Trapezius muscle load as a risk indicator for occupational shoulder-neck complaints

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.

Quantification of physical stress experienced by obstetrics and gynecology sonographers: A comparative study of two ultrasound devices

This study aims to quantify the stresses of sonographers using two different ultrasound devices, one of conventional and one of ergonomic design. A total of 20 obstetricians and gynecologists participated in this study, and two types of tasks (scanning and positioning) were evaluated while using the two different devices. To quantify workload, four dependent variables (muscle activity, estimated grip force, subjective comfort rating, and task time) were measured. The muscular activity required while using the conventional device was 14.4% MVC (Maximum voluntary contraction) for the scanning task, which was significantly higher than that of the ergonomic device. The subjective comfort rating for the conventional design was lower than that of the ergonomic design. For the positioning task, the ergonomic device (33.2% MVC) resulted in significantly higher muscle activity in the extensor digitorum (ED) and flexor digitorum superficialis (FDS) than the conventional design (22.2% MVC), whereas the deltoid muscle showed significantly lower activity than in users of conventional design (4.5% MVC). Ergonomically-designed ultrasound devices improve ease of moving and the probe's supporters, reduce physical load and increase ease of use for sonographers. Our results may be used as guidelines for usability testing of ultrasound devices.

Surgeons' posture and muscle strain during laparoscopic and robotic surgery

Background

It is assumed that conventional laparoscopy (LAP) and robotic-assisted laparoscopic surgery (RALS) differ in terms of the surgeon's comfort. This study compared muscle workload, work posture and perceived physical exertion of surgeons performing LAP or RALS.

Methods

Colorectal surgeons with experience in advanced LAP and RALS performed one of each operation. Bipolar surface electromyography (EMG) recordings were made from forearm, shoulder and neck muscles, and expressed relative to EMG maximum (%EMGmax). The static, median and peak levels of muscle activity were calculated, and an exposure variation analysis undertaken. Postural observations were carried out every 10 min, and ratings of perceived physical exertion before and after surgery were recorded.

Results

The study included 13 surgeons. Surgeons performing LAP showed higher static, median, and peak forearm muscle activity than those undertaking RALS. Muscle activity at peak level was higher during RALS than LAP. Exposure variation analysis demonstrated long-lasting periods of low-level intensity muscle activity in the shoulders for LAP, in the forearms for RALS, and in the neck for both procedures. Postural observations revealed a greater need for a change in work posture when performing LAP compared with RALS. Perceived physical exertion was no different between the surgical modalities.

Conclusion

Minimally invasive surgery requires long-term static muscle activity with a high physical workload for surgeons. RALS is less demanding on posture.

Effects of using a document holder when typing on head excursion and neck muscle activity among computer users with and without neck pain

Purpose. The aim of this study was to examine the effects of using a document holder while typing on head excursion and neck muscle activity among computer users with and without neck pain. Method. An experimental study including 52 individuals with (n = 26) and without (n = 26) neck pain was conducted. Head excursion and neck muscle activity were measured using an accelerometer and surface electromyography, respectively. Two-way analysis of variance was conducted to examine the effects of using a document holder between computer users with and without neck pain. Results. The results demonstrated a decrease in head excursion (p < 0.001) and muscle activity of both the right and left upper trapezius and the left lower trapezius and right anterior deltoid (p < 0.05) with the use of a document holder. Computer users with neck pain had significantly (p < 0.001) higher right lower trapezius muscle activity compared to the group without neck pain. Conclusion. The results of the study supported the use of a document holder to assist in decreasing head excursion and neck muscle activity. The use of a document holder while typing may be beneficial in preventing repetitive strain injuries among computer users.

An investigation into four different sit-stand workstation use schedules

Twelve office workers participated in a study investigating effects of four sit/stand schedules (90-min sit/30-min stand, 80/40, 105/15, and 60/60) via several objective and subjective measures (muscle fatigue, foot swelling, spinal shrinkage, and self-reported discomfort). Results showed that there were no significant differences in shoulder and low back static muscle activities between sitting and standing. Muscle fatigue was developed during workday under all schedules. The longest standing schedule seemed to have a tendency of reducing muscle fatigue. None of the schedules helped or worsened foot swelling and spinal shrinkage. More active break-time activities seemed reducing muscle fatigue and foot swelling. While the self-reported bodily discomfort levels were generally low, the preferred schedules among the participants were varied, although the least standing schedule was the least preferred. We may conclude that effects of using sit-stand workstation to improve musculoskeletal health may be limited but promoting more active break-time activities can help. Practitioner Summary: Sit-stand workstations are used to reduce work-related musculoskeletal disorders. This study shows that office workers prefer sit/stand durations in the range between 1:1 and 3:1. Longer standing may have the potential to reduce muscle fatigue. However, active break-time activities may be more effective in reducing muscle fatigue and foot swelling.

Part I: Participatory Ergonomics Approach to Waste Container Handling Utilizing a Multidisciplinary Team

This multidisciplinary team approach to waste container handling, developed within the Grassroots Ergonomics process, presents participatory ergonomic interpretations of quantitative and qualitative aspects of this process resulting in a peer developed training. The lower back, shoulders, and wrists were identified as frequently injured areas, so these working postures were a primary focus for the creation of the workers' training. Handling procedures were analyzed by the team to identify common cycles involving one 5 gallon (60 pounds), two 5 gallons (60 and 54 pounds), 30 gallon (216 pounds), and 55 gallon (482 pounds) containers: lowering from pallet, transporting to/from transport vehicles, loading/unloading on transport vehicles, and loading onto pallet. Eleven experienced waste container handlers participated in this field analysis. Ergonomic exposure assessment tools measuring these field activities included posture analysis, posture targeting, Lumbar Motion Monitor™(LMM), and surface electromyography (sEMG) for the erector spinae, infraspinatus, and upper trapezius muscles. Posture analysis indicates that waste container handlers maintained non-neutral lower back postures (flexion, lateral bending, and rotation) for a mean of 51.7% of the time across all activities. The right wrist was in non-neutral postures (radial, ulnar, extension, and flexion) a mean of 30.5% of the time and the left wrist 31.4%. Non-neutral shoulder postures (elevation) were the least common, occurring 17.6% and 14.0% of the time in the right and left shoulders respectively. For training applications, each cycle had its own synchronized posture analysis and posture target diagram. Visual interpretations relating to the peak force modifications of the posture target diagrams proved to be invaluable for the workers' understanding of LMM and sEMG results (refer to Part II). Results were reviewed by the team's field technicians and their interpretations were developed into ergonomic training that address the issues originally raised. This training includes intervention methods, ergonomic tools used, data acquired, and effects of waste container handling techniques on lower back, shoulder, and wrists and methods to help proactively reduce injuries associated with this profession.

Surface EMG Recordings as a Risk Assessment Tool for Musculoskeletal Disorders

Injury models for musculoskeletal disorders are considered. It is suggested that several injury mechanisms may coexist in the same body region. This has implications for data reduction procedures used in risk evaluation by surface EMG (SEMG). Biomechanical exposure as an injury risk may conceptually be characterized by the three exposure dimensions: amplitude, duration and repetitiveness. The time dimension has become increasingly important since many work tasks are performed at low exposure amplitude. For practical purposes, static activity levels higher than 2% EMGmax appear to signify increased risk of musculoskeletal disorders. However, a lower activity level does not guarantee good health, as risk factors apparently unrelated to muscle activity also exist. Mental stress may be one such risk factor.

Experimental Induction of Muscle Tenderness and Headache in Tension-Type Headache Patients

The aim of the present study was to investigate the impact of static contraction of the shoulder and neck muscles on muscle tenderness and headache in patients with tension-type headache. Twenty patients with frequent episodic tension-type headache and 20 healthy age- and sex-matched controls were examined using a placebo-controlled cross-over design. The subjects performed static contraction of the trapezius muscles (active procedure) or the anterior tibial muscles (placebo procedure) with 10% of maximal force for 30 min. Total tenderness score, local tenderness score and headache intensity were evaluated before and after the static work. Changes in headache intensity were followed for 24 h. Pericranial tenderness increased significantly more in patients than in controls after the active procedure ( P = 0.04). The increase in pericranial tenderness tended to be higher after the active procedure than after the placebo procedure in patients ( P = 0.08) and in controls ( P = 0.07). Sixty per cent of the patients and 20% of the healthy controls developed headache after the active procedure. Fifty per cent of the patients and none of the controls developed headache after the placebo procedure. There was no significant difference in headache development between the active and the placebo procedure in patients or controls. These findings demonstrate that tension-type headache patients are more liable to develop shoulder and neck pain in response to static exercise than healthy controls.

Seated postural neck and trunk reactions to sideways perturbations with or without a cognitive task

Driving on irregular terrain will expose the driver to sideways mechanical shocks or perturbations that may cause musculoskeletal problems. How a cognitive task, imposed on the driver, affects seated postural reactions during perturbations is unknown. The aim of the present study was to investigate seated postural reactions in the neck and trunk among healthy adults exposed to sideways perturbations with or without a cognitive task. Twenty-three healthy male subjects aged 19-36 years, were seated on a chair mounted on a motion system and randomly exposed to 20 sideways perturbations (at two peak accelerations 5.1 or 13.2m/s(2)) in two conditions: counting backwards or not. Kinematics were recorded for upper body segments using inertial measurement units attached to the body and electromyography (EMG) was recorded for four muscles bilaterally in the neck and trunk. Angular displacements (head, neck, trunk and pelvis) in the frontal plane, and EMG amplitude (normalised to maximum voluntary contractions, MVC) were analysed. The cognitive task provoked significantly larger angular displacements of the head, neck and trunk and significantly increased EMG mean amplitudes in the upper neck during deceleration, although 10% of MVC was never exceeded. A cognitive task seems to affect musculoskeletal reactions when exposed to sideways perturbations in a seated position.

Physical workload, trapezius muscle activity, and neck pain in nurses' night and day shifts: a physiological evaluation

The purpose of this study was to compare physical workload, electromyography (EMG) of the trapezius muscle, neck pain and mental well-being at work between night and day shifts in twenty Swiss nurses. Work pulse (average increase of heart rate over resting heart rate) was lower during night (27 bpm) compared to day shifts (34 bpm; p < 0.01). Relative arm acceleration also indicated less physical activity during night (82% of average) compared to day shifts (110%; p < 0.01). Rest periods were significantly longer during night shifts. Trapezius muscle rest time was longer during night (13% of shift duration) than day shifts (7%; p < 0.01) and the 50th percentile of EMG activity was smaller (p = 0.02), indicating more opportunities for muscle relaxation during night shifts. Neck pain and mental well-being at work were similar between shifts. Subjective perception of burden was similar between shifts despite less physical burden at night, suggesting there are other contributing factors.

Variation in muscle activity among office workers when using different information technologies at work and away from work

OBJECTIVE

To determine differences in muscle activity amplitudes and variation of amplitudes when using different information and communication technologies (ICT).

BACKGROUND

Office workers use different ICT to perform tasks. Upper body musculoskeletal complaints are frequently reported by this occupational group. Increased muscle activity and insufficient variation are potential risk factors for musculoskeletal complaints.

METHOD

Muscle activity of right and left upper trapezius and right wrist extensor muscle bundle (extensor carpi radialis longus and brevis) of 24 office workers (performing their usual tasks requiring different ICT at work and away from work) were measured continuously over 10 to 12 hours. Muscle activity variation was quantified using two indices, amplitude probability distribution function and exposure variation analysis.

RESULTS

There was a trend for electronics-based New ICT tasks to involve less electromyography (EMG) variation than paper-based Old ICT tasks. Performing Combined ICT tasks (i.e., using paper- and electronics-based ICT simultaneously) resulted in the highest muscle activity levels and least variation; however, these Combined ICT tasks were rarely performed. Tasks involving no ICT (Non-ICT) had the greatest muscle activity variation.

CONCLUSION

Office workers in this study used various ICT during tasks at work and away from work. The high EMG amplitudes and low variation observed when using Combined ICT may present the greatest risk for musculoskeletal complaints, and use of Combined ICT by workers should be kept low in office work. Breaking up combined, New, and Old ICT tasks, for example, by interspersing highly variable Non-ICT tasks into office workers' daily tasks, could increase overall muscle activity variation and reduce risk for musculoskeletal complaints.

Predicting maximum acceptable efforts for repetitive tasks: an equation based on duty cycle

OBJECTIVE

The objective was to develop an equation, for repetitive tasks, that uses frequency and/or duty cycle (DC) to predict maximum acceptable efforts (MAE) relative to maximum voluntary efforts (MVE).

BACKGROUND

Ergonomists must determine acceptable physical demands for a wide variety of tasks. Although a large database exists in the literature for maximum single-effort strength, far fewer repetitive tasks have psychophysical and/or physiological data available to guide the prediction of acceptable submaximal, repeated efforts.

METHOD

DC represents the total effort duration divided by the cycle time. MAEs were calculated by dividing average psychophysics-based acceptable loads by corresponding single-effort maximum strength using 69 values from studies of the upper extremities. The author developed an equation to characterize the relationship between MAE and DC.

RESULTS

The resulting equation had DC taken to the exponent 0.24, and it predicted MAE very well (r2 = 0.87%, root mean square [RMS] difference = 7.2% of the maximum strength). At higher DC values, the equation also demonstrated good agreement with the published physiological data.

CONCLUSION

The limited psychophysical database in the literature makes it difficult for ergonomists and engineers to recommend acceptable efforts for the large variety of repetitive tasks they evaluate. However, the proposed equation now allows for a correction of the large strength database to estimate acceptable force and torque limits for repetitive occupational tasks.

APPLICATION

The proposed equation will have wide applications for ergonomic practitioners performing evaluations of repetitive tasks.

Bias and imprecision in posture percentile variables estimated from short exposure samples

Background

Upper arm postures are believed to be an important risk determinant for musculoskeletal disorder development in the neck and shoulders. The 10^th and 90^th percentiles of the angular elevation distribution have been reported in many studies as measures of neutral and extreme postural exposures, and variation has been quantified by the 10^th-90^th percentile range. Further, the 50^th percentile is commonly reported as a measure of "average" exposure. These four variables have been estimated using samples of observed or directly measured postures, typically using sampling durations between 5 and 120 min.

Methods

The present study examined the statistical properties of estimated full-shift values of the 10^th, 50^th and 90^th percentile and the 10^th-90^th percentile range of right upper arm elevation obtained from samples of seven different durations, ranging from 5 to 240 min. The sampling strategies were realized by simulation, using a parent data set of 73 full-shift, continuous inclinometer recordings among hairdressers. For each shift, sampling duration and exposure variable, the mean, standard deviation and sample dispersion limits (2.5% and 97.5%) of all possible sample estimates obtained at one minute intervals were calculated and compared to the true full-shift exposure value.

Results

Estimates of the 10^th percentile proved to be upward biased with limited sampling, and those of the 90^th percentile and the percentile range, downward biased. The 50^th percentile was also slightly upwards biased. For all variables, bias was more severe with shorter sampling durations, and it correlated significantly with the true full-shift value for the 10^th and 90^th percentiles and the percentile range. As expected, shorter samples led to decreased precision of the estimate; sample standard deviations correlated strongly with true full-shift exposure values.

Conclusions

The documented risk of pronounced bias and low precision of percentile estimates obtained from short posture samples presents a concern in ergonomics research and practice, and suggests that alternative, unbiased exposure variables should be considered if data collection resources are restricted.

Comparison of the cortisol awakening response in women with shoulder and neck pain and women with fibromyalgia

Shoulder and neck pain (SNP) and fibromyalgia syndrome (FMS), two musculoskeletal conditions of unknown pathogenesis, share some common features in terms of altered neuroendocrine responses, pain and stress perception. However, the pain distribution in SNP is localized, whereas in FMS is more widespread. Because regional musculoskeletal pain may represent an intermediate stage along a continuum towards widespread musculoskeletal pain we compared the cortisol awakening response (CAR) in women with SNP with the CAR in FMS patients and healthy controls (HC) in a controlled hospital-hotel setting. The aim of the study was to investigate whether SNP is related to a deviant regulation of the hypothalamic-pituitary-adrenal (HPA) axis. Eighteen women with SNP, 29 female FMS patients, and 27 female HC participated in the study. Cortisol samples were collected upon awakening, 30 and 60 min later. Questionnaires measuring pain levels, sleeping problems, perceived stress, and psychological characteristics were administered to the participants. Compared with HC, women with SNP had a tendency towards higher cortisol levels, whereas FMS had lower cortisol levels. Adjustment for potential confounders did not influence the results. Women with SNP and FMS patients reported more health complaints, pain, and perceived stress than the HC, but women with SNP were less affected than the FMS patients. Women with SNP showed a tendency towards an elevated HPA axis activity compared with HC. The current findings may indicate that the hypercortisolism in regional musculoskeletal pain represent an intermediate stage towards the development of a hypocortisolism in widespread musculoskeletal pain.

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.

Comparison of biomechanical loading during use of conventional stud welding equipment and an alternate system

We investigated the effect of an alternative welding system designed to reduce exposure to extreme trunk flexion on measures of trunk inclination and muscle activity. Among 10 participants, data were collected while using conventional stud welding equipment and while using the alternate system. Paired t-tests were used to compare results between the two welding systems. Mean trunk inclination angle was reduced with the alternate system (34.4° versus 9.7°, p < 0.01). Percent time with trunk inclination angles greater than 60° was also reduced (40.0% versus 4.7%, p < 0.01). In general, the alternate system resulted in less desirable upper trapezius muscle activity levels. The alternate system appears to be effective in reducing exposure to extreme trunk flexion among stud welders. Continued development of the system should explore features designed to reduce shoulder forces and improve productivity.

Effects of postural and visual stressors on myofascial trigger point development and motor unit rotation during computer work

Musculoskeletal complaint rates are high among those performing low-level static exertions (LLSEs), such as computer users. However, our understanding of the causal mechanisms is lacking. It was hypothesized that myofascial trigger point (MTrP) development might be one causal mechanism to help explain these complaints and that static postural and visual demands may be contributing factors. Therefore, the purpose of this experiment was to examine MTrP development and the behavior of multiple parts of the trapezius muscle under postural and mental stress (represented by visual stress) conditions during computer work. Twelve subjects (six male and six female) were monitored for MTrP development via expert opinion, subject self-report, and cyclic changes in EMG median frequency across fourteen spatial locations. Results showed that MTrPs developed after one hour of continuous typing, despite the stress condition. Interestingly, both the high postural and high visual stress conditions resulted in significantly fewer median frequency cycles (3.76 and 5.35 cycles, respectively), compared to the baseline low stress condition (6.26 cycles). Lastly, the MTrP location as well as locations more medial to the spine showed significantly fewer cycles than other locations. Findings suggest that MTrPs may be one causal pathway for pain during LLSEs and both postural and visual demands may play a role in muscle activation patterns, perhaps attributing to MTrP development and resultant discomfort.

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.

Evidence-based guidelines for the wise use of computers by children: physical development guidelines

Computer use by children is common and there is concern over the potential impact of this exposure on child physical development. Recently principles for child-specific evidence-based guidelines for wise use of computers have been published and these included one concerning the facilitation of appropriate physical development. This paper reviews the evidence and presents detailed guidelines for this principle. The guidelines include encouraging a mix of sedentary and whole body movement tasks, encouraging reasonable postures during computing tasks through workstation, chair, desk, display and input device selection and adjustment and special issues regarding notebook computer use and carriage, computing skills and responding to discomfort. The evidence limitations highlight opportunities for future research. The guidelines themselves can inform parents and teachers, equipment designers and suppliers and form the basis of content for teaching children the wise use of computers. STATEMENT OF RELEVANCE: Many children use computers and computer-use habits formed in childhood may track into adulthood. Therefore child-computer interaction needs to be carefully managed. These guidelines inform those responsible for children to assist in the wise use of computers.

Association between numbers of long periods with sustained low-level trapezius muscle activity and neck pain

The purpose of this prospective cohort study was to evaluate relationships between sustained low-level muscle activity (SULMA) in the neck and pain after 1 year among machine operators of forest harvesters (n = 19), forwarders (n = 20) and forest researchers (n = 20). Surface electromyography of the right upper trapezius muscle was measured during one working day. Continuous muscle activity (SULMA periods) were analysed in predetermined time intervals. Neck pain was assessed by the Borg's category-ratio scale and the Standardized Nordic Questionnaire (dichotomised into pain duration <or=30 or >30 d). Harvesters reported significantly more pain than researchers. A higher number of long SULMA periods >8 min duration increased the risk of neck pain >30 d during the successive year (odds ratio 3.0, 95% CI 1.2-7.8). Perceived personal economy above average was associated with less pain, while other potential confounders or intermediate variables were not significant. Low-level trapezius muscle activity in periods longer than 8 min may constitute a risk for neck pain.

A method to quantify frequency and duration of sustained low-level muscle activity as a risk factor for musculoskeletal discomfort

AIM

The purpose of this paper was to describe and evaluate different aspects of muscle activity patterns associated with musculoskeletal discomfort/pain.

METHOD

Surface electromyography (sEMG) of the right upper trapezius and the right extensor digitorum muscles was conducted continuously during one working day in 19 male forest machine operators driving harvesters, 20 driving forwarders and 20 researchers at the Forest Research Institute. Perceived discomfort/pain in the right side of the neck and the right forearm was rated morning, noon and afternoon with Borg's CR-10 scale. Static, median and peak levels of muscle activity were analyzed and the number and total duration of EMG gaps (muscular rest) were calculated. Sustained low-level muscle activity (SULMA) was defined as continuous muscle activity above 0.5% of the maximal EMG activity quantified into 10 periods of predetermined duration intervals from 1.6 to 5s up to above 20 min. The number of SULMA periods is presented within each interval and as cumulative periods above the already determined levels. The operators handled control levers seated in a fixed position while the researchers performed mainly PC work and other varied tasks.

RESULTS

A positive correlation was found between discomfort/pain in the right upper trapezius muscle region in the afternoon and cumulative SULMA periods above 10 min duration, and a negative correlation to cumulative SULMA periods also including the short durations. No specified patterns were found for discomfort/pain in the right extensor digitorum or for the other EMG measurements. All EMG measurements distinguished to some extent between the occupational groups, especially between machine operators driving harvesters and researchers.

CONCLUSIONS

Number of SULMA periods longer than 10 min per hour was positively correlated, and predominantly short periods were negatively correlated, to complaints in the neck region. This seems promising in order to find duration limits for sustained low-level muscle activity as a risk factor for musculoskeletal disorders.

Applications of biomechanics for prevention of work-related musculoskeletal disorders

This paper summarises applications of biomechanical principles and models in industry to control musculoskeletal disorders of the low back and upper extremity. Applications of 2-D and 3-D biomechanical models to estimate compressive force on the low back, the strength requirements of jobs, application of guidelines for overhead work and application of strain index and threshold limit value to address distal upper extremity musculoskeletal disorders are presented. Several case studies applied in the railroad industry, manufacturing, healthcare and warehousing are presented. Finally, future developments needed for improved biomechanical applications in industry are discussed. The information presented will be of value to practising ergonomists to recognise how biomechanics has played a significant role in identifying causes of musculoskeletal disorders and controlling them in the workplace. In particular, the information presented will help practising ergonomists with how physical stresses can be objectively quantified.

Muscular load characterization during isometric shoulder abductions with varying force

This study sought to characterize muscle loading and fatigue during static shoulder abductions with varying force. In a supine posture, participants maintained fixed shoulder abductions against a time-varying external resistance, generated by a dynamometer-spring mechanism. Patterns (cumulative distribution) of the external resistance were varied by selecting different 10th and 90th percentiles of the distribution. Dynamometer angular velocities were also varied, to reflect different rates of cyclic muscle contraction. The degree of local fatigue development was assessed by common measures, including endurance time, strength reduction, and perceived discomfort. Myoelectric (EMG) signals were continuously obtained from the middle deltoid muscle throughout experimental exercise (60min max). Changes in EMG root-mean-square (RMS) and spectral measures (derived from 1-s windows at peaks in the cyclic contractions) were used as manifestations of muscle fatigue. For each minute, the RMS signal was further reduced using two methods, the cumulative probability distribution of EMG (CPDE) and exposure variation analysis (EVA). The former resulted in three percentile values (10th, 50th, and 90th), whereas the latter method resulted in 10 different measures (grouped by EMG activity level and duration). A main finding of the study was the applicability of several common fatigue indicators for these cyclic, repetitive exertions. Overall, the use of CPDE and EVA to characterize task differences and predict muscle fatigue was found to have limited value.

Electromyographic assessment of apple bucket intervention designed to reduce back strain

The authors previously developed an apple bucket that was modified by use of a hip belt to reduce muscle fatigue. The intervention of belt use was accepted by workers and shown not to interfere with productivity. However, use of this intervention did not appear to reduce muscle fatigue when measured by tests of voluntary muscle strength. The purpose of the present study was to evaluate the intervention's effect on muscle fatigue employing surface electromyographic (EMG) amplitude. Amplitude measurements on 15 muscles were taken from 10 laboratory volunteers who were carrying a full bucket of apples, once while wearing the intervention belt and once without the intervention. These measurements were taken for seven different postures (four angles of trunk flexion (0 degrees , 20 degrees , 45 degrees , 90 degrees ) and three raised-arm positions (both up, dominant up, non-dominant up)) common to apple harvest work. Participants were measured in these conditions both with the bucket carried in front and with the bucket carried to the side. Significant reductions in amplitude favouring the intervention were seen for 11 of the 15 muscles in models considering the four body flexion angles. Ten of these were of the middle and lower back. These control/intervention differences were seen with both bucket-carrying positions (front vs. side) and tended to increase with increasing flexion angle. In contrast, no significant intervention effects were observed in models considering treatment by arm-raised position. One significant main effect (upper trapezius, side bucket) showed an amplitude reduction in the treatment condition. Another main effect showing increased amplitude in the intervention condition use was observed in the dominant levator scapulae (side bucket). Thus, the use of the intervention belt reduces EMG amplitude among a number of mid- and lower-back muscles. This is suggestive of a protective effect against back strain.

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.

Effect of joystick stiffness, movement speed and movement direction on joystick and upper limb kinematics when using hydraulic-actuation joystick controls in heavy vehicles

Despite the widespread use of hydraulic-actuation joysticks in mobile North American construction, mining and forestry vehicles, the biomechanical effects that joysticks have on their human operators has not been studied extensively. Using nine unskilled joystick operators and a laboratory mock-up with a commonly used North American heavy off-road equipment hydraulic-actuation joystick and operator seat, the purpose of this work was to quantify and compare the effects of three hydraulic-actuation joystick stiffnesses and two movement speeds on upper limb and joystick kinematics as one of the initial steps towards the development of a hydraulic-actuation joystick design protocol. In addition to providing a detailed description of the kinematics of a constrained occupational task, coupled with the corresponding effects of the task on operator upper limb kinematics, results from principal component analysis and ANOVA procedures revealed a number of differences in joystick and upper limb angle ranges and movement curve shapes resulting from the various joystick stiffness-speed combinations tested. For the most part, these joystick motion alterations were caused by small, insignificant changes in one or more upper limb joint angles. The two exceptions occurred for forward movements of the joystick; the fast speed - light stiffness condition movement pattern shape change was caused primarily by an alteration of the elbow flexion-extension movement pattern. Similarly, the fast speed - normal stiffness condition movement curve shape perturbation - was caused principally by a combination of significant movement curve shape alterations to elbow flexion-extension, external-internal shoulder rotation and flexion-extension of the shoulder. The finding that joystick stiffness and speed alterations affect joystick and upper limb kinematics minimally indicates that the joystick design approach of modelling the joystick and operator upper limb as a closed linkage system should be pursued. This approach would allow one to simulate the upper limb and joystick kinematics that result from virtual changes to upper limb and joystick lengths.

Relationship between perceived exertion and mean power frequency of the EMG signal from the upper trapezius muscle during isometric shoulder elevation

The aim of the study was to investigate the relationship between a fatigue-induced increase of perceived exertion in the neck with a decrease of mean power frequency (MPF) in the surface electromyography (sEMG) signal during repeated shoulder elevation endurance tasks. About Thirty-two healthy women (age range 20-62) performed two maximum 6-min shoulder elevation endurance tasks at 30% of their maximal voluntary contraction (MVC) level, separated by a rest of 6 min. During these exercises, perceived exertion was estimated using the Borg scale (range 0-10), whereas the MPF of the sEMG signal from the upper trapezius was simultaneously detected. Linear regression analysis was applied over time for each trial and subject for both MPF and Borg scale rating values. The MPF was normalized by the intercept of the linear regression analysis. The resulting slopes of normalized mean power frequency (nMPF) and Borg scale rating were correlated with each other by linear regression for both trials. In order to investigate the individual behavior of fatigue effects between trials, Delta (trial 2-trial 1) slopes of nMPF and Borg scale ratings were calculated for each subject. These slopes of nMPF and Borg scale ratings were correlated with each other as well by linear regression. The increase of Borg scale ratings, as well as the decrease of nMPF, were significantly higher in trial 2 than trial 1 (P<0.01). The results show a linear correlation between slopes of nMPF and Borg scale ratings for both trials 1 and 2 (r=0.76, P<0.01). Trial-to-trial slopes (Delta (trial 2-trial 1)) of nMPF and Borg scale rating, were also significantly correlated (r=0.68, P<0.05). Thus, the individually sensed increase of perceived exertion in the neck during trial 2 was accompanied by a simultaneously higher detected decrease of nMPF. These findings indicate a close relationship between subjective perception of exertion in the neck and objectively assessed muscle fatigue of the upper trapezius.

Normalization procedures using maximum voluntary isometric contractions for the serratus anterior and trapezius muscles during surface EMG analysis

The serratus anterior and trapezius muscles are considered to be the only upward rotators of the scapula and are very important for normal shoulder function. A variety of methods have been used to produce a maximum voluntary isometric contraction (MVIC) of these muscles for normalization of EMG data. The purpose of this study was to quantify the surface EMG activity of the serratus anterior muscle and the upper, middle, and lower parts of the trapezius during 9 manual muscle tests performed with maximum effort in 30 subjects. It was found that no one muscle test produced a MVIC for all individuals. Therefore, to perform normalization within each subject, it is suggested that the 2 or 3 tests identified in this study that produce high levels of EMG activity for each muscle be performed. The scapular protraction muscle test that is often used to normalize data for the serratus anterior muscle produced relatively low levels of EMG activity and was not found to be an optimal test. Muscle tests in which an attempt was made to de-rotate the scapula from an upwardly rotated position produced much higher levels of EMG activity in the serratus anterior muscle.

An integrated analysis of ergonomics and time consumption in Swedish 'craft-type' car disassembly

Car disassembly is at the edge of extensive rationalization due to increased legislative demands for recycling. This study focused on (1) assessing current mechanical exposures (physical work loads) for comparison with future rationalised systems, with particular emphasis on time aspects, (2) analysing disassembly work in terms of time consumption and exposures in constituent tasks as defined by a loss analysis technique, and (3) predicting the consequences of car disassembly rationalisation for mechanical exposures. The study showed that disassembly implied pronounced circulatory loads, and that more walking and higher lumbar peak loads were found than in studies of assembly work. Value-adding tasks comprised 30% of the total working time, and implied higher postural exposures for the head, arm, trunk and wrist, as well as less opportunities to recover, as compared to non-value-adding tasks. Organisational-type rationalization can be expected to increase the time spent in value-adding work, thus increasing local exposures for the average worker, while a concurrent increase in mechanization level might reduce circulatory exposures, the amount of walking, and peak lumbar loads.

The effect of sampling frequency on EMG measures of occupational mechanical exposure

The purpose of the present study was to determine how much it is possible to downsample EMG without loosing the accuracy of some EMG measures of occupational mechanical exposure frequently used in the literature. The EMG signals of four muscles (two trapezius, right deltoid and right extensor digitorum) were collected (sampling frequency: 1024 Hz) from 20 subjects while performing a 20 min computer work task. The EMG RMS amplitude was computed from 0.125 s successive time-windows for the original (1024 Hz) and four additional digitally resampled (at 512, 256, 128 and 64 Hz) EMG signals. Three of the most frequently used data reduction methods (1. gaps analysis, 2. amplitude probability distribution function and 3. exposure variation analysis) were used to summarise the 20 min EMG activation profiles. The results from the exposure variation analysis were summarized into three variables using a new method detailed here. In general, the ANOVA for repeated measures demonstrated that a decrease of the sampling frequency significantly changed the EMG measures (relative to the 1024 Hz sampling condition) most of the time at 64 and 128 Hz, occasionally at 256 Hz, but practically never at 512 Hz. An analysis of the maximal errors (relative to the 1024 Hz condition) across all subjects supported these findings. Consequently, it was concluded that 512 Hz is quite conservative and should practically never lead to invalid EMG parameters estimations. Conversely, 256 Hz represents the lowest limit tolerable for some EMG parameters (gaps analysis, amplitude probability distribution function) while it is unacceptable for others (modified exposure variation analysis).

Single motor unit and spectral surface EMG analysis during low-force, sustained contractions of the upper trapezius muscle

Intramuscular and surface electromyographic (EMG) activities were recorded from the left and right upper trapezius muscle of eight healthy male subjects during 5-min long static contractions at 2% and 5% of the maximal voluntary contraction (MVC) force. Intramuscular signals were detected by wire electrodes while surface EMG signals were recorded with linear adhesive electrode arrays. The surface EMG signals were averaged using the potentials extracted from the intramuscular EMG decomposition as triggers. The conduction velocity of single motor units (MUs) was estimated over time from the averaged surface potentials while average rectified value and mean power spectral frequency were computed over time from 0.5 s epochs of surface EMG signal. It was found that (1) MUs were progressively recruited after the beginning of sustained contractions of the upper trapezius muscle at 2% and 5% MVC, (2) the conduction velocity of the MUs active since the beginning of the contraction significantly decreased over time, and (3) although the CV of single MUs significantly decreased, the mean power spectral frequency of the surface EMG did not show a consistent trend over time. It was concluded that spectral surface EMG analysis, being affected by many physiological mechanisms, may show limitations for the objective assessment of localized muscle fatigue during low force, sustained contractions. On the contrary, single motor unit conduction velocity may provide an early indication of changes in muscle fiber membrane properties with sustained activity.

Precision of measurements of physical workload during standardised manual handling. Part I: Surface electromyography of m. trapezius, m. infraspinatus and the forearm extensors

Though surface electromyography (EMG) has been widely used in studies of occupational exposure, its precision in terms of the variance between-days and between-subjects has seldom been evaluated. This study aimed at such an evaluation. Six women performed three different work tasks: 'materials picking', 'light assembly', and 'heavy assembly', repeated on 3 different days. EMG was recorded from m. trapezius, m. infraspinatus and the forearm extensors. Normalisation was made to a maximal (MVE), and a submaximal (RVE), reference contraction. Variance components between days (within subjects) and between subjects were derived for the 10th, 50th and 90th percentiles, as well as for muscular rest parameters. For the task 'heavy assembly', the coefficient of variation between days (CV(BD)) was 8% for m. trapezius (right side, 50th percentile, MVE normalised values). Larger variabilities were found for m. infraspinatus (CV(BD) 15%), and the forearm extensors (CV(BD) 33%). Between-subjects variability (CV(BS)) was greater, 16% for m. trapezius and 57% for m. infraspinatus, 29% for the forearm extensors. RVE normalisation resulted in larger CV(BD), while reducing CV(BS). The between-days and between-subjects variability may be used to optimise sampling strategy, and to assess the bias in epidemiological studies. The bias caused by measurement procedures per se is acceptable.

Will visual discomfort among visual display unit (VDU) users change in development when moving from single vision lenses to specially designed VDU progressive lenses?

PURPOSE

Three types of progressive additions lenses (PAL) specially designed for VDU-work and one single vision lens were compared in a prospective field study. The aim was to investigate if these progressive lenses created a difference in the development of visual discomfort compared to single vision lenses when working on an optimized VDU-workstation.

METHODS

The study had a prospective, parallel group design, with four groups of VDU-workers. Approximately 40 subjects in each group, selected after careful task analysis with special attention towards the visual angles and distances to the work tasks. The groups were followed over one year. A questionnaire concerning visual conditions, working conditions, discomfort in different body areas, the status of the subjects' optometric corrections, psychological factors both at work and at home, amount, frequency and duration of VDU-work etc. was filled in before the intervention, after six months and after one year. No other contact was made with the subjects. The VDU-lenses included were Interview (Essilor), Gradal RD (Zeiss) and Technica (American Optical). Pain intensity and duration were assessed on a 100 mm Visual Analogue Scale (VAS) before the intervention, and six and twelve months after the intervention. All subjects were given a complete optometric examination.

RESULTS

Only small changes in the development of headache and visual discomfort were registered. However, the subjective evaluation of area of clear vision and overall satisfaction was significantly improved for the Interview and Gradal RD lens (p < 0.05). There were no significant changes for Technica and single vision lenses.

CONCLUSION

Lens designs that cover viewing distances from near and out to approximately 2 meters work well compared to lens designs trying to cover greater range of clear vision. When tasks analysis shows that single vision correction may be used, this is still an acceptable solution.

Effect of side dominance on myoelectric manifestations of muscle fatigue in the human upper trapezius muscle

The purpose of this study was to investigate whether differences in the peripheral and control properties of the neuromuscular system due to long-term preferential use, related to side dominance, affect postural muscles, such as the upper trapezius. Therefore, fatigability properties of the upper trapezius muscles of the dominant and non-dominant side were assessed. Surface EMG signals were detected from the upper trapezius muscles of both sides of nine right- and five left-handed subjects with adhesive linear electrode arrays consisting of eight contact points. Static constant force contractions with the arms 90 degrees abducted were performed by the subjects while holding hand loads of 0 kg, 0.5 kg, and 1 kg. Surface EMG spectral and amplitude variables were computed from the recorded signals. EMG spectral variable rate of change (indicating fatigue) showed a statistically significant difference between the two sides, with the dominant side less fatigable than the non-dominant one. The observed differences held for both the right- and left-handed subject group. A possible explanation for the results is that long preferential use of one side with respect to the other leads to changes in muscle fiber membrane and control properties, in agreement with previous results on limb muscles.

Variability in mechanical exposure within and between individuals performing a highly constrained industrial work task

Data on exposure variability is an important remedy for designing and interpreting studies of occupational ergonomics. The present study aimed at retrieving the within- and between-subjects variance of several mechanical exposure parameters in a repeated, short-cycle task. Seven experienced operators repeatedly secured joints using two types of hand-held nutrunners. The joints were placed in three different locations on a rack, simulating automotive assembly. Bilateral muscle activity from the upper trapezius and the lower arm extensors, as well as head and upper arm inclination was continuously monitored. Exposure levels and their variance components were assessed in several data subsets using ANOVA. The results were interpreted in terms of statistical precision and power, and discussed as markers of important ergonomic qualities. A substantial exposure variability was found within and between subjects in all joint locations and for both tools. For mixed work across tools and locations, the necessary number of subjects to arrive at a group mean exposure with 95% confidence limits corresponding to +/- 10% of the mean ranged between 8 and 158, with posture recordings tending to require smaller populations than muscle activity recordings. Within-subject variance increased 2- to 37-fold, depending on exposure parameter, when work was 'enlarged' from securings with a specified location and tool to a mix of all locations and tools. Systematic differences between subjects in variability and responsiveness to 'work enlargement' indicated individualized motor control strategies. The results illustrate the importance of exposure variability data to the design of proper measurement strategies. They also suggest that the sizes of exposure variability per se can be interpreted as operational indices of what is thought to be important ergonomic risk indicators, such as the 'sameness' of repeated operations and the allowance for flexible working techniques.

Activation imbalances in lumbar spine muscles in the presence of chronic low back pain

Paraspinal electromyographic (EMG) activity was recorded bilaterally from three lumbar levels during 30-s isometric trunk extensions [40 and 80% of maximum voluntary contraction (MVC)] in 20 healthy men and 14 chronic low back pain patients in pain. EMG parameters indicating neuromuscular fatigue and contralateral imbalances in EMG root-mean-square amplitude and median frequency were analyzed. Patients in pain showed less fatigue than controls at both contraction levels and produced only 55% of their MVC. Patients in pain likely did not produce a "true" maximum effort. A low MVC estimate would mean lower absolute contraction levels and less neuromuscular fatigue, thus explaining lower scores in the patients. Contralateral root-mean-square amplitude imbalances were present in both categories of subjects although such imbalances, when averaged across lumbar levels, were significantly larger in patients. Median frequency imbalances were significantly larger in the patients, at segmental as well as across lumbar levels. These results suggest that the presence of pain in these patients caused a redistribution of the activation behavior between synergistic muscles of the lumbar back.

Low-frequency oscillations (<0.3 Hz) in the electromyographic (EMG) activity of the human trapezius muscle during sleep

The surface electromyographic (EMG) signal from right and left trapezius muscles and the heart rate were recorded over 24 h in 27 healthy female subjects. The root-mean-square (RMS) value of the surface EMG signals and the heartbeat interval time series were calculated with a time resolution of 0.2 s. The EMG activity during sleep showed long periods with stable mean amplitude, modulated by rhythmic components in the frequency range 0.05-0.2 Hz. The ratio between the amplitude of the oscillatory components and the mean amplitude of the EMG signal was approximately constant over the range within which the phenomenon was observed, corresponding to a peak-to-peak oscillatory amplitude of approximately 10% of the mean amplitude. The duration of the periods with stable mean amplitude ranged from a few minutes to approximately 1 h, usually interrupted by a sudden change in the activity level or by cessation of the muscle activity. Right and left trapezius muscles presented the same pattern of FM. In supplementary experiments, rhythmic muscle activity pattern was also demonstrated in the upper extremity muscles of deltoid, biceps, and forearm flexor muscles. There was no apparent association between the rhythmic components in the muscle activity pattern and the heart rate variability. To our knowledge, this is the first time that the above-described pattern of EMG activity during sleep is documented. On reanalysis of earlier recorded trapezius motor unit firing pattern in experiments on awake subjects in a situation with mental stress, low-FM of firing with similar frequency content was detected. Possible sources of rhythmic excitation of trapezius motoneurons include slow-wave cortical oscillations represented in descending cortico-spinal pathways, and/or activation by monoaminergic pathways originating in the brain stem reticular formation. The analysis of muscle activity patterns may provide an important new tool to study neural mechanisms in human sleep.

The prevalence of musculoskeletal symptoms among British Columbia sonographers

A survey (n = 211, 92% response rate) was carried out to determine baseline prevalence of musculoskeletal symptoms and identify related biomechanical, psychosocial, work organization and demographic factors among the population of sonographers in British Columbia, Canada. Ninety-one percent of respondents reported musculoskeletal pain or discomfort that they associated with the work tasks of scanning. Almost half reported frequent and severe symptoms. The neck, shoulder, and upper back were the main symptomatic body sites. A very small minority were absent from work due to the symptoms, with more than two thirds of respondents reporting working in pain. Bivariate analyses found significant associations between scanning time, static postures, psychosocial factors, and degree of musculoskeletal symptoms.

Daytime trapezius muscle activity and shoulder-neck pain of service workers with work stress and low biomechanical exposure

AIM

The study aims to look for an association between trapezius muscle activity and shoulder-neck pain in work situations characterized by work stress and low biomechanical exposure.

METHODS

Female subjects (n = 93) were recruited from four occupational groups: health care, shop assistants, banking, and university secretaries. Thirty-eight workers were interviewed and classified as pain-afflicted by their shoulder-neck pain score in the last 6 months; 55 were pain-free. Shoulder-neck pain, bodily state of tension and fatigue, and subjective exposure assessments were monitored by repeated hourly measurements over 24 hr. Trapezius muscle activity was recorded bilaterally by surface EMG.

RESULTS

Shoulder-neck pain, perceived tension, work stress ("stressful environment"), and mental fatigue was augmented over the workday and reduced leisure time. Physical fatigue was low and stable. Thus, work stress appeared to be the dominant exposure associated with shoulder-neck pain. Muscle activity was low and similar for pain-afflicted and pain-free subjects during working hours. It was unchanged from work to leisure for the pain-afflicted workers and was significantly reduced for the pain-free group.

CONCLUSIONS

Stress-induced shoulder and neck pain is not necessarily associated with elevated trapezius muscle activity, but pain-free workers may benefit from better muscle relaxation in leisure.

Do specially designed visual display unit lenses create increased postural load compared with single-vision lenses during visual display unit work?

Three different types of spectacle lenses, specially designed for visual display unit (VDU) work, were compared with single-vision lenses regarding postural load. The different corrections effect on postural load was measured by using electromyography. Muscle loads were recorded from the trapezius muscle and the infraspinatus muscle. Continuous measurements of body posture were measured by using three dual axis inclinometers attached to the head, back, and upper arm. No significant differences were found between the single-vision lenses and the specially designed VDU lenses regarding muscle load. Small differences were found regarding the head angle. The study concludes that these new lens designs create interesting opportunities for the optometrist to optimize the visual conditions for VDU workers.

Statistical power and measurement allocation in ergonomic intervention studies assessing upper trapezius EMG amplitude. A case study of assembly work

The present study aimed at exploring the statistical power of ergonomic intervention studies using electromyography (EMG) from the upper trapezius muscle. Data from a previous study of cyclic assembly work were reanalyzed with respect to exposure variability between subjects, between days, and within days. On basis of this information, the precision and power of different data collection strategies were explored. A sampling strategy comprising four registrations of about two min each (i.e. two work cycles) for one day per subject resulted in coefficients of variation between subjects on the 10-, 50-, and 90-APDF-percentiles of 0.44, 0.31, and 0.29, respectively. The corresponding necessary numbers of subjects in a study aiming at detecting a 20% exposure difference between two independent groups of equal size were 154, 78, and 68, respectively (p< or = 0.05, power 0.80). Multiple measurement days per subject would improve power, but only to a marginal extent beyond 4 days of recording. Increasing the number of recordings per day would have minor effects. Bootstrap resampling of the data set revealed that estimates of variability and power were associated with considerable uncertainty. The present results in combination with an overview of other occupational studies showed that common-size investigations using trapezius EMG percentiles are at great risk of suffering from insufficient statistical power, even if the expected intervention effect is substantial. The paper suggests a procedure of how to retrieve and use exposure variability information as an aid when studies are planned, and how to allocate measurements efficiently.

Computer terminal work and the benefit of microbreaks

Microbreaks are scheduled rest breaks taken to prevent the onset or progression of cumulative trauma disorders in the computerized workstation environment. The authors examined the benefit of microbreaks by investigating myoelectric signal (MES) behavior, perceived discomfort, and worker productivity while individuals performed their usual keying work. Participants were randomly assigned to one of three experimental groups. Each participant provided data from working sessions where they took no breaks, and from working sessions where they took breaks according to their group assignment: microbreaks at their own discretion (control), microbreaks at 20 min intervals, and microbreaks at 40 min intervals. Four main muscle areas were studied: the cervical extensors, the lumbar erector spinae, the upper trapezius/supraspinatus, and the wrist and finger extensors. The authors have previously shown that when computer workers remained seated at their workstation, the muscles performing sustained postural contractions displayed a cyclic trend in the mean frequency (MNF) of the MES (McLean et al., J. Electrophysiol. Kinesiol. 10 (1) (2000) 33). The data provided evidence (p < 0.05) that all microbreak protocols were associated with a higher frequency of MNF cycling at the wrist extensors, at the neck when microbreaks were taken by the control and 40 min protocol groups, and at the back when breaks were taken by the 20 and 40 min protocol groups. No significant change in the frequency of MNF cycling was noted at the shoulder. It was determined (p < 0.05) that microbreaks had a positive effect on reducing discomfort in all areas studied during computer terminal work, particularly when breaks were taken at 20 min intervals. Finally, microbreaks showed no evidence of a detrimental effect on worker productivity. The underlying cause of MNF cycling, and its relationship to the development of discomfort or cumulative trauma disorders remains to be determined.

Motor control of low-threshold motor units in the human trapezius muscle

The firing pattern of low-threshold motor units was examined in the human trapezius and first dorsal interosseous (FDI) muscles during slowly augmenting, low-amplitude contractions that were intended to mimic contractile activity in postural muscles. The motor unit activity was detected with a special needle electrode and was analyzed with the assistance of computer algorithms. The surface electromyographic (EMG) signal was recorded. Its root-mean-square (RMS) value was calculated and presented to the subject who used it to regulate the muscle force level. In the trapezius, there was minimal, if any, firing rate modulation of early recruited motor units during slow contractions (< or =1% EMG(max)/s), and later recruited motor units consistently presented higher peak firing rates. As the force rate of the contraction increased (3% EMG(max)/s), the firing rates of the motor units in the trapezius approached an orderly hierarchical pattern with the earliest recruited motor units having the greatest firing rate. In contrast, and as reported previously, the firing rates of all motor units in the FDI always presented the previously reported hierarchical "onion-skin" pattern. We conclude that the low-threshold motor units in the postural trapezius muscle, that is the motor units that are most often called on to activate the muscle in postural activities, have different control features in slow and fast contractions. More detailed analysis revealed that, in the low force-rate contractions of the trapezius, recruitment of new motor units inhibited the firing rate of active motor units, providing an explanation for the depressed firing rate of the low-threshold motor units. We speculate that Renshaw cell inhibition contributes to the observed deviation of the low-threshold motor units from the hierarchical onion-skin pattern.