Cognitive performance and muscle activation in workers with chronic shoulder myalgia.

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

Human muscle activity related to non-biomechanical factors in the workplace.

This paper presents current knowledge on low-level, long-lasting work-related muscle activity, focusing on the shoulder and the upper part of the trapezius muscle, and on mental, rather than biomechanical reasons for the muscle activation. The paper identifies three sources of vocational muscle activity: the biomechanical need for force production in order to perform movements or maintain postures against the force of gravity, the biomechanical need to stabilise body parts as a reference for performing movements and securing a stable visual field, and finally, muscle activity without obvious biomechanical purposes. This last category has been labelled non-biomechanical muscle activity in this review. Non-biomechanical muscle activity is related to the mental load, the emotional load and the individual characteristics of the subject, and is identified as having a low-level and a low second-to-second variability, resembling a static muscle contraction. Recent research has indicated that the size principle for motor unit recruitment order puts a strain on a limited number of low-threshold motor units which might be heavily taxed despite the overall low level of this muscle activity. However, the paper also cites a recent report showing that motor unit substitution may occur in prolonged low-level muscle activation (longer than a few minutes). Evaluations of muscle load at work usually omit the possibility of extra muscle activation due to nonbiomechanical factors, and thus may often give estimates of the muscle load that are too low, or misinterpret nonbiomechanical muscle activity as biomechanical muscle load.

Attention-related muscle activity in different body regions during VDU work with minimal physical activity.

The purpose of this study was to characterize regional variation in task-irrelevant muscle activity. Surface EMG was recorded at 20 different muscle sites during a complex choice-reaction task. Up to ten muscle sites were recorded in parallel and always including the upper trapezius muscles. The experimental setting was designed to minimize the physical activity while performing a VDU-based task demanding continuing attention. Consequently, the label 'attention-related muscle activity' has been adopted. The presence of this activity varied between subjects and between the different muscle sites. It tended to decrease in a caudal direction and to be low in the muscles of the extremities, with the exception of the extensor muscles of the hand and foot. Attention-related activity was prevalent in the frontalis and upper trapezius muscles. This result is discussed with respect to a potentially harmful health effect of task-irrelevant muscle activity.

Activity of single motor units in attention-demanding tasks: firing pattern in the human trapezius muscle.

Activity of single motor units in relation to surface electromyography (EMG) was studied in 11 subjects in attention-demanding work tasks with minimal requirement of movement. In 53 verified firing periods, single motor units fired continuously from 30 s to 10 min (duration of the experiment work task) with a stable median firing rate in the range of 8-13 Hz. When the integrated surface EMG were stable, the motor units identified as a rule were continuously active with only small modulations of firing rate corresponding to low-amplitude fluctuations in surface EMG. Marked changes in the surface EMG, either sudden or gradual, were caused by recruitment or derecruitment of motor units, and not by modulations of the motor unit firing rate. Motor unit firing periods (duration 10 s-35 s) in low-level voluntary contractions (approximately 1%-5% EMGmax) performed by the same subjects showed median firing rates (7-12 Hz) similar to the observations in attention-related activation.

[Psychogenic muscular activity as a risk factor of muscular pain]. / Psykogen muskelaktivitet som risikofaktor for muskelsmerte.

Recent research indicates that any continuous load on the shoulder and upper limb muscles is a risk factor for occupational muscle pain. When the muscle load is moderate to low, the duration seems far more important than the actual load level, possibly because of a fixed recruitment order of motor units. Research at the Norwegian National Institute of Occupational Health has shown that a mentally challenging task at a visual display terminal may give a low level of muscle activation with apparently no purpose as regards posture or movement ("psychogenic" muscle tension), based on continuous firing in a few motor units. This indicates a possible mechanism for the often observed link between psychosocial stress and muscle pain.

[Occupational and individual risk factors of muscular pain]. / Arbeidsrelaterte og individuelle risikofaktorer for muskelsmerte.

It is suggested that occupational exposure to muscle load should be described by three factors to indicate health risks: level, repetitiveness and duration. A reduction in level is beneficial if the level is high (> 10-15% MVC). However, even a low level involves risk of musculoskeletal complaints if the exposure is repetitive. A reduction of the occupational exposure from 7-8 hours to 4-5 hours per day delays the development of musculoskeletal complaints, but does not provide long-term reduction of risk with frequent repetitions. When interventions are carried out to reduce the risk of occupational musculoskeletal complaints, it is necessary to consider psychosocial and individual constitutional factors in addition to the three factors constituting the occupational exposure to muscle load.

The effect of motivation on shoulder-muscle tension in attention-demanding tasks.

This paper evaluates motivation as a task-related factor causing the development of psychologically mediated ('psychogenic') shoulder-muscle tension. In four separate experiments using a visual display unit (VDU) based complex reaction-time task, motivation was established by means of continuous feedback on the level of performance (relative to a preset target performance level) or by means of a money reward for good performance. In all experiments motivation improved the performance, but an increase in the psychogenic shoulder-muscle tension was only observed in the money-reward condition. It is discussed whether this difference in the muscle-tension response is due to changes in the subjects' attitude unrelated to performance or due to a superior performance in the money-reward condition.

Working hours as a risk factor in the development of musculoskeletal complaints.

The length of daily working hours as a risk factor for the development of musculoskeletal complaints was studied by comparing the sick leave statistics of 408 sewing machine operators on full-time schedules (8 h working day) with 210 operators on part-time schedules (5 h working day). Working part-time was shown to postpone the occurrence of sick leave due to musculoskeletal disorders by approximately half a year. There was no lasting effect on the reduction in working hours on sick leave due to shoulder-neck complaints, but a reduction in low back complaints was indicated. It is suggested that any reorganization of work activities to counteract musculoskeletal injuries from repetitive work should aim to break up the muscular activity patterns over time periods considerably shorter than the 5 h working day of the part-time workers in the present study.

Shoulder muscle tension induced by two VDU-based tasks of different complexity.

This paper evaluates task complexity as a task-related factor causing the development of psychologically mediated ('psychogenic') shoulder muscle tension. Eighteen subjects performed an experimental work session, responding to simple and complex reaction time tasks which were presented on a VDU screen. Most subjects generated low-level static muscle tension during the tests. On a group level the two tasks did not have a differential effect on muscle tension. However, a subgroup of eight subjects which consistently generated higher muscle tension in the complex tests, was identified. It is argued that for these subjects the difference in muscle tension is due to an increased mental effort invested, because of the higher computational demands in the complex task.