Gunnar Palmerud

Psychophysiological stress responses, muscle tension, and neck and shoulder pain among supermarket cashiers.

: This study examined psychological and physiological stress, as well as muscle tension and musculoskeletal symptoms, among 72 female supermarket cashiers. Stress levels were found to be significantly elevated at work, as reflected in the catecholamines, blood pressure, heart rate, electromyographic (EMG) activity, and self-reports. Fifty cashiers (70%) suffering from neck-shoulder pain (trapezius myalgia) were found to have higher EMG activity at work and reported more tension after work. Women who kept a diary for 1 week and reported more musculoskeletal pain (above the median) were older, had higher blood pressure, and reported more work stress and psychosomatic symptoms. The elevated stress levels at work are consistent with data from workers involved in other types of repetitive tasks and can be important for the high prevalence of neck and shoulder symptoms among the cashiers.

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.

Voluntary redistribution of muscle activity in human shoulder muscles

Four shoulder muscles (the supraspinatus, the infraspinatus, the anterior and middle portion of the deltoid, and the descending part of the trapezius) were examined with electromyography in abducted arm positions. By using feedback techniques, we found that the subjects could reduce the EMG activity voluntarily by 22-47% in the trapezius muscle while keeping different static postures. This was not true for any other muscle investigated. When the trapezius activity was reduced there was a tendency towards an increase of EMG activity in some other shoulder muscles, particularly the infraspinatus. The findings may be related to relaxation from an initial overstabilization of the shoulder, or redistribution of load among synergists. It is suggested that the possibility of reducing trapezius activity may be of ergonomic significance. It is also noted that EMG trapezius activity may not serve as a universal descriptor of total muscular load in the shoulder.

Hand grip increases shoulder muscle activity An EMG analysis with static handcontractions in 9 subjects

We examined 4 shoulder muscles--the supraspinatus, infraspinatus, the middle portion of the deltoid and the descending part of the trapezius--with electromyography (EMG) in adducted and flexed arm positions, in 9 healthy subjects. The subjects were asked to produce a static handgrip force of 30% and 50% of maximal voluntary contraction (MVC) in 8 different arm positions. In all positions, the subjects held a dynamometer in the hand. The myoelectric activity in the shoulder muscles with only the dynamometer in the hand was compared to the EMG activity when static contractions were added. There was an association between static handgrip and shoulder muscle activity, as revealed by E < G. The EMG activity increased in the supraspinatus muscle in humeral flexion from and above 60 degree in 120 degree abduction. In the infraspinatus muscle, the changes were less; a significant increase, however, was noticed in flexion. In the deltoid muscle there was a tendency towards increased activity in positions lower than 90 degree, in the higher arm positions, the activity decreased. There was no significant alteration regarding the EMG activity of the trapezius. Our findings imply that high static handgrip force, particularly in elevated arm positions, increases the load on some shoulder muscles. The stabilizing muscles (the rotator cuff) were more influenced than the motor muscles by hand activity. Handgrip activity is important to evaluate while assessing shoulder load in manual work an in clinical evaluations of patients with shoulder pain.

The effect of light manual precision work on shoulder muscles— an EMG analysis

In order to clarify if light manual precision work influences the activity of shoulder muscles, seven shoulder muscles were examined using electromyography (EMG) in five different arm positions in ten subjects. The subjects were asked to perform light precision work with the hand while maintaining the same arm position. The EMG activity in the shoulder muscles with the arm in specific positions was compared to the activity when manual precision work was added. The results were analysed statistically using a non-parametric method. We found an increased EMG activity in almost all arm positions and muscles. The increase was significant in four out of five arm positions regarding the infraspinatus and in three out of five positions concerning the levator scapulae. In the supraspinatus the increase was significant with the arm in its highest location. The average increase in shoulder muscle activity amounted to 22% of the resting activity.

Consequences of trapezius relaxation on the distribution of shoulder muscle forces: an electromyographic study

This study was focused on the ability to reduce voluntarily the muscle activity in the descending part of the trapezius muscle without changing the arm position or hand load, and its consequences on the distribution of shoulder muscle forces. Visual feedback techniques were used. Six different arm positions were investigated in 11 subjects. Electromyography was used for monitoring the muscle involvement. The selection of relevant muscles was performed by a model simulation of the shoulder using a newly developed biomechanical model of the human shoulder. The ability to reduce the muscle activity of the descending part of the trapezius was confirmed and the reduction ranged from 60 to 76% depending on the arm position. Among the muscles studied the rhomboid major and minor and the transverse part of the trapezius were affected the most, increasing their activity on the average to 232, 175 and 201% respectively, compared to the initial activity. The anterior part of the deltoid and the medial part of the serratus anterior also intensified their activity. The influence on the levator scapulae was, contrary to simulation results and to empirical knowledge, a decrease of the muscle activity. It is suggested that attention is given to the rhomboids and the transverse part of the trapezius when muscle activity is reduced in the descending part of the trapezius, for instance in biofeedback-based therapy. In conclusion, the study showed that reducing the trapezius activity caused a redistribution of muscle forces in the shoulder.

Intramuscular pressure of the infra- and supraspinatus muscles in relation to hand load and arm posture

Abstract In work engaging the upper extremities, the musculoskeletal system of the shoulder is sometimes exposed to prolonged excessive load, leading to musculoskeletal disorders of the shoulder. One way of reducing work-related shoulder disorders is to establish guidelines for working postures. The purpose of this study was to identify harmful working positions, by performing a comprehensive survey of the intramuscular pressure (IMP) in the infra- and supraspinatus muscles in relation to different arm positions and external loads. Ten healthy males participated, and the IMP in the infra- and supraspinatus muscles was studied in a total of 112 combinations of arm positions and hand loads at levels that occur frequently in industrial work. High-precision spatial recordings were accomplished with a three-dimesional motion-analysis system, and the IMP was measured using the microcapillary infusion technique. The mean IMP of the infraspinatus muscle as well as that of the supraspinatus muscle increased continuously from a resting pressure at 0° of upper arm elevation to a maximal pressure at 90° of upper arm elevation, for all elevation planes. The mean IMP of the supraspinatus muscle appeared to be more dependent upon the elevation plane and less dependent upon the hand load, compared to the infraspinatus muscle. Even during only moderate arm elevation, the mean IMP of the infra- and supraspinatus muscles, presented here in polar diagrams, had already exceeded the levels of reduced recovery from local muscle fatigue and blood flow impairment. The elevation angle and the hand load primarily influence the development of IMP in the infra- and supraspinatus muscles.

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.

Influences of handgrip on shoulder muscle activity

Four shoulder muscles (the supraspinatus, the infraspinatus, the middle portion of the deltoid and the descending part of the trapezius muscle) were examined using electromyography (EMG) in abducted and flexed arm positions, in nine subjects who had no history of illness from arm or shoulder. The subjects were asked to supply an intermittent isometric handgrip force of 30% and 50% of maximal voluntary contraction, in a total of eight different arm positions. The EMG activity with and without hand activity was compared in all positions. There was a statistically significant increase in the EMG activity in the supraspinatus muscle in humeral flexion from and above 60°. In the infraspinatus muscle the changes were less; a significant decrease was however noticed in abduction. In the deltoid muscle there was a significant decrease with hand activity in flexion from and above 90°. There was no statistically significant alteration regarding the EMG activity of the trapezius muscle. The result of this study implies that high demands on handgrip force, particularly while using hand tools in elevated arm positions, adds further to the already high load on some shoulder muscles. This factor should be considered in the design of manual work and in the places of work.

The effect of arm support on supraspinatus muscle load during simulated assembly work and welding.

The effect of arm support, by a suspension device, on muscle load in the supraspinatus muscle was evaluated with simultaneous intramuscular pressure measurement and electromyography (EMG) in nine healthy subjects. Two work situations, a low load assembly type of work, and welding with a higher shoulder muscle load, were simulated in the laboratory. Each subject performed three work-cycles of each type, with and without arm support. Arm suspension reduced supraspinatus muscle load in both work situations with reduction in pressure of 34% and 22% respectively, and reduction in normalized EMG of 20% and 17% respectively. The reduction of muscle load was significant, but in the welding situation with arm-suspension 10-15 N, average muscle pressure was still high enough to reduce muscle blood flow. The interpretation of the importance of this load reduction for the development of work-related shoulder pain is problematic.