Recruitment of low threshold motor-units in the trapezius muscle in different static arm positions

A prevailing hypothesis for development of myalgia symptoms in the trapezius muscle is based on observations that in stereotypic activation of muscle the same low-threshold motor-units are the first ones to be recruited, and that these units are active throughout the contraction, until total relaxation. The theory suggests that these units are the first ones in monotonous repetitive work to be subject to degenerative processes, causing pain. The present project was undertaken to evaluate to what extent recruitment of motor-units in the trapezius muscle is position-specific, i.e. if there are motor-units that are recruited over a wide range of arm postures. Fine wire electrode pairs, inserted 2 cm lateral to the midpoint between the acromion and the C7 spinous processus, were used for signal acquisition. Methods for decomposition of the interference patterns were developed, allowing identification of single motor-units in signals registered in different arm positions. Voluntary recruitment of motor-units in the descending portion of the trapezius muscle was studied in 24 different arm positions (0-90 degrees shoulder flexion, 0-45 degrees humeral abduction, and 45-135 degrees elbow flexion) in three subjects. The results showed that the wire electrode technique and the signal processing algorithm employed allowed motor-unit identification in non-isometric conditions. It was found that low threshold motor-units in the trapezius muscle, active over a wide range of arm positions, could be identified in all three subjects. We refer to those as Cinderella, or C, units. These results support the research hypothesis addressed.     

An electromyographic study of dental work

Musculoskeletal disorders are common among dentists, and have been ascribed to the demands of high precision work and sustained static loading in the neck-shoulder region, combined with a flexed and rotated cervical spine. In order to determine muscular load levels during dentistry, activity in neck, shoulder, and arm muscles was recorded using an electromyography technique (EMG). Normalized mean, median, 10th and 90th percentile EMG amplitude levels (% maximal reference contraction, %max-RVC) were calculated during ordinary dental work. Among the muscles investigated, the trapezius muscle on both sides had the highest mean (the right trapezius 9.0% and the left 7.6% of max-RVC) and 10th percentile amplitude levels (both about 2% of max-RVC). The trapezius muscles showed similar myoelectric activity on the right and left side, probably because of similar muscular static load on the both sides. The right extensor carpi radialis muscle had a significantly higher muscular load level than the left one, possibly due to stabilization demands on the dominant wrist during demanding precision work. The infraspinatus muscle had low activity level on both sides, reflecting that the dentists worked with a small degree of arm elevation and external rotation. The dentistry work thus seems to generate relatively high muscular load on both trapezius and dominant extensor-carpi-radialis, and relatively low load on the infraspinatus muscle.     

Interactive effects of acute experimental pain in trapezius and sored wrist extensor on the electromyography of the forearm muscles during computer work

We investigated the interactive effects of shoulder pain and wrist extensor muscle soreness on surface electromyography (EMG) during computer mouse work. On day one, subjects (N = 12) performed computer work with/without acute muscle pain induced in the trapezius muscle. Subsequently, eccentric exercise was performed to induce delayed onset muscle soreness (DOMS) in wrist extensor muscles. In presence of DOMS on day two, computer work recordings with/without pain were repeated. EMG signals were recorded from the descending part of trapezius bilaterally, flexor carpi ulnaris and extensor carpi radialis brevis. Experimental muscle pain in trapezius led to a decrease in the muscular activity of the wrist extensor (P < 0.02) and decreased the relative rest time in the wrist flexor even in presence of DOMS (P < 0.01). The present result suggests that shoulder pain plays a role in the coordination of wrist flexors and extensors during computer work.     

Neck and shoulder muscle activity among ophthalmologists during routine clinical examinations

Muscular demands during common clinical ophthalmologic activities may contribute to the high prevalence of musculoskeletal health outcomes observed among ophthalmologists and other eye care physicians. Characterizations of the muscle activation patterns in the live ophthalmologic environment are lacking. This study was conducted to (i) characterize the frequencies and durations of common activities performed by ophthalmologists during routine clinical examinations, (ii) characterize neck and shoulder muscle activation patterns during the whole clinical work day, and (iii) explore differences in neck and shoulder muscle activation patterns between common clinical activities. Fifteen ophthalmologists performed routine patient examinations in an outpatient ophthalmology clinic while continuous surface electromyography measurements of the upper trapezius and anterior deltoid muscles were obtained. Results indicated that while computer use was the most frequently performed clinical activity, use of the indirect ophthalmoscope, followed by use of the slit lamp biomicroscope, required greater muscular demands than computer use or other clinical activities. Results provide evidence that the clinical activities of indirect ophthalmoscope and slit lamp biomicroscope use are appropriate for ergonomic intervention.     

Assessment of workload and arm position during different work sequences: a study with portable devices on construction workers

It is recognised that work related shoulder pain is overrepresented among construction workers compared to other occupations. Studies have shown that working with hands above shoulder level increases the shoulder load. Most studies have been confined to the laboratory. The present project was carried out to map the muscular engagement and postures of construction workers undertaking ceiling fitting, and to compare the results to those of the laboratory studies. Two ambulatory devices were used, one allowing recording of electromyographic (EMG) signals bilaterally from the trapezius muscle, and the other to record the position of both arms and back by means of measuring the angles between the vertical line and the back and both upper arms. These recordings were performed during 1.5–2 h work sequences. The results show that the work was mostly performed in an upright position, that both arms were used to a similar amount and that the workers for a large proportion of their working time had their upper arms at levels that are considered harmful in view of shoulder load. The EMG data showed that nearly 50% of the work was spent with trapezius activity that exceeded that of the reference contraction used (about 15% of maximal voluntary contraction) and that the time spent in muscular relaxation was 10%. It was concluded that the exposure of construction workers undertaking ceiling fitting meets the criteria formulated on the basis of laboratory experiments with respect to a high risk of acquiring chronic shoulder pain, due to rotator cuff tendinitis.     

Work load and fatigue in repetitive arm elevations

Six females performed continuous series of concentric and eccentric flexions in the shoulder between 0 and 90 degrees with 0 to 3.1 kg weights held in a powergrip. Heart rate (H R), perceived exertion (RPE) and myoelectric activity (EMG) from the descending part of the trapezius muscle, the anterior part of the deltoid muscle, and the biceps brachii were measured during the tasks. The increase of RPE was faster than the increase of HR with work load indicating an increased importance of local factors (i.e. strain on muscles and tendons) with load in the perceived exertion. The local muscular load determined by EMG on the trapezius muscle was closely correlated with the external torque produced in the glenohumeral joint. The time constants of EMG amplitude increase were correlated with work load, endurance time and with slope coefficients of RPE-HR linear regression. Symptoms and complaints 24 hours after the task were often localized to the descending part of the trapezius muscle. It is suggested that exertion of the descending part of the trapezius muscle in tasks involving repetitive shoulder flexion may promote discomfort and complaints referred to the neck.     

Influence of mouse position on muscular activity in the neck, shoulder and arm in computer users

Anecdotal reports of shoulder and arm discomfort related to computer mouse use are common. The aim of this study of ten subjects was to examine the influence of mouse position, relative to the keyboard, on shoulder and arm muscular activity and working posture. Surface electromyography and the Rapid Upper Limb Assessment were used to determine the differences in muscle activity and posture during an editing task in each of three mouse positions. Significantly less anterior (p = 0.01) and middle (p = 0.03) deltoid electromyographic activity was demonstrated when the computer mouse was positioned adjacent to a keyboard without a numeric pad when compared to performance of an identical task with the mouse adjacent to a standard keyboard. Electromyographic activity in trapezius muscle did not differ between mouse positions. Working posture of right handed mouse users is improved by removal of the numeric keypad.     

Muscular loading and subjective ratings of muscular tension by novices when typing with standard and split‐design computer keyboards

The objective of this research project was to compare two different computer keyboard designs with respect to their effect on the extent of muscular loading in the right and left trapezius and extensor muscles during typing. The two computer keyboards used in this study were (1) a classic or standard keyboard, and (2) a split‐design keyboard. Evaluation of muscular loading was done using the electromyography (EMG) technique and subjective ratings of muscular tension. Sixteen women, between 18 and 26 years of age, with similar secretarial experience and limited typing abilities, took voluntary part in the experiment. Each subject used both keyboards for 15 min. The EMG signals recorded during typing sessions were those of the left and right sides of trapezius (m. trapezius pars descendeus) and extensor (m. extensor carpi radialis brevis et longus) muscles. Upon completion of each task, subjects were asked to evaluate perceived levels of muscular tension in the shoulder‐neck area and forearms. The results showed that the split‐design computer keyboard significantly reduces muscular loading of the trapezius muscle and subjective feelings of muscular tension in the shoulder‐neck area.     

Upper trapezius muscle activity in healthy office workers: reliability and sensitivity of occupational exposure measures to differences in sex and hand dominance

Patterns of cervical muscle activity may contribute to overuse injuries in office workers. The purpose of this investigation was to characterise patterns of upper trapezius muscle activity in pain-free office workers using traditional occupational exposure measures and a modified Active Amplitude Probability Distribution Function (APDF), which considers only periods of active muscle contraction. Bilateral trapezius muscle activity was recorded in 77 pain-free office workers for 1–2 full days in their natural work environment. Mean amplitude, gap frequency, muscular rest and Traditional and Active APDF amplitudes were calculated. All measures demonstrated fair to substantial reliability. Dominant muscles demonstrated higher amplitudes of activity and less muscular rest compared to non-dominant, and women demonstrated less muscular rest with no significant difference in amplitude assessed by Active APDF compared to men. These findings provide normative data to identify atypical motor patterns that may contribute to persistence or recurrence of neck pain in office workers.

Practitioner Summary: Upper trapezius muscle activity was characterised in a large cohort of pain-free workers using electromyographic recordings from office environments. Dominant muscles demonstrated higher activity and less rest than non-dominant, and women demonstrated less rest than men. Results may be used to identify atypical trapezius muscle activity in office workers.     

The size and structure of arm movement variability decreased with work pace in a standardised repetitive precision task

Increased movement variability has been suggested to reduce the risk of developing musculoskeletal disorders caused by repetitive work. This study investigated the effects of work pace on arm movement variability in a standardised repetitive pipetting task performed by 35 healthy women. During pipetting at slow and fast paces differing by 15%, movements of arm, hand and pipette were tracked in 3D, and used to derive shoulder and elbow joint angles. The size of cycle-to-cycle motor variability was quantified using standard deviations of several kinematics properties, while the structure of variability was quantified using indices of sample entropy and recurrence quantification analysis. When pace increased, both the size and structure of motor variability in the shoulder and elbow decreased. These results suggest that motor variability drops when repetitive movements are performed at increased paces, which may in the long run lead to undesirable outcomes such as muscle fatigue or overuse.     

Integration of active pauses and pattern of muscular activity during computer work

Abstract

Submaximal isometric muscle contractions have been reported to increase variability of muscle activation during computer work; however, other types of active contractions may be more beneficial. Our objective was to determine which type of active pause vs. rest is more efficient in changing muscle activity pattern during a computer task. Asymptomatic regular computer users performed a standardised 20-min computer task four times, integrating a different type of pause: sub-maximal isometric contraction, dynamic contraction, postural exercise and rest. Surface electromyographic (SEMG) activity was recorded bilaterally from five neck/shoulder muscles. Root-mean-square decreased with isometric pauses in the cervical paraspinals, upper trapezius and middle trapezius, whereas it increased with rest. Variability in the pattern of muscular activity was not affected by any type of pause. Overall, no detrimental effects on the level of SEMG during active pauses were found suggesting that they could be implemented without a cost on activation level or variability.

Practitioner Summary: We aimed to determine which type of active pause vs. rest is best in changing muscle activity pattern during a computer task. Asymptomatic computer users performed a standardised computer task integrating different types of pauses. Muscle activation decreased with isometric pauses in neck/shoulder muscles, suggesting their implementation during computer work.     

Muscular Activity in Relation to Support of the Upper Extremity in Work With a Computer Mouse

The objective of this study was to compare the effects of using a forearm or wrist support on muscular load in the forearm and neck-shoulder region during work with a computer mouse. Fourteen 35- to 50-year-old healthy women who worked with a video display unit for at least 4 hr a day in an office and who also used a computer mouse for at least 2 hr a day participated in this experimental study. In the laboratory, computer mouse use with the forearm supported was compared with computer mouse use with the wrist supported. The participants edited a text table by moving symbols from one square to another in a fixed order. Each trial was a set of 64 movements. The trials were repeated 10 times in random order, 5 trials with each method of support. The dependent variable was electromyographic activity measured from muscles in the preferred lower arm and in the neck-shoulder region. Normalization to the individual mean of each participant was found to be more effective in reducing the interindividual variation than normalization to the maximal electrical activity (MEMG) obtained during isometric maximal voluntary contraction using the variation coefficient. The amplitudes normalized to the individual mean of each participant were used to evaluate the statistical significance of the difference between the methods of support. To interpret the magnitude and the practical meaning of the statistically significant differences, the corresponding mean percentages of the MEMG were used. The results of the study indicate that changing the position and support of the upper extremity affects muscular activity in the forearm and neck-shoulder region. The load on the trapezius muscle is significantly lower when the forearm is supported, and therefore such support may offer one means of preventing common disorders in the neck-shoulder region in office work. The load on the infraspinatus muscle and on forearm flexor muscles was, on the contrary, higher when the forearm was supported. The result may indicate the difficulty that workers have in adopting new hand and upper arm movements when using a computer mouse.     

Impact of physical exposure on neck and upper limb disorders in female workers

Physical workload [muscular load of the trapezius and infraspinatus muscles using electromyography (EMG), wrist positions and movements by electrogoniometers] and neck and upper limb disorders (from, for example, a physical examination) were studied in women with repetitive industrial work (n = 95) and referents (n = 74). The repetitive work displayed higher ratings for wrist movements, but not for EMG. The prevalences of neck, shoulder and wrist/hand disorders were elevated for women with repetitive work [age-adjusted prevalence odds ratios (PORs) 2.0-7.5]. For the left hand, high frequency of wrist movements (mean power frequency 0.53 Hz) was associated with a high prevalence of disorders (56%), as compared to low (0.28 Hz and 26%; POR 3.5). We found no consistent and significant effect of muscular load, on either neck or shoulder disorders. However, selection and other bias may have diminished our possibility to observe such effects. Psychosocial work environment factors were not confounding the results. Measurements of wrist movements may be used for identification of high-risk work tasks.     

Shoulder muscle fatigue development in young and older female adults during a repetitive manual task

Age may modify the association between occupational physical demand and muscle loading, and ultimately increase the risk of musculoskeletal disorders. The goal of this study was to investigate age-related differences in shoulder muscle fatigue development during a repetitive manual task. Twenty participants in two age groups completed an 80-minute simulated low-intensity assembly task. Electromyographic (EMG) manifestation of muscle fatigue was observed in the upper trapezius, deltoid and infraspinatus muscles in both age groups, and coincided with an increase in the subjective ratings of perceived exertions. Compared with the younger group, older group showed a more monotonic decrease in EMG power frequency in the upper trapezius and deltoid muscles. However, the age-related difference in EMG amplitude was less consistent. Relative rest time of the upper trapezius muscle in the older group was less than the young group throughout the task. The observed patterns of EMG measures suggest that older participants may have disadvantages in fatigue resistance in the upper trapezius and posterior deltoid muscles during the simulated repetitive manual task.     

Disorders of the cervicobrachial region among female workers in the electronics industry

Ninety-six female employees in the electronics industry were investigated in a cross-sectional study. Symptoms from the neck, shoulder-neck angle, shoulder, arm and thoracic spine regions were coded according to four degrees of severity, and related to individual characteristics. Static muscle strength of five shoulder and arm muscle groups, as well as static endurance were not related to symptoms; nor were medical and work history, productivity, leisure physical activity or hobbies. For the evaluation of postures and movements of the neck, shoulder and upper arm during work a systematic video technique (VIRA) was developed. Large interindividual differences in working technique, and thereby in work load, were obtained. Some of these measures, especially those describing flexion and abduction of the upper arm, were related to symptoms in the neck and shoulder. Stepwise multiple regression analyses confirmed and strengthened these relationships.     

Muscle activity during patient transfers: a preliminary study on the influence of lift assists and experience

The purpose of this study was to examine muscle activity patterns during patient handling during manual transfers, and transfers using floor and ceiling lifts. EMG patterns during transfers from bed to wheelchair and wheelchair to bed as well as patient repositioning in novices and experienced participants were examined. Surface EMG was recorded from the upper and lower erector spinae, latissimus dorsi and trapezius muscles bilaterally. Overall, normalized mean and peak muscle activity were lowest using the ceiling lift, increasing with the floor lift, which were lower than manual transfers (novices: all p < 0.01). Experienced patient handlers demonstrated approximately two times greater trapezius and latissimus dorsi activity than novices, combined with lower mean erector spinae activity (p < 0.05, for most tasks). Integrated EMG for all muscles was directly proportional to the transfer time and was lowest during the manual transfer followed by the ceiling lift, with the floor lift being highest. The difference between the muscle activity patterns between the experienced and novice patient handlers may suggest a learned behaviour to protect the spine by distributing load to the shoulder. Further examination of the muscle activation patterns differences between experience levels could improve training techniques to develop better patient handling strategies.     

Effects of arm suspension in simulated assembly line work: muscular activity and posture angles.

The electromyogram and posture angles of the shoulder and arm were recorded during sitting manipulative work in three positions, with and without the forearm suspended by a balancer. A Selspot system was used to record movements. The results from 12 female subjects (mean age 29 yr) showed that the activity in deltoideus anterior decreased significantly by using a balancer. Lower mean values were also noted for deltoideus lateralis and the trapezius muscles. The neck inclination increased with higher work area position, but was independent of the use of a balancer.     

Muscle activity during patient transfers: a preliminary study on the influence of lift assists and experience

The purpose of this study was to examine muscle activity patterns during patient handling during manual transfers, and transfers using floor and ceiling lifts. EMG patterns during transfers from bed to wheelchair and wheelchair to bed as well as patient repositioning in novices and experienced participants were examined. Surface EMG was recorded from the upper and lower erector spinae, latissimus dorsi and trapezius muscles bilaterally. Overall, normalized mean and peak muscle activity were lowest using the ceiling lift, increasing with the floor lift, which were lower than manual transfers (novices: all p?<?0.01). Experienced patient handlers demonstrated approximately two times greater trapezius and latissimus dorsi activity than novices, combined with lower mean erector spinae activity (p?<?0.05, for most tasks). Integrated EMG for all muscles was directly proportional to the transfer time and was lowest during the manual transfer followed by the ceiling lift, with the floor lift being highest. The difference between the muscle activity patterns between the experienced and novice patient handlers may suggest a learned behaviour to protect the spine by distributing load to the shoulder. Further examination of the muscle activation patterns differences between experience levels could improve training techniques to develop better patient handling strategies.     

Activity in neck-shoulder and lower arm muscles during computer and smartphone work

Relevance to industryThere is emerging evidence of musculoskeletal problems related to smartphone work and a rapid transition to mobile workplaces, where smartphones are key working tools. Aim: The study's aim was to compare muscle activity during computer work with smartphone work and to see what possible effects ergonomic recommendations for smartphone usage have on muscle activity.MethodsActivity was measured bilaterally from the upper trapezius muscle and from lower arm muscles on the right hand side, on twelve participants with surface electromyography who performed e-mail work on the computer using ergonomic recommendations, smartphone in a self-chosen way of working and on smartphone with ergonomic recommendations. Effects on productivity was not assessed in this study.ResultsActivity in m. trapezius and m. extensor digitorum was significantly higher during computer work (p < 0.05) than during the two smartphone usages and activity in m. interossei dorsalis 1 was, vice versa, significantly higher during smartphone work (p < 0.05). Comparison of smartphone in self-chosen way of working and smartphone with ergonomic recommendations showed no significant differences.ConclusionsPrevious research has highlighted the benefits of variation of work postures. This paper indicates that replacing the computer with a smartphone gives the trapezius muscle an opportunity to rest.