.85 (95% CI: 2.06-11.79), (e) difficulties to do heavy labour, OR: 3.70 (95% CI: 1.24-11.64); (4) beliefs in future work disability, OR: 2.64 (95% CI: 1.11-6.48). The results indicate that development of chronic neck pain is influenced by pre-accident neck and/or shoulder pain, the impact of the collision, as well as post-accident symptoms, perceived impaired function and pessimism for the future ability to work.

Low-amplitude trapezius activity in work and leisure and the relation to shoulder and neck pain

The aim of this study is to obtain evidence supporting or negating the hypothesis that muscle pain is associated with sustained activation of low-threshold motor units. Long-term surface electromyographic (EMG) recordings of trapezius activity pattern were related to subjectively reported shoulder and neck pain in work and leisure. Recordings from 118 female subjects (73 recorded both during work and leisure) were analyzed. Computer operators, secretaries, and health care and retail workers were represented in the material. The recordings were calibrated by the root-mean-square-detected response at maximal voluntary contraction (%maximum EMG). The analysis was performed by quantifying duration and amplitude of surface EMG activity exceeding 2% maximum EMG (“EMG bursts”). Three response categories were defined by duration of the burst periods during work: low- (<50%), intermediate- (50–70%), and high-response (>70%) groups. Shoulder and neck pain was assessed by hourly visual analog score throughout work and leisure and by pain score for the last 6 mo. Shoulder and neck pain was higher at work than leisure for subjects with long-term pain in both the high- and the low-response groups. Persistent pain, defined by the 6-mo score, was more prevalent in the high- than the low- and intermediate-response groups (73 vs. 37%); relative risk was 2.0. Trapezius activity was reduced from work to leisure for the high- but not the low-response group. The activity pattern is consistent with low-threshold motor unit overexertion for the high- but not the low-response group. We speculate that different mechanisms of muscle pain causation, dependent and independent of motor activity pattern, coexist.

A comparison of symptomatic and asymptomatic office workers performing monotonous keyboard work--2: neck and shoulder kinematics

Prolonged static posture has been identified as a major risk factor for work-related neck and upper limb disorders (WRNULD) in computer users. Previous research has mainly examined working postures in healthy pain-free individuals. The present study examined whether symptomatic subjects exhibited the same kinematic patterns as asymptomatic controls during a prolonged computer task. In a Case-Control comparison, female office workers performed the same computer task using the same adjustable computer workstation for 1h. Three-dimensional (3D) kinematics were measured in the head-neck, thorax and shoulder (upper arm) segments. Case Group subjects (n=21) displayed trends for increased head-neck flexion angles and greater ranges of movements than the Control Group (n=17). There were also small but significant differences between groups in side flexion and rotation angles of the head-neck region. The shoulder joints displayed significantly greater flexion and abduction angles on the right in both groups, although no group differences were observed. The increased neck flexion angles were associated with significantly higher activity in the upper trapezius muscle and with neck and shoulder discomfort. The individual differences in postural habits appeared to be independent of the physical environment. These results suggest motor control changes are associated with the presence of WRNULD.

Associação entre distúrbios do ombro e trabalho: breve revisão da literatura

Este artigo apresenta os resultados de uma breve revisão da literatura focalizada nos distúrbios do ombro. As publicações foram pesquisadas nos sites do Medline, Ovid, Lilacs e periódicos Capes, no período de janeiro de 1993 a julho de 2004, com as seguintes palavras-chave: shoulder pain, rotator cuff, job, dor no ombro, manguito rotador, trabalho. Foram selecionados os artigos de acordo com os seguintes critérios: estudo epidemiológico publicado em inglês, português ou espanhol, com informações sobre a carga física ou sobre os fatores psicossociais do trabalho, explicitação do método de avaliação da exposição, presença de dor no ombro ou tendinopatia do manguito rotador. Vinte artigos preencheram todos os critérios de inclusão. Encontrou-se associação positiva entre os distúrbios do ombro e o trabalho praticado pelos sujeitos dos estudos revisados. Restaram como limite da revisão: a heterogeneidade dos artigos quanto à avaliação da exposição, ao tipo de diagnóstico, à metodologia utilizada na condução da pesquisa, e aos meios para avaliação da dor. Os autores discutem a necessidade de aprofundamento dos métodos de investigação de problemas musculoesqueléticos relacionados ao trabalho.

Paraspinal muscles and intervertebral dysfunction: part two

BACKGROUND

One of the diagnostic characteristics of the manipulable spinal lesion--a musculoskeletal disturbance that is claimed to be detected with manual palpation and corrected with manipulation--is said to be altered segmental tissue texture. Little evidence for the nature of abnormal paraspinal tissue texture exists, but indirect evidence from experimental studies supports the plausibility of the concept of protective muscle spasm, although investigations of increased paraspinal electromyography (EMG) associated with low back pain suggests complex changes in motor control rather than simple protective reflexes.

OBJECTIVES

To review the literature for evidence that may support or refute proposed explanations for clinically observed altered paraspinal tissue texture associated with the manipulable spinal lesion. This review aims to highlight areas that require further research and make recommendations for future studies. Data Source MEDLINE and CINAHL databases were searched using various combinations of the keywords paraspinal, muscle, palpation, EMG, spine, low back pain, pain, myofascial, hardness, manipulation, reliability, and somatic dysfunction, along with searching the bibliographies of selected articles and textbooks. Data Extraction All relevant data were used.

RESULTS

Decreased paraspinal muscle activity and strength associated with low back pain is well established, and there is evidence of changes in muscle fiber composition and localized selective multifidus atrophy. Disturbances in microcirculation have been implicated in nonparaspinal muscle pain. The effect of spinal manipulation on paraspinal EMG activity is inconclusive but promising.

CONCLUSION

Little direct evidence exists to support the existence or nature of paraspinal tissue texture change that is claimed to be detected with palpation. The proposal of segmental reflex paraspinal muscle contraction was not supported, at least in association with low back pain. There appears to be a complex relationship between deep paraspinal muscle inhibition during dynamic activity and nonvoluntary guarding behavior during static activity. The relationship between these findings and palpable tissue change is speculative, but increased activity, decreased activity, or both may be responsible for paraspinal tissues detected as abnormal with palpation. Recommendations are outlined for future research.

Understanding work related musculoskeletal pain: does repetitive work cause stress symptoms?

BACKGROUND

Pain in the neck and upper extremity is reported with high frequency in repetitive work. Mechanical overload of soft tissues seems a plausible mechanism, but psychological factors have received considerable attention during the past decade. If psychological factors are important for development of regional pain in repetitive work, stress symptoms would likely be on the causal path.

AIMS

To examine whether objective measures of repetitive monotonous work are related to occurrence and development of stress symptoms.

METHODS

In 1994-95, 2033 unskilled workers with continuous repetitive work and 813 workers with varied work were enrolled. Measures of repetitiveness and force requirements were quantified using video observations to obtain individual exposure estimates. Stress symptoms were recorded at baseline and after approximately one, two, and three years by the Setterlind Stress Profile Inventory.

RESULTS

Repetitive work, task cycle time, and quantified measures of repetitive upper extremity movements including force requirements were not related to occurrence of stress symptoms at baseline or development of stress symptoms during three years of follow up.

CONCLUSIONS

The findings do not indicate that repetitive work is associated with stress symptoms, but small effects cannot be ruled out. Thus the results question the importance of mental stress mechanisms in the causation of regional pain related to repetitive work. However, the findings should be interpreted with caution because the stress inventory has not been validated against a gold standard.

Linking work factors to neck myalgia: the nitric oxide/oxygen ratio hypothesis

The pathophysiological link between work-related exposures and neck myalgia remains a puzzle. According to the hypothesis presented here, neck myalgia is evoked when low-level contractions in the trapezius muscle are combined with sympathetic vasoconstriction due to psychological stress or prolonged head-down neck flexion at work. These ischemic contractions increase nitric oxide/oxygen concentration ratio in the muscle fibres, enhancing herewith the reversible inhibition of mitochondrial cytochrome oxidase by nitric oxide. The result is depletion of adenosine triphosphate, which elicits production/efflux of lactic acid, in turn activating and sensitising proton-sensitive nociceptive fibres in the connective tissue, causing myalgic pain and tenderness. High estrogen-level, which gives a high expression of nitric oxide synthase in the muscle, accentuates the situation. During episodes of sustained inhibition of cytochrome oxidase by nitric oxide, peroxynitrite may be produced and cause irreversible inactivation of several enzymes in the mitochondrial electron-carrier chain. With repeated episodes, an increasing part of the enzymatic capacity for cellular respiration is inactivated. Even if this process only takes place within a small portion of the muscle fibres, it may contribute to frequent exacerbations of pain. Effects of peroxinitrite may also explain the mitochondrial abnormalities found in the trapezius muscle of many neck myalgia patients. Adrenergic antagonists and nitric oxide synthase inhibitors could reduce symptoms. Ascorbic acid, alpha tocopherol, and flavonoids, which are safe and effective scavengers of peroxynitrite, could prevent chronicity. The most effective non-pharmacological measure may be to reduce exposure to prolonged head-down neck flexions and psychosocial stress at work.

Influence of an outpatient multidisciplinary pain management program on the health-related quality of life and the physical fitness of chronic pain patients

BACKGROUND

Approximately 10 to 20 percent of the population is suffering from chronic pain. Since this represents a major contribution to the costs of the health care system, more efficient measures and interventions to treat these patients are sought.

RESULTS

The development of general health and physical activity of patients with chronic pain was assessed in an interdisciplinary outpatient pain management program (IOPP). 36 patients with an average age of 48 years were included in the IOPP. Subjective assessment of well-being was performed at five time points (baseline, post intervention and 3, 6, and 12 months thereafter) by using standardized questionnaires. The study focused on the quality of life survey Medical Outcomes Study Short Form-36, which is a validated instrument with established reliability and sensitivity. In addition, the patients participated in physical assessment testing strength, power, endurance, and mobility. Prior to therapy a substantial impairment was found on different levels. Marked improvements in the psychological parameters were obtained by the end of the program. No success was achieved with regard to the physical assessments.

CONCLUSION

Although many different studies have evaluated similar programs, only few of them have attained positive results such as improvements of general quality of life or of physical strength. Often no difference from the control group could be detected only some months after the intervention. In the present study no significant persistent improvement of well-being occurred. Possible reasons are either wrong instruments, wrong selection of patients or wrong interventions.

Gnawing behavior of a mouse in a narrow cylinder: a simple system for the study of muscle activity, fatigue, and stress

When a mouse is put into a cylinder too narrow for it to turn (its front end being blocked with a thin plastic strip), the mouse gnaws away the plastic to escape. Hence, the weight reduction in the plastic can be used as an index of 'gnawing activity (GA).' GA was high at first, but decreased with time. Training augmented GA, but not the activity of the histamine-forming enzyme (histidine decarboxylase [HDC]: a proposed marker of muscle fatigue) in the masseter muscle. In trained mice, GA was higher at night than in the daytime, and was decreased by starvation. In mice prevented from reaching the strip, the elevation of serum cortisol was greater than that seen in mice able to gnaw at it. As such gnawing is a form of voluntary behavior, these results suggest that our experimental system may be useful for (i) the quantitative study of voluntary muscle activity associated with physical or mental fatigue or motivation, and (ii) the study of an animal's response to stress when it has, or alternatively does not have, an apparent way of escape.

Work environment and neck and shoulder pain: the influence of exposure time. Results from a population based case-control study

OBJECTIVES

To study associations between long term and short term exposure to different work environmental conditions and the incidence of neck or shoulder pain. The results were obtained as part of the MUSIC-Norrtälje study, which is a population based case-control study conducted in Sweden in 1993-7.

METHODS

The cases were people from the study base who sought medical care or treatment for neck or shoulder pain. Information on physical and psychosocial conditions in the work environment, currently and 5 years ago, and lifestyle factors, was obtained by self administered questionnaires from 310 cases and 1277 randomly selected referents.

RESULTS

Associations between both physical and psychosocial exposures in the work environment and seeking care for neck or shoulder pain were found. The risk patterns differed for the sexes, and risk ratios exceeding 1.5 were more often found among women than among men. Generally, subjects who had experienced a recent increase of exposure were more likely (relative risk (RR) 2.1-3.7) to seek care than those who had been exposed long term (RR 1.5-1.8). Among women, an increased amount of visual display terminal (VDT) work, work above shoulder level, and reduced opportunities to acquire new knowledge, and among men, an increased amount of seated work were associated with neck or shoulder pain. This might indicate short induction periods for neck or shoulder pain for these exposures. However, for repetitive work with the hands and hindrance at work among women, and possibly also local vibrations among men, the induction periods seem to be longer. Interactive effects between factors, both at work and in the family, were found, but only among women.

CONCLUSIONS

Associations between some exposures in the work environment and seeking care for neck or shoulder pain were found. The high RRs for short term exposure might indicate that for many factors the induction period for neck or shoulder pain is short.

A field comparison of neck and shoulder postures in symptomatic and asymptomatic office workers

Poor neck and shoulder postures have been suggested to be a cause of neck and shoulder pain in computer workers. The present study aimed to evaluate and compare the head, neck and shoulder postures of office workers with and without symptoms in these regions, in their actual work environments. The two all female subject groups reported significantly different discomfort scores across five trials repeated in a single working day. The results of repeated video capture and two-dimensional motion analysis showed that there were trends for increased head tilt and neck flexion postures in the symptomatic subjects (n = 8), compared to the asymptomatic subjects (n = 8). Symptomatic subjects also tended to have more protracted acromions compared with asymptomatic subjects and showed greater movement excursions in the head segment and the acromion. All subjects demonstrated an approximately 10% increase in forward head posture from their relaxed sitting postures when working with the computer display, but there were no significant changes in posture as a result of time-at-work.

Shoulder and neck complaints in customer relations: individual risk factors and perceived exposures at work

Perceived psychosocial and biomechanical exposures, individual factors and pain in the shoulder and neck were recorded in two groups of female service workers (healthcare and shopping centre workers). The jobs investigated were characterized by 'much' direct human relations, 'little' sitting and 'much' standing, and were light work by physiological or biomechanical criteria but potentially psychosocially demanding. A screening survey (n = 400 females) was the basis for the selected sample (n = 66 females), which was the object of the main investigation of this study. Reliability of the questionnaires was tested in a separate group of female healthcare workers (n = 29). Heart rate recordings through the work day estimated workload. There was a high prevalence of shoulder and neck pain (> 50%) for both work groups. In the two populations it proved difficult to explain shoulder and neck pain by reported physical and psychosocial exposures or individual factors, except by the variable 'perceived general tension', which clearly differentiated workers with and without pain. The findings in this study indicated, first, that perceived general tension might be an independent risk factor for muscle pain and, second, that this might be related to personality factors. However, this putative relationship must be verified in a longitudinal study. As no variable describing exposures in the working environment was associated with shoulder and neck pain, the question is posed whether such complaints can be considered work-related. Alternatively, the variables used to describe mechanical and psychosocial exposures in this study may have low specificity in characterizing work-related risk factors for service workers with customer relations.

Trapezius muscle activity as a risk indicator for shoulder and neck pain in female service workers with low biomechanical exposure

Electromyographic activity of the upper trapezius muscles was recorded over the workday for two groups of service workers, shopping centre (n = 22) and healthcare workers (n = 44), both with low observed biomechanical exposure. Static and median EMG activity level, number of EMG gaps and gap time were determined. The variability of these variables over the workday was examined by calculating the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) of 1-h consecutive recording periods. All variables except gap time showed acceptable reliability (ICC = 0.69-0.78), i.e. the largest fraction of variance in the data set was due to intersubject variance, despite relatively large hour-to-hour variation (CV = 0.21 0.62). The EMG activity level in the trapezius muscles was low (static activity level < 1% EMGmax), despite the high prevalence of shoulder and neck pain for both groups of workers. In addition to the work recordings, tests were performed to determine intersubject variation in muscle activity when adopting a standardized resting posture, and in a dynamic muscle activity pattern during paced arm movement. Neither the EMG variables from the work recordings nor the tests with EMG recording indicated higher trapezius EMG activity levels for workers with pain in the shoulders and neck in this study. The low EMG levels are interpreted to indicate a low risk of developing shoulder and neck complaints due to biomechanical exposure for both groups of workers. The possibility of pain-initiating mechanisms, associated with stress and not mediated through muscle activity, is considered in the discussion.

Cognitive performance and muscle activation in workers with chronic shoulder myalgia

The present study determined whether workers with chronic shoulder myalgia exhibit impaired cognitive performance and elevated muscle activation. Ten workers with and 10 without shoulder myalgia performed three sessions, each consisting of three different cognitive tasks. Subjective pain was reported on visual analogue scales (VAS). Surface electromyography was recorded bilaterally from the trapezius and forearm extensor muscles. Speed and error rates of the cognitive work were almost the same in the two groups, but the myalgia subjects reported more stress during the work. In two of the cognitive tasks, both groups increased their speed of performance to the same extent from the first to the third session, despite pain increase in the myalgia subjects. In the biomechanically demanding task, the speed fell from the first to the third session in both groups. Muscle activation was equal in the two groups and constant throughout the protocol despite changes in speed of performance and pain. It is concluded that localized chronic muscle pain does not significantly alter cognitive performance, or muscle activation level during cognitive work. However, the perceived burden of work is larger, as indicated by the higher stress reported during cognitive work in the myalgia subjects compared with their pain-free controls.

Elevation of histidine decarboxylase activity in skeletal muscles and stomach in mice by stress and exercise

The effects of different types of stress (water bathing, cold, restraint, and prolonged walking) on histidine decarboxylase (HDC) activity in masseter, quadriceps femoris, and pectoralis superficial muscles, and in the stomach were examined in mice. All of these stresses elevated gastric HDC activity. Although water bathing, in which muscle activity was slight, was sufficiently stressful to produce gastric hemorrhage and to increase gastric HDC activity, it produced no detectable elevation of HDC activity in any of the muscles examined. The other stresses all elevated HDC activity in all three muscles. We devised two methods of restraint, one accompanied by mastication and the other not. The former elevated HDC activity in the masseter muscle, but the latter did not. These results suggest that 1) HDC activity in the stomach is an index of responses to stress, 2) the elevation of HDC activity in skeletal muscles during stress is induced partly or wholly by muscle activity and/or muscle tension, and 3) stress itself does not always induce an elevation of HDC activity in skeletal muscles.

Muscular rest and gap frequency as EMG measures of physical exposure: the impact of work tasks and individual related factors

Owing to an orderly recruitment of motor units, low threshold type I fibres are presumed to be vulnerable in contractions of long duration. To study load on these fibres muscular rest was registered as the time fraction of electromyographic (EMG) activity below a threshold. Moreover, the frequency of periods with muscular rest, EMG gaps, was derived, since a low gap frequency has been shown to be a risk factor for musculoskeletal disorders. Trapezius EMG was registered in 24 female hospital cleaners, 21 female office workers and 13 male office workers during one working day. Cleaners have a high risk of neck/shoulder pain and had much less muscular rest than office workers measured as a percentage of total registered time (median value = 1.5%, range = 0.2-13% vs. median value = 12%, range = 0.0-32%, respectively). Gap frequency showed no difference between the two occupational groups. Both measures displayed a wide inter-individual variation. For the cleaners, some of the variance was explained by body mass index (BMI) and age, with lower values of muscular rest for older subjects with a high BMI. Among the office workers, low values of muscular rest and a high gap frequency were registered in subjects with a low subjective muscular tension tendency. Gender, strength, smoking, job strain, employment time and musculoskeletal symptoms had no impact on either EMG measure.

Job stress, upper extremity pain and functional limitations in symptomatic computer users

BACKGROUND

Research suggests that exposure to occupational stressors are related to the presence and/or exacerbation of work-related upper extremity symptoms in office workers. Also worker's response to work demands and/or job stressors (i.e., workstyle) may exacerbate symptom severity and impact function. The present study examines the association among work demands, job stress and workstyle on pain and function.

METHODS

124 symptomatic female office workers completed a questionnaire measuring demographics, medical history, work demands, perception of the work environment, workstyle, pain intensity, functional impact, and time lost from work.

RESULTS

Heightened job stress and the tendency to continue to work in a way that contributes to pain to ensure high quality (dimension of workstyle) were related to pain intensity at work and decreased function. These variables, in addition to hours worked per year, were related to increased pain experienced across the work week. The model tested did not predict the occurrence of lost time.

CONCLUSIONS

The present findings provide support for the association between job stress, workstyle, upper extremity pain and function. While it is not possible to determine the exact direction of the observed relationships, these results are consistent with prior research indicating the potential significance of job stress and workstyle on symptom exacerbation and functional limitations. Implications for evaluation and intervention are discussed.

Muscle activation in the contralateral passive shoulder during isometric shoulder abduction in patients with unilateral shoulder pain

Studies have shown an increased muscle activation at the opposite passive side during unilateral contractions. The purpose of the present study was to examine the influence of pain on muscle activation in the passive shoulder during unilateral shoulder abduction. Ten patients with unilateral rotator tendinosis of the shoulder and nine healthy controls performed unilateral maximal voluntary contractions (MVC) and sustained submaximal contractions with and without subacromial injections of local anaesthetics of the afflicted shoulder. Muscle activation was recorded by electromyography (EMG) from the trapezius, deltoid, infraspinatus and supraspinatus muscles in both shoulders. During MVCs, the EMG amplitude from muscles of the passive afflicted side was not different in patients and controls, and was not influenced by pain alterations. In contrast, the EMG amplitude from the muscles of the passive unafflicted side was lower in the patients and increased after pain reduction. During the sustained submaximal contraction the EMG amplitude increased gradually in the passive shoulder to 15-30% of the EMG amplitude observed during MVC. This response was not influenced by differences in pain. We conclude that muscle activation of the passive shoulder was closely related to the activation of the contracting muscles and thus related to central motor drive, and not directly influenced by changes in pain.

Surface EMG mapping of the human trapezius muscle: the topography of monopolar and bipolar surface EMG amplitude and spectrum parameters at varied forces and in fatigue

OBJECTIVES

To investigate the factors affecting the topography of trapezius muscle EMG, multichannel recordings were made at different forces of isometric shoulder elevation and during fatiguing exercise.

METHODS

Twenty-eight channels of monopolar EMG were recorded from an array of 4 x 7 electrodes placed on the upper trapezius muscle. From the monopolar EMG and the bipolar derivations the root mean square (RMS(monopolar), RMS(bipolar)) and power spectrum median frequency (MF(monopolar), MF(bipolar)) were calculated.

RESULTS

The maximum RMS(monopolar) was located above the middle part of the trapezius muscle, where a minimum was found for RMS(bipolar). The cranial-caudal RMS distribution shifted upwards when the force was increased from 50 to 100% MVC and during fatigue. MF(bipolar) showed a peak above the endplate region, where the MF(monopolar) was low. During fatigue the normalized MF slope was independent of the cranial-caudal electrode position, but MF(monopolar) decreased most strongly at positions above the endplate region, where MF(bipolar) decreased less.

CONCLUSIONS

While the changes in MF reflected metabolic properties and volume conduction phenomena in the muscle, changes in RMS reflected a compensation for the fatigue processes within the muscle. The RMS changes in fatigue can be explained by the direction of the fibres involved in shoulder elevation.

Psychological aspects of pain

Manual therapy is based on a biomedical model of illness and places considerable reliance on the patient's report of pain. Reported pain intensity is assumed to bear a close relationship with underlying nociception but research has shown that the experience of pain is also influenced by a wide range of psychological factors. Firstly, response to pain provocation (whether palpation or induction of biomechanical stress) can be affected by fear of an adverse outcome (such as pain) and fear of injury. Secondly, a patient's global rating of their pain may be widely influenced by factors in addition to nociception such as distress, fear and mistaken beliefs about the nature of pain and likely outcome of treatment. The manual therapist needs, therefore, to conduct and understand biomedical assessment within a biopsychosocial framework. In appraising the patient's response, the therapist may find it helpful to incorporate specific assessment of subjectively reported fear or behavioural indicators of fear such as guarded movements or behavioural signs. Therapists need to understand that in manual therapy, they are frequently managing the patient's pain behaviour and distress, rather than simply the nociceptive component of their pain.

Job characteristics as predictors of neck pain. A 4-year prospective study

Data from a community-based 4-year prospective study were used to investigate job characteristics as predictors of neck pain. Of 1791 working responders who completed a questionnaire in 1990, 1429 (79.8%) returned a second questionnaire 4 years later (1994). In responders without neck pain during the previous 12 months in 1990, the "little influence on own work situation" factor predicted neck pain during the previous 12 months (odds ratio = 2.21; 95% confidence interval, 1.18 to 4.14) and previous 7 days in 1994 (OR = 2.85; 95% confidence interval, 1.21 to 6.73) after adjustment for a series of potential confounders. In responders with neck pain in 1990, the little influence on own work situation factor was associated with persistent neck pain 4 years later. The study indicates that having little influence on one's own work situation is a predictor of neck pain.

Pathophysiological mechanisms of tension-type headache: a review of epidemiological and experimental studies

In this present thesis I have discussed the epidemiology and possible pathophysiological mechanisms of tension-type headache. A population-based study of 1000 subjects randomly selected from a general population, two clinical studies, and a method study of EMG recordings, were conducted. Tension-type headache was the most prevalent form of headache, with a life-time prevalence of 78% in a general adult population. Thirty percent were affected more than 14 days per year and 3% were chronically affected, i.e. had headache at least every other day. Females were more frequently affected than males, and young subjects more frequently affected than older subjects. Females were more sensitive to mechanical pressure pain and revealed more tenderness from pericranial muscles and tendon insertions than males, and young subjects were more pain-sensitive than older subjects. Significantly higher tenderness in pericranial muscles was found in subjects with tension-type headache compared to migraineurs and to subjects without any experience of headache. Tenderness increased significantly with increasing frequency of tension-type headache in both males and females, whereas no such relation was found for mechanical pain thresholds. The applied EMG methodology was fairly reliable and nonpainful, but due to intersubject variability paired studies should be preferred. Subjects with chronic tension-type headache had slightly increased EMG levels during resting conditions and decreased levels during maximal voluntary contraction compared with headache-free subjects, indicating insufficient relaxation at rest and impaired recruitment at maximal activity. In a subsequent clinical, controlled study, the effect of 30 min of sustained tooth clenching was studied. Within 24 h, 69% of patients and 17% of controls developed a tension-type headache. Shortly after clenching, tenderness was increased in the group who subsequently developed headache, whereas tenderness was stable in the group of patients who remained headache-free, indicating that tenderness might be a causative factor of the headache. Likewise, psychophysical and EMG parameters were studied in 28 patients with tension-type headache, both during and outside of a spontaneous episode of tension-type headache. It was concluded that a peripheral mechanism of tension-type headache is most likely in the episodic subform, whereas a secondary, segmental central sensitization and/or an impaired supraspinal modulation of incoming stimuli seems to be involved in subjects with chronic tension-type headache. Prolonged nociceptive stimuli from myofascial tissue may be of importance for the conversion of episodic into chronic tension-type headache. The author emphasizes that tension-type headache is a multifactorial disorder with several concurrent pathophysiological mechanisms, and that extracranial myofascial nociception may constitute only one of them. The present thesis supplements the understanding of the balance between peripheral and central components in tension-type headache, and thereby, hopefully, leads us to a better prevention and treatment of the most prevalent type of headache.

Interactions between physical and psychosocial risk factors at work increase the risk of back disorders: an epidemiological approach

OBJECTIVES

To investigate the possible interactions between physical and psychosocial risk factors at work that may be associated with self reported back disorders.

METHODS

891 of 1514 manual workers, delivery drivers, technicians, customer services computer operators, and general office staff reported risk factors at work and back disorders with a self administered questionnaire (59% return rate). Of the 869 respondents with a valid questionnaire, 638 workers were classified in to one of four exposure groups: high physical and high psychosocial; high physical and low psychosocial; low physical and high psychosocial; and low physical and low psychosocial. Low physical and low psychosocial was used as an internal reference group. The exposure criteria were derived from existing epidemiological publications and models for physical and psychosocial work factors. The frequency and amplitude of lifting and the duration spent sitting while experiencing vibration were used as physical exposure criteria. Ordinal values of mental demands, job control, and social support from managers and coworkers were used as psychosocial exposure criteria.

RESULTS

The highest increase in risk was found in the high physical and high psychosocial exposure group for symptoms of back disorders. In the crude and multivariate analyses, a departure from an additive risk model was found for the 7 day prevalence of symptoms of a low back disorder and also for a recurrent back disorder not present before the current job but also experienced in the past 7 days.

CONCLUSION

This study suggests that an interaction between physical and psychosocial risk factors at work may exist to increase the risk of self reported back disorders. Ergonomic prevention strategies that aim to minimise the risks of symptoms of work related back disorders should not only focus on physical but also on psychosocial risk factors at work. The greatest benefits are likely to be realised when both physical and psychosocial factors are put right.

Workplace changes associated with a reduction in musculoskeletal symptoms in office workers

The purpose of this study was to identify factors associated with reductions observed in musculoskeletal symptoms when office workers were moved to a new building. A questionnaire including items regarding symptoms and aspects of the work environment was administered to 577 office workers before and after they were moved from nine buildings to a single new facility in 1992. Employees working in two reference buildings, where they remained throughout the study period, were also surveyed. Two musculoskeletal outcomes, hand/arm and neck/shoulder/back, were selected for study. In matched multivariate analyses, the reduction in hand/arm symptoms from 1992 to 1993 was associated with improved satisfaction with the physical workstation (odds ratio [OR] = 2.0); the reduction in neck/shoulder/back symptoms was associated with improved chair comfort (OR = 1.8), fewer housekeeping responsibilities (OR = 3.6), female gender (OR = 1.8), and low pay range (OR = 1.7). Longitudinal results suggested that changes in workstations resulted in decreased symptoms. Results of this investigation might be used to develop workplace changes that result in reductions of musculoskeletal disorders.