Psychophysiological mechanisms of pain in computer work: The blood vessel-nociceptor interaction hypothesis

Effects of anti-fatigue lenses on performance, muscle activity and subjective discomfort responses during a seated computer task

Anti-fatigue lenses (AFL) intend to provide health benefits in association with computer work. Their effects on visual and muscle discomfort mechanisms and task performance remain unclear. Twenty-three computer users (n = 12 males) underwent two 90-min computer sessions with AFL or placebo lenses. Eye strain, body discomfort, typing performance, upper trapezius (UT) activation amplitude and variability, and neck posture were analyzed for time, condition, and sex effects. No significant effects of condition were observed on the dependent measures evaluated. Discomfort increased over time (neck/shoulder: p < .001), more so in females (eye strain: p < .001). Females' UT activation amplitude was negatively correlated to eye strain in the placebo condition (p = .05).

The impaired healing hypothesis: a mechanism by which psychosocial stress and personal characteristics increase MSD risk?

While the effects of physical risk factors on MSD development have been a primary focus of musculoskeletal research, psychological stressors, and certain personal characteristics (e.g. ageing, sex, and obesity) are also associated with increased MSD risk. The psychological and personal characteristics listed above share a common characteristic: all are associated with disruption of the body's neuroendocrine and immune responses resulting in an impaired healing process. An impaired healing response may result in reduced fatigue life of musculoskeletal tissues due to a diminished ability to keep pace with accumulating damage (perhaps reparable under normal circumstances), and an increased vulnerability of damaged tissue to further trauma owing to the prolonged healing process. Research in engineered self-healing materials suggests that decreased healing kinetics in the presence of mechanical loading can substantially reduce the fatigue life of materials. A model of factors influencing damage accrual and healing will be presented. Practitioner summary: This article provides a potential reason why musculoskeletal disorder risk is affected by psychosocial stress, age, sex, and obesity. The reason is that these factors are all associated with a slower than normal healing response. This may lead to faster damage development in musculoskeletal tissues resulting in higher MSD risk.

Effects of the COVID-19 lockdown on musculoskeletal pain, physical activity, and work environment in Estonian office workers transitioning to working from home

BACKGROUND

COVID-19 lockdown caused a sudden change in the work culture and environment.

OBJECTIVE

The aim of this study was to evaluate the impact of COVID-19 lockdown caused changes in musculoskeletal pain (MSP), physical activity (PA), workplace properties, and their in-between relationships among office workers.

METHODS

A total of 161 office workers (64.6%female) with a mean age of 38.2±9.5 years participated. The study was conducted as an online form. Baecke Physical Activity Questionnaire, NORDIC Questionnaire, and questions about the work environment were used. The participants were asked to describe the current situation and retrospectively the situation 3 months before.

RESULTS

We found no significant differences in the prevalence of MSP or between the mean number of body regions suffering from MSP before and during the lockdown. During the lockdown, a significant (p < 0.001) reduction in total PA and sport-related PA and a significant (p < 0.001) increase in work-related PA was noted. A significant drop in both workplace comfort rating (p < 0.05) and workplace ergonomics rating (p < 0.001) was suggested during the lockdown. Our data also suggested several individual factors influencing the MSP among office workers during the lockdown.

CONCLUSIONS

Maintaining habitual physical activity level and preparing a more comfortable and ergonomic workspace can play a role in a healthier transition to working from home.

Effects of cycling while typing on upper limb and performance characteristics

Active computer workstations may help reduce workplace sedentarism. However, their impact on the upper limb musculoskeletal system is unknown. Subjects participated in two 60-min computer laptop-based Bike-and-Type sessions at different cycling intensities (LOW, HIGH). Upper trapezius and wrist extensor muscle blood flow, sensitivity and pain thresholds, and typing performance were measured intermittently. Neck/shoulder discomfort increased over time (p < 0.001), and was higher in the HIGH intensity (p = 0.036). Blood flow to the trapezius (p = 0.041) and wrist (p = 0.021) muscles were higher during HIGH, and wrist blood flow increased over time (p = 0.01). Trapezius sensitivity threshold significantly decreased over time (p = 0.003). There were no effects on pressure pain thresholds (interaction p = 0.091). Average typing speed was greater during HIGH (p = 0.046) and increased over time (p < 0.001). Time spent biking while typing effectively improved performance, which may facilitate muscle regeneration, although effects depend on biking intensity.

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.

Reliability of muscle blood flow measurements in orbicularis oculi

PURPOSE

Orbicularis oculi muscle tension and muscle blood flow have been shown to be objective measures of eyestrain during visually demanding activities, such as computer work. In line with this, positive associations between eye-related pain and muscle blood flow in orbicularis oculi have been observed. A hypothesis regarding work situations with cognitive tasks and low-level muscle activity, such as computer work, proposes that muscle pain originates from the blood vessel-nociceptor interactions of the connective tissue of the muscle. Noninvasive muscle blood flow measurements in the orbicularis oculi muscle are preferable to using an invasive technique. The aim of this study was to test reproducibility and stability of muscle blood recordings in orbicularis oculi using photoplethysmography.

METHODS

In the reproducibility tests, 12 subjects were tested twice within 1 to 5 weeks. To study the stability of the method, six of the subjects were randomly selected and tested four more times within 2 to 6 weeks. Test subjects were doing identical visually demanding computer work for 10 minutes in each test.

RESULTS

The short-term repeatability of muscle blood flow measurements was considered good, but the stability of blood flow recordings over time in orbicularis oculi was low because of a greater within-subject maximum variability compared with between-subject average variability.

CONCLUSIONS

Investigators should be aware of the effect of time, possibly attributed to confounding factors such as environmental changes and mental stress, when comparing photoplethysmography muscle blood flow recordings.

Comparative metabolomics of muscle interstitium fluid in human trapezius myalgia: an in vivo microdialysis study

Purpose

The mechanisms behind trapezius myalgia are unclear. Many hypotheses have been presented suggesting an altered metabolism in the muscle. Here, muscle microdialysate from healthy and myalgic muscle is analysed using metabolomics. Metabolomics analyse a vast number of metabolites, enabling a comprehensive explorative screening of the cellular processes in the muscle.

Methods

Microdialysate samples were obtained from the shoulder muscle of healthy and myalgic subjects that performed a work and stress test. Samples from the baseline period and from the recovery period were analysed using gas chromatography—mass spectrometry (GC–MS) together with multivariate analysis to detect differences in extracellular content of metabolites between groups. Systematic differences in metabolites between groups were identified using multivariate analysis and orthogonal partial least square discriminate analysis (OPLS-DA). A complementary Mann–Whitney U test of group difference in individual metabolites was also performed.

Results

A large number of metabolites were detected and identified in this screening study. At baseline, no systematic differences between groups were observed according to the OPLS-DA. However, two metabolites, l-leucine and pyroglutamic acid, were significantly more abundant in the myalgic muscle compared to the healthy muscle. In the recovery period, systematic difference in metabolites between the groups was observed according to the OPLS-DA. The groups differed in amino acids, fatty acids and carbohydrates. Myristic acid and putrescine were significantly more abundant and beta-d-glucopyranose was significantly less abundant in the myalgic muscle.

Conclusion

This study provides important information regarding the metabolite content, thereby presenting new clues regarding the pathophysiology of the myalgic muscle.

Shoulder muscle fatigue during repetitive tasks as measured by electromyography and near-infrared spectroscopy

OBJECTIVE

The objective of this study was to quantify shoulder muscle fatigue during repetitive exertions similar to motions found in automobile assembly tasks.

BACKGROUND

Shoulder musculoskeletal disorders (MSDs) are a common and costly problem in automotive manufacturing.

METHOD

Ten subjects participated in the study. There were three independent variables: shoulder angle, frequency, and force. There were two types of dependent measures: percentage change in near-infrared spectroscopy (NIRS) measures and change in electromyography (EMG) median frequency. The anterior deltoid and trapezius muscles were measured for both NIRS and EMG. Also, EMG was collected on the middle deltoid and biceps muscles.

RESULTS

The results showed that oxygenated hemoglobin decreased significantly due to the main effects (shoulder angle, frequency, and force). The percentage change in oxygenated hemoglobin had a significant interaction attributable to force and repetition for the anterior deltoid muscle, indicating that as repetition increased, the magnitude of the differences between the forces increased. The interaction of repetition and shoulder angle was also significant for the percentage change in oxygenated hemoglobin. The median frequency decreased significantly for the main effects; however, no interactions were statistically significant.

CONCLUSIONS

There was significant shoulder muscle fatigue as a function of shoulder angle, task frequency, and force level. Furthermore, percentage change in oxygenated hemoglobin had two statistically significant interactions, enhancing our understanding of these risk factors.

APPLICATION

Ergonomists should examine interactions of force and repetition as well as shoulder angle and repetition when evaluating the risk of shoulder MSDs.

A prospective study of the relationship between musculoskeletal or psychological complaints and muscular responses to standardized cognitive and motor tasks in a working population

The present study sought to determine the relationship between musculoskeletal or psychological complaints and muscular responses to standardized cognitive and motor tasks. The prospective study design examined (i) whether complaint severity predicts muscular responses during standardized tasks and (ii) whether the muscular responses predict changes in complaint severity over one year. Musculoskeletal and psychological complaints were recorded by monthly reports the four months preceding and 12 months succeeding a work session in the laboratory; complaint-severity indices were computed from complaint-severity scores (intensity scorexduration score). Surface electromyography (EMG) was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles in 45 post-office workers (30 women) during two identical task series. Between the series, exhausting submaximal muscle contractions (25% of peak torque) were performed. In adjusted regression models, no relations between musculoskeletal complaints the last four months and muscle activity during the task series were found. However, psychological complaints the last four months predicted higher muscle activity levels and a steeper rise in muscle activity in the muscles not engaged in motor task performance. Sleep disturbance was the strongest individual predictor of increased muscle responses. In contrast, psychological complaints the last four months predicted lower EMG levels in the task-engaged muscle during the complex-choice-reaction-time tasks. None of the muscle-activity responses to the standardized tasks predicted changes in severity of musculoskeletal or psychological complaints over the subsequent one-year period. In conclusion, psychological complaints predict different responses in task-engaged and non-involved muscles during cognitive and motor tasks. Musculoskeletal complaints did not predict responses to the tasks.

Autonomic activation and pain in response to low-grade mental stress in fibromyalgia and shoulder/neck pain patients

OBJECTIVE

Psychosocial stress is a risk factor for musculoskeletal pain, but how stress affects musculoskeletal pain is poorly understood. We wanted to examine the relationship between low-grade autonomic activation and stress-related pain in patients with fibromyalgia and localised chronic shoulder/neck pain.

METHODS

Twenty-three female patients with fibromyalgia, 29 female patients with chronic shoulder-neck pain, and 35 healthy women performed a stressful task lasting 60min. With a blinded study design, we recorded continuous blood pressure, heart rate, finger skin blood flow and respiration frequency before (10min), during (60min) and after (30min) the stressful task. The physiological responses were compared with subjective reports of pain.

RESULTS

The increase in diastolic blood pressure and heart rate in response to the stressful task were smaller in fibromyalgia patients compared with the healthy controls. Furthermore, fibromyalgia patients had reduced finger skin blood flow at the end of the stressful task compared to healthy controls. We also found an inverse relationship between the heart rate response and development and recovery of the stress-related pain in fibromyalgia patients.

CONCLUSION

We found abnormal cardiovascular responses to a 60min long stressful task in fibromyalgia patients. Furthermore, we found a negative association between the heart rate response and the pain which developed during the stressful task in the fibromyalgia group, possibly a result of reduced stress-induced analgesia for fibromyalgia patients.

Deep tissue hyperalgesia after computer work

Background and purpose

A growing number of people are using computers. Shoulder and neck pain occur commonly during computer work. Peripheral and central sensitization may play a major role in establishing and maintaining several chronic pain conditions. We have previously reported that a 90 min simulated computer office-work induced substantial pain in the shoulders and neck. We hypothesized that the development of pain during the computer work may be related to sensitization. The aim of the present study was to examine if the 90 min computer work induced deep tissue muscle hyperalgesia manifested as altered pressure pain thresholds (PPTs).

Methods

Twenty-two subjects with chronic shoulder and neck pain (pain group) and 26 healthy and pain free subjects (reference group) performed a standardized computer office-work task with use of a computer-mouse and with time pressure and high precision demands continuously for 90 min. The pressure pain threshold was measured with a pressure algometer in shoulder and forearm muscles (bilaterally in upper trapezius and extensor carpi radialis), and at sternum, before and 15 min and 30 min after the computer work task.

Results

The PPTs before starting the computer work were not different between the groups at any of the five locations. In both groups, the PPTs in the active and inactive side of the upper trapezius as well as in the extensor carpi radialis of the forearm operating the computer mouse were significantly reduced after the 90 min computer work compared with the pre-work levels. In the pain group, also the PPT in the inactive resting forearm was significantly reduced. The changes seen in PPTs from pre-to post-work were not significantly different between the groups, except for the inactive resting forearm where the groups exhibited different time course.

Conclusion

A decrease in pressure pain thresholds of involved muscles suggests that computer office-work can induce deep tissue hyperalgesia within 90 min. The development of pain during the computer work indicates peripheral sensitization as the predominant mechanism. Decreased pressure pain thresholds also in sites distant from pain areas may indicate a contribution from central sensitization in the subjects with chronic shoulder and neck pain.

Implications

The lasting pain after work and the reduced PPTs both in involved and distant musculature may indicate need for frequent pauses during computer work, especially when performed with time pressure and high precision demands, in order to avoid pain to increase and sustain after work, and thus to prevent the possibility of pain to become chronic.

Behavioral and psychophysiological responses to job demands and association with musculoskeletal symptoms in computer work

INTRODUCTION

The purpose of this study was to identify and compare individual behavioral and psychophysiological responses to workload demands and stressors associated with the reporting of musculoskeletal symptoms with computer work. Evidence is growing that the prevalence of musculoskeletal symptoms increases with longer hours of computer work and exposure to psychosocial stressors such as high workloads and unrealistic deadlines. Workstyle, or how an individual worker behaves in response to such work demands, may also be an important factor associated with musculoskeletal symptoms in computer operators.

METHOD

Approximately 8,000 employees of the Australian Public Service were invited to complete an on-line survey if they worked with a computer for 15 or more hours per week. The survey was a composite of three questionnaires: the ASSET to measure perceived organizational stressors, Nordic Musculoskeletal Questionnaire to measure reported prevalence of musculoskeletal symptoms and additional questions to measure individual work behaviors and responses.

RESULTS

934 completed surveys were accepted for analyses. Logistic regression was used to identify significant behavioral and work response predictors of musculoskeletal symptoms. Reporting of heightened muscle tension in response to workload pressure was more strongly associated, than other physical behavioral factors, with musculoskeletal symptoms for all body areas, particularly the neck (OR = 2.50, 95% CI: 2.09-2.99).

CONCLUSIONS

Individual workstyles in response to workload demands and stressors, including working with heightened muscle tension and mental fatigue, were significantly associated with musculoskeletal symptoms. Future risk management strategies should have a greater focus on the identification and management of those organizational factors that are likely to encourage and exacerbate adverse workstyles.

Combining microdialysis and near-infrared spectroscopy for studying effects of low-load repetitive work on the intramuscular chemistry in trapezius myalgia

Epidemiological research provides strong evidence for a link between repetitive work (RW) and the development of chronic trapezius myalgia (TM). The aims were to further elucidate if an accumulation of sensitising substances or impaired oxygenation is evident in painful muscles during RW. Females with TM (n = 14) were studied during rest, 30 minutes RW and 60 minutes recovery. Microdialysate samples were obtained to determine changes in intramuscular microdialysate (IMMD) [glutamate], [PGE₂], [lactate], and [pyruvate] (i.e., [concentration]) relative to work. Muscle oxygenation (%StO₂) was assessed using near-infrared spectroscopy. During work, all investigated substances, except PGE₂, increased significantly: [glutamate] (54%, P < .0001), [lactate] (26%, P < .005), [pyruvate] (19%, P < .0001), while the %StO₂ decreased (P < .05). During recovery [PGE₂] decreased (P < .005), [lactate] remained increased (P < .001), [pyruvate] increased progressively (P < .0001), and %StO₂ had returned to baseline. Changes in substance concentrations and oxygenation in response to work indicate normal increase in metabolism but no ongoing inflammation in subjects with TM.

Pain induced by a single simulated office-work session: time course and association with muscle blood flux and muscle activity

The present study aimed to assess the development of pain during computer work with high precision demand and time pressure, and the association between pain and muscle blood fluxes and muscle activity. Twenty-eight healthy subjects (range 22-44 years) performed a 90-min standardized task of correcting a text on a word-processor. Monetary reward was given according to productivity in order to induce time pressure. Pain intensity, general tension, and eye strain were reported on visual analogue scales before, during and after the computer session. Intramuscular trapezius blood fluxes were recorded by laser-Doppler flowmetry (LDF) and analyzed as % of baseline level. Muscle activity was measured from the upper trapezius and forearm extensors by surface electromyography (EMG), analyzed as % of a maximal calibration contraction, %EMG(max). Pain, tension, and eye strain increased considerably during the computer session with different time course for pain and tension. The LDF of the active side of trapezius was elevated for 30 min followed by a falling trend. There was an initial short-lasting increase in the inactive trapezius. The upper trapezius EMG was low (<3 % of EMG(max)) but increased during the work, similar in both the active and passive side. There was a significant time and blood flux interaction effect on pain in the shoulders/neck, but no association with EMG. In conclusion, considerable neck/shoulder pain may develop in healthy pain-free subjects during 90 min of office-work and seems to be related to the regulation of trapezius muscle blood flow.

The impact of a computerized work environment on professional occupational groups and behavioural and physiological risk factors for musculoskeletal symptoms: a literature review

INTRODUCTION

Computers have become an essential tool for many office based professional occupations, but their use is also accompanied by change to work demands and psychosocial work environment. Whilst considerable research exists relating to the potential health risks associated with computer work amongst semi-skilled occupations, there is a paucity of knowledge regarding the impact of an increasingly computerized workplace on the physical and psychological wellbeing of professional occupations.

METHODS

A literature search was conducted using OVID Medline, PsycINFO and Cinahl databases. Papers published between 1980 and 2007 were selected for review. These included epidemiological and experimental studies that explored the relationships among occupational demands and stressors, work behaviours and musculoskeletal health in workers operating in a computerized work environment.

RESULTS

In response to workload, deadline and performance monitoring pressures, many professional workers are often encouraged to perform long hours of computer work with high mental demands; work at a hectic workpace resulting in heightened muscle tension and forces, and with inadequate work breaks. These factors were identified in this review as risk factors for work related musculoskeletal symptoms.

CONCLUSION

As new technology continues to computerise the way professionals do their work, it is important for organizations to identify and measure the risks to health and wellbeing associated with these changes. Further research with professional groups is needed to support effective risk management decisions.

Reliability of bloodflux measurements from the upper trapezius muscle during muscle contractions

The aim of this study was to assess the blood flux responses to isometric contractions and recovery. We also wanted to assess the variations in Laser Doppler blood fluxes (LDF) within the upper trapezius muscles, the concordance between the left and right sides and the reliability of blood-flux measurements between contractions and over measurement days. Ten subjects, mean age 25 years (21-35 years), performed two isometric shoulder abduction contractions at 10% of maximal voluntary contraction torque (MVC), repeated on two separate days. LDF were measured in three sites in the upper trapezius muscles bilaterally. Mean arterial pressure, heart rate and electromyography from the upper trapezius muscles was also recorded. The reliability of blood flux measurements during rest and contractions are good; however, considerable variations between positions and between measurements in different muscles were observed. Laser-Doppler measurement of microcirculation is a promising method. Considerable variations between measurement sites, muscles, and variations over measurement days indicate caution with interpretation of blood flux responses between conditions and individuals.

The influence of body posture, arm movement, and work stress on trapezius activity during computer work

The study aimed to determine the influence of arm posture and movement on trapezius activity of computer workers, considering the full workday. A second aim was to investigate if work periods perceived as stressful were associated with elevated or more sustained muscle activity pattern. Twenty-six computer workers performing call-center (n=11), help desk (n=7), or secretarial (n=8) work tasks participated. Bilateral trapezius surface electromyographic (sEMG) activity and heart rate was recorded throughout the workday. Simultaneous inclinometer recordings from left thigh and upper arms identified periods with sitting, standing, and walking, as well as arm posture and movement. Perceived work stress and tension were recorded on visual analog scales (VAS) every hour. Trapezius sEMG activity was low in seated posture [group median 1.8 and 0.9% of activity at maximal voluntary contraction (%EMGmax) for dominant and non-dominant side] and was elevated in standing (3.0 and 2.5% EMGmax) and walking (3.9 and 3.4% EMGmax). In seated posture (mean duration 79% of workday) arm movement consistently influenced trapezius activity, accounting for approximately 20% of intra-individual variation in trapezius activity. Arm elevation was on average not associated with trapezius activity when seated; however, considerable individual variation was observed. There was no indication of increase in trapezius activity or more sustained activity pattern, nor in heart rate, in high-stress versus low-stress periods, comparing periods with seated posture for the subjects reporting contrasts of at least two VAS units in stress (n=16) or tension (n=14) score.

Prospective research on musculoskeletal disorders in office workers (PROMO): study protocol

Background

This article describes the background and study design of the PROMO study (Prospective Research on Musculoskeletal disorders in Office workers). Few longitudinal studies have been performed to investigate the risk factors responsible for the incidence of hand, arm, shoulder and neck symptoms among office workers, given the observation that a large group of office workers might be at risk worldwide. Therefore, the PROMO study was designed. The main aim is to quantify the contribution of exposure to occupational computer use to the incidence of hand, arm, shoulder and neck symptoms. The results of this study might lead to more effective and/or cost-efficient preventive interventions among office workers.

Methods/Design

A prospective cohort study is conducted, with a follow-up of 24 months. In total, 1821 participants filled out the first questionnaire (response rate of 74%). Data on exposure and outcome is collected using web-based self-reports. Outcome assessment takes place every three months during the follow-up period. Data on computer use are collected at baseline and continuously during follow-up using a software program.

Discussion

The advantages of the PROMO study include the long follow-up period, the repeated measurement of both exposure and outcome, and the objective measurement of the duration of computer use. In the PROMO study, hypotheses stemming from lab-based and field-based research will be investigated.

Effects of 30 versus 60 min of low-load work on intramuscular lactate, pyruvate, glutamate, prostaglandin E(2) and oxygenation in the trapezius muscle of healthy females

The aim of this study was to investigate the effects of duration of low-load repetitive work on intramuscular lactate, pyruvate, glutamate and prostaglandin E(2) (PGE(2)), and oxygen saturation in the trapezius muscle. Twenty healthy females were studied during baseline rest, during low-load repetitive work for either 30 (REP 30) or 60 min (REP 60) and 60 min recovery. Intramuscular microdialysate (IMMD) samples were obtained, and local muscle tissue oxygenation (%StO(2)) assessed with near-infrared spectroscopy (NIRS). Subjects rated their perceived exertion (Borg CR-10 scale) and capillary blood was sampled for lactate analysis. The results showed a significant increase in IMMD lactate in response to both REP 30 and REP 60 (P < 0.05 and P < 0.01, respectively) and glutamate (P < 0.0001), but no progressive increase with increasing work duration. Both IMMD pyruvate and lactate tended to be significantly increased during the recovery period. No corresponding increase in blood lactate was found. Local muscle %StO(2) did not change significantly in response to work and was not correlated to the IMMD lactate concentration. The ratings of perceived exertion increased in response to work, and remained increased after recovery for REP 60. In conclusion, the results of this study show significantly increased IMMD lactate and, glutamate concentrations in the trapezius muscle of healthy females in response to low-load work, but no progressive increase with increased work duration. Further, they do not indicate that the increased IMMD lactate concentration was caused by a locally decreased or insufficient muscle tissue oxygenation.

Basis for an FCE methodology for patients with work-related upper limb disorders

A reported reduction in work-related functional capacity in Work-related Upper Limb Disorders (WRULD) patients is among the most common problems in WRULD. The extent to which this reduction in functional capacity can be objectified remains unknown. A validated instrument to test functional capacity in this patient group is unavailable. The objective of this study was to design a Functional Capacity Evaluation (FCE) for WRULD patients working with Visual Display Units (VDU) and provide evidence for content validity. A review to epidemiological literature was conducted to identify physical risk factors for VDU-related WRULD. The results indicate that physical risk factors were related to repetition, duration, working in awkward and static positions and forceful movements of the upper extremity and neck. An FCE was designed based on the risk factors identified. Eight tests were selected to cover all risk factors: the overhead lift, overhead work, repetitive reaching, handgrip strength, finger strength, wrist extension strength, fingertip dexterity, and a hand and forearm dexterity test. Content validity of this FCE was established by providing the rationale, specific objectives and operational definitions of the FCE. Further research is needed to establish reliability and other aspects of validity of the WRULD FCE.

Pathophysiology of upper extremity muscle disorders

A review of the literature on the pathophysiology of upper extremity muscle disorders (UEMDs) was performed. An overview is given of clinical findings and hypotheses on the pathogenesis of UEMDs. The literature indicates that disorders of muscle cells and limitations of the local circulation underlie UEMDs. However, these disorders identified do not necessarily lead to symptoms. The following mechanisms have been proposed in the literature: (1) selective recruitment and overloading of type I (Cinderella) motor units; (2) intra-cellular Ca(2+) accumulation; (3) impaired blood flow; (3b) reperfusion injury; (3.3c) blood vessel-nociceptor interaction; (4a) myofascial force transmission; (4b) intramuscular shear forces; (5) trigger points; (6) impaired heat shock response. The results of the review indicate that there are multiple possible mechanisms, but none of the hypotheses forms a complete explanation and is sufficiently supported by empirical data. Overall, the literature indicates that: (1) sustained muscle activity, especially of type I motor units, may be a primary cause of UEMDs; (2) in UEMDs skeletal muscle may show changes in morphology, blood flow, and muscle activity; (3) accumulation of Ca(2+) in the sarcoplasm may be the cause of muscle cell damage; (4) it seems plausible that suboptimal blood flow plays a role in pathogenesis of UEMDs; (5) since the presence of fiber disorders is not a sufficient condition for the development of UEMSDs additional mechanisms, such as sensitization, are assumed to play a role.

Kinin peptides in human trapezius muscle during sustained isometric contraction and their relation to pain

To determine the muscular concentration of bradykinin and kallidin during static contraction, microdialysis probes were implanted bilaterally in the trapezius muscles of healthy women. Three hours after probe implantation, 200 μM of the angiotensin-converting enzyme (ACE) inhibitor enalaprilat were added to the perfusion solution in one of the sides for 30 min. Thirty minutes later, the subjects performed a sustained bilateral shoulder abduction at 10% of the maximal voluntary contraction until exhaustion. This protocol was repeated twice, with an interval of at least 17 days. High intersession repeatability was observed in the concentration of bradykinin but not of kallidin. Enalaprilat induced a significant increase in bradykinin levels in the dialysate, without affecting kallidin levels. The sustained contraction induced a significant increase in dialysate levels of both kinin peptides. The contraction also induced a significant increase in pain ratings, as measured by a visual analog scale. During contraction, positive correlations were found between pain ratings and levels of kinin peptides in dialysate, predominantly in the side previously perfused with enalaprilat. Subjects with the higher pain ratings also showed larger increases in kinin peptides in the side previously perfused with enalaprilat. The present results show that both plasma and tissue kinin-kallikrein are activated during muscle contraction, but that their metabolic pathways are differently regulated during rest and contraction, because they showed a different response to ACE inhibition. They also indicate that intramuscular kinin peptides levels, and ACE activity, may contribute to muscle pain.

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.