Mikael Forsman

Effects of experimentally induced mental and physical stress on motor unit recruitment in the trapezius muscle

Mental stress may induce muscle tension and it is thought to contribute to the development of musculoskeletal disorders. Prolonged activity of a muscles motor units (the smallest functional units) having low activation thresholds, may cause metabolic disturbances, degenerative processes and pain. The present experiment was aimed at investigating if the same motor units are activated by mental stress as by physical demands. Seventeen participants were exposed to mental stress tests (mental arithmetic, Stroop color word) and physical demands (standardized reference contraction, force ramp contraction) in the laboratory. In addition, they were exposed to the cold pressor test. Bipolar intramuscular electromyographic (EMG) activity was measured in the trapezius muscle, which covers the upper part of the back, the neck and the shoulder, and an automatic classification programme was used to separate the contributions of the individual motor units in the compex EMG signal. Overall muscle activity was measured by bipolar surface EMG and heart rate was recorded as an indicator of stress. The mental stress induced significant increases in muscle activity ( p < .05) and in heart rate ( p < .05). In 12 out of 14 participants, one or more motor units were found to be active in mentally as well as in physically induced muscle contraction. The results indicate that mentally induced stress may contribute to keeping low threshold motor units active, even in the absence of physical demands. Considering the health risks of prolonged motor unit activity, it was concluded that lack of mental rest is an important risk factor for the development of muscular pain.

MTM-based ergonomic workload analysis

Abstract Production engineers are urged to take ergonomic considerations into account when planning for production. However, the means to do so are often inadequate with respect to training as well as support. There is a lack of methods to predict, at a reasonable cost, biomechanical load on an operator performing a task not yet observable on a product and in a workplace not yet existing. The purpose of this study was to develop an ergonomic complement to a modern MTM system called SAM that gives the production engineer a first insight into the future ergonomic quality of a planned production. A method was developed that requests the engineer to supply two additional pieces of information to the analysis: the zone relative to the operators body in which the movement takes place or ends, and the weight or force involved in the operation. As method of comparison for validation purposes was selected the operator self-evaluation method VIDAR. The method was tested at the Torslanda final assembly plant of Volvo Car Corporation and at the ITT Flygt plant for large submersible pumps at three different balances and compared with two different methods of assessing biomechanical load. The results show that the method identifies the events causing high biomechanical load on the operator so that they can be redesigned. The suggested method has proven to be a useful tool and is being introduced at Volvo Car Corporation. Relevance to industry Under the concept of concurrent engineering the production engineer will have less time for experimenting with new production layouts. Given access to early design drawings of the product she or he must be able to decide on the future layout. There is a need for a tool giving the possibility to estimate simultaneously the consumption of time in the envisaged production, and the biomechanical load inherent in the planned tasks.

Variation between seated and standing/walking postures among male and female call centre operators

BackgroundThe dose and time-pattern of sitting has been suggested in public health research to be an important determinant of risk for developing a number of diseases, including cardiovascular disorders and diabetes. The aim of the present study was to assess the time-pattern of seated and standing/walking postures amongst male and female call centre operators, on the basis of whole-shift posture recordings, analysed and described by a number of novel variables describing posture variation.MethodsSeated vs. standing/walking was recorded using dichotomous inclinometers throughout an entire work shift for 43 male and 97 female call centre operators at 16 call centres. Data were analysed using an extensive set of variables describing occurrence of and switches between seated and standing/walking, posture similarity across the day, and compliance with standard recommendations for computer work.ResultsThe majority of the operators, both male and female, spent more than 80% of the shift in a seated posture with an average of 10.4 switches/hour between seated and standing/walking or vice versa. Females spent, on average, 11% of the day in periods of sustained sitting longer than 1 hour; males 4.6% (p = 0.013). Only 38% and 11% of the operators complied with standard recommendations of getting an uninterrupted break from seated posture of at least 5 or 10 minutes, respectively, within each hour of work. Two thirds of all investigated variables showed coefficients of variation between subjects above 0.5. Since work tasks and contractual break schedules were observed to be essentially similar across operators and across days, this indicates that sedentary behaviours differed substantially between individuals.ConclusionsThe extensive occurrence of uninterrupted seated work indicates that efforts should be made at call centres - and probably in other settings in the office sector - to introduce more physical variation in terms of standing/walking periods during the work day. We suggest the metrics used in this study for quantifying variation in sedentary behaviour to be of interest even for other dichotomous exposures relevant to occupational and public health, for instance physical activity/inactivity.

Motor-unit recruitment in the trapezius muscle during arm movements and in VDU precision work

Abstract It has been hypothesized that shoulder myalgia, which is common in many occupations involving light-to-moderate manual work, is due to an overuse of low threshold muscle fibers, causing damage at the muscle cell level. The present study was undertaken to investigate in the trapezius muscle, (a) to what extent it is possible to track low threshold motor-units in non-isometric conditions, and (b) to study the effect of arm movements on recruitment and firing patterns. Intramuscular four lead fine wire electrodes were inserted in the trapezius muscle of four volunteers, who performed arm movements (shoulder abduction–adduction 0–90–0° in the scapula plane), as well as standardized work using computer mouse. Three channels of electromyography (EMG) were used for motor-unit action potential (MUAP) identification and signal decomposition. Results showed (a) that it was possible to perform signal tracking during slow movements, and (b) that there were motor-units that were active over a wide range of shoulder abduction movement, as well as in work with computer mouse. The findings indicate that stereotypic recruitment patterns exist in the trapezius muscle not only in static work, but also in work situations involving arm and shoulder movements. Relevance to industry Occupationally related chronic shoulder pain is an increasing problem in industry, in manual work as well as in computer work. This paper presents results of studies on muscular activity patterns during movements which support the notion that chronic pain may be evoked by overuse syndromes at the muscular fiber level. Practical precautions to reduce such risks are discussed.

Thumb joint movement and muscular activity during mobile phone texting - A methodological study.

PURPOSE The extended intensive use of mobile phones and other devices for Information and Communication Technology (ICT) could expose the thumbs and fingers to operational stresses beyond their intended function which may generate pain and musculoskeletal disorders in the thumbs and the associated joints. Studies characterizing biomechanical exposures associated with mobile phone use are limited to date and the use of electrogoniometry and EMG was assessed for characterizing thumb-based activities. SCOPE The purpose was to investigate whether there were associations between thumb-based exposures registered with relatively easy-to-use electrogoniometric methods and more complicated EMG methods. Our hypothesis was that some outcome measures from the simpler electrogoniometric methods are related/correlated to and may be used as surrogate measure for the more complicated EMG methods. CONCLUSIONS Due to low associations and the complementary nature of exposures collected with electrogoniometry and EMG, the results indicated both measures are necessary for characterizing thumb exposure during mobile phone text messaging. Only in a few, selected abduction/adduction activities, could electrogoniometry be used in lieu of EMG for assessing thumb exposures.

Modified method time measurements for ergonomic planning of production systems in the manufacturing industry

The paper reports on an evaluation of a method called ErgoSAM. This method is based on SAM, a higher-level method-time-measurement (MTM) system, and is for use by production planners, e.g. production engineers. In addition to the SAM information, the ErgoSAM method considers information on weight handled or forces applied and work zone. The method is designed to predict the physical demands of work postures, force and repetitivity, according to a scientific model, the Cube model. In co-operation with the Volvo Car Corporation, six work-stations of an assembly line were analysed by a production engineer using ErgoSAM. The total assembly time analysed was about 17 minutes. The results were compared to results from ergonomic analyses made by an ergonomist using the Volvo Car Corporations standards. The results showed that ErgoSAM predicts work situations of high physical stress for the workers. However, the method does not consider stressful positions for the hand, wrist and neck or mental stress. The production engineer judged the ErgoSAM analyses to require about 5% more time than SAM analyses alone. It is concluded that the method shows considerable promise for predicting physically stressful work situations, but needs to be further evaluated and refined.

Effects of visually demanding near work on trapezius muscle activity

Poor visual ergonomics is associated with visual and neck/shoulder discomfort, but the relation between visual demands and neck/shoulder muscle activity is unclear. The aims of this study were to investigate whether trapezius muscle activity was affected by: (i) eye-lens accommodation; (ii) incongruence between accommodation and convergence; and (iii) presence of neck/shoulder discomfort. Sixty-six participants (33 controls and 33 with neck pain) performed visually demanding near work under four different trial-lens conditions. Results showed that eye-lens accommodation per se did not affect trapezius muscle activity significantly. However, when incongruence between accommodation and convergence was present, a significant positive relationship between eye-lens accommodation and trapezius muscle activity was found. There were no significant group-differences. It was concluded that incongruence between accommodation and convergence is an important factor in the relation between visually demanding near work and trapezius muscle activity. The relatively low demands on accommodation and convergence in the present study imply that visually demanding near work may contribute to increased muscle activity, and over time to the development of near work related neck/shoulder discomfort.

Stabilization of gaze: a relationship between ciliary muscle contraction and trapezius muscle activity.

In an experimental study four levels of oculomotor load were induced binocularly. Trapezius muscle activity was measured with bipolar surface electromyography and normalized to a submaximal contraction. Twenty-eight subjects with a mean age of 29 (range 19-42, std 8) viewed a high-contrast fixation target for four 5-min periods through: (i) -3.5 dioptre (D) lenses; (ii) 0 D lenses; (iii) individually adjusted prism D lenses (1-2 D base out); and (iv) +3.5 D lenses. The target was placed close to the individuals age-appropriate near point of accommodation in conditions (i-iii) and at 3m in condition (iv). Each subjects ability to compensate for the added blur was extracted via infrared photorefraction measurements. A bitwise linear regression model was fitted on group level with eye-lens refraction on the x-axis and normalized trapezius muscle EMG (%RVE) on the y-axis. The model had a constant level of trapezius muscle activity--where subjects had not compensated for the incurred defocus by a change in eye-lens accommodation--and a slope, where the subjects had compensated. The slope coefficient was significantly positive in the -D (i) and the +D blur conditions (iv). During no blur (ii) and prism blur (iii) there were no signs of relationships. Nor was there any sign of relationship between the convergence response and trapezius muscle EMG in any of the experimental conditions. The results appear directly attributable to an engagement of the eye-lens accommodative system and most likely reflect sensorimotor processing along its reflex arc for the purpose of achieving stabilization of gaze.

Musculoskeletal health and work ability in physically demanding occupations: study protocol for a prospective field study on construction and health care workers

BackgroundMusculoskeletal disorders have a profound impact on individual health, sickness absence and early retirement, particularly in physically demanding occupations. Demographics are changing in the developed countries, towards increasing proportions of senior workers. These senior workers may have particular difficulties coping with physically demanding occupations while maintaining good health.Previous studies investigating the relationship between physical work demands and musculoskeletal disorders are mainly based on self-reported exposures and lack a prospective design. The aim of this paper is to describe the background and methods and discuss challenges for a field study examining physical demands in construction and health care work and their prospective associations with musculoskeletal disorders, work ability and sickness absence.Methods and designThis protocol describes a prospective cohort study on 1200 construction and health care workers. Participants will answer a baseline questionnaire concerning musculoskeletal complaints, general health, psychosocial and organizational factors at work, work demands, work ability and physical activity during leisure. A shorter questionnaire will be answered every 6th months for a total of two years, together with continuous sickness absence monitoring during this period. Analysis will prospectively consider associations between self-reported physical demands and musculoskeletal disorders, work ability and sickness absence. To obtain objective data on physical exposures, technical measurements will be collected from two subgroups of N = 300 (Group A) and N = 160 (Group B) during work and leisure. Both group A and B will be given a physical health examination, be tested for physical capacity and physical activity will be measured for four days. Additionally, muscle activity, ground reaction force, body positions and physical activity will be examined during one workday for Group B. Analysis of associations between objectively measured exposure data and the outcomes described above will be done separately for these subpopulations.DiscussionThe field study will at baseline produce objectively measured data on physical demands in the construction and health care occupations. In combination with clinical measurements and questionnaire data during follow-up, this will provide a solid foundation to prospectively investigate relationships between physical demands at work and development of musculoskeletal disorders, work ability and sickness absence.

Oxygenation and hemodynamics do not underlie early muscle fatigue for patients with work-related muscle pain

Patients suffering from work-related muscle pain (WRMP) fatigue earlier during exercise than healthy controls. Inadequate oxygen consumption and/or inadequate blood supply can influence the ability of the muscles to withstand fatigue. However, it remains unknown if oxygenation and hemodynamics are associated with early fatigue in muscles of WRMP patients. In the present study we applied near-infrared spectroscopy (NIRS) on the extensor carpi radialis (ECR) and trapezius (TD) muscles of patients with WRMP (n = 18) and healthy controls (n = 17). Our objective was to determine if there were group differences in endurance times for a low-level contraction of 15% maximal voluntary contraction (MVC) – sustained for 12–13 min, and to see if these differences were associated with differences in muscle oxygenation and hemodynamics. At baseline, oxygen saturation (StO2%) was similar between groups for the ECR, but StO2% was significantly lower for TD for the WRMP patients (76%) compared to controls (85%) (P<0.01). Also, baseline ECR blood flow was similar in the two groups. For both muscles there were a larger number of patients, compared to controls, that did not maintain the 15% MVC for the allotted time. Consequently, the endurance times were significantly shorter for the WRMP patients than controls (medians, ECR: 347 s vs. 582 s; TD: 430 s vs. 723 s respectively). Responses in StO2% during the contractions were not significantly different between groups for either muscle, i.e. no apparent difference in oxygen consumption. Overall, we interpret our findings to indicate that the early fatigue for our WRMP patients was not associated with muscle oxygenation and hemodynamics.