Mats Hagberg

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 exe...

Occupational musculoskeletal stress and disorders of the neck and shoulder: a review of possible pathophysiology.

SummaryDisorders and complaints in the neck and shoulder regions are common among industrial workers and are often attributed to occupational musculoskeletal stress. The possible pathophysiological mechanism of occupational stress on the neck and shoulders is reviewed. A mechanical origin for cervical disc degeneration and osteoarthrosis is reported for a few occupational groups. However, a mechanical origin for osteoarthrosis is debatable. A work posture involving elevated arms may accelerate degeneration of shoulder tendons through impairment of circulation due to static tension and humeral compression against the coracoacromial arch. Furthermore, work tasks with repetitive arm movements may evoke shoulder tendinitis or tendovaginitis, probably due to friction. Three possible routes to neck-shoulder muscular pain are discussed; mechanical failure, local ischemia and energy metabolism disturbance.

Discomfort and load on the upper trapezius muscle when operating a wordprocessor.

Abstract Surface electromyogram recordings from the descending part of the trapezius muscle and discomfort ratings were assessed for six female VDT wordprocessor operators during their work. Each operator was studied during three work periods of 3 to 5 hours on different days. During one of the work periods, short pauses (15 s) were introduced every sixth minute. A static local muscular load of 32 and 30% of the maximal voluntary contraction was found on the right and the left side, respectively. The median and peak muscular loads during work were low. There was a significant negative correlation between pauses and static load on the right upper trapezius muscle. The rating of perceived discomfort was less after the work periods with short pauses than after the work periods without.

The effects of different pause types on neck and shoulder EMG activity during VDU work

Surface electromyographic recordings on the right side from the descending part of the trapezius muscle and from the levator scapulae muscle and discomfort ratings were assessed for twelve female word processor operators. The operators were studied during three work periods of 30 min each. Three different kinds of pauses, active pauses with pause gymnastic movements, passive pauses with rest and diverting pauses, were introduced into the work every six minutes. The static muscular load was low and about the same for the three work periods. High contraction levels with a duration of at least 1% of the registration time differed significantly between the three types of pauses, reflecting the activity during the pauses itself. Active pauses could thus change the muscle activity pattern. There was a tendency to prefer pauses with activity to passive pauses.

Myoelectric power spectrum dependence on muscular contraction level of elbow flexors

SummaryThe influence of the strength of contraction on surface recorded myoelectric power spectra was studied for three elbow flexors. Four subjects performed brief (3–5 s) isometric contraction levels (5–80% MVC). The experiment was repeated 23–26 times on different days. The surface myoelectric signal was recorded from the biceps brachii, the brachialis and the brachioradialis. By fast Fourier transform the myoelectric power spectrum was computed. The mean power frequency (MPF) was calculated and used as a single estimate of the myoelectric power spectrum. The MPF was found to increase with contraction strength with low level contractions. At levels in excess of 25–30% of MVC, the MPF became independent of contraction level. This dependence of the MPF on low level contractions is explained by tissue filtering effects and the recruitment order and distribution of motor units.

Trunk posture and load on the trapezius muscle whilst sitting at sloping desks

Abstract Sitting posture in relation to three different desk slopes of 45,22 and 0° (horizontal) was examined by statometry on 10 subjects who were reading. With increasing desk slope the cervical as well as the lumbar spine were extended, and the head and trunk changed towards a more upright posture. Electromyography (EMG) from the descending part of the trapezius muscle was also recorded during both reading and writing. This muscular load was low and did not change with varying desk slopes for any of the two tasks. A rating for acceptability, performed for both tasks on each desk inclination, favoured a steep slope of the desk for reading, while the opposite was favoured for writing. The study suggests that the reading matter should be placed on a sloping desk, and the paper used for writing on the horizontal tabletop between the desk and the person.

Serum creatine kinase as an indicator of local muscular strain in experimental and occupational work.

SummarySerum creatine kinase (SCK) was measured in ten subjects in the laboratory before and after the performance of bicycle ergometry and a lifting task. SCK was significantly increased 24 h and 48 h after the lifting work but not after the bicycle ergometry, although the work performed on the latter was four-times as great as on the former. The lifting work resulted in muscle pain and tenderness and, for six subjects, in clinical signs of shoulder tendinitis. In a field study, an increase in SCK was noted among assemblers/welders and cash-register operators, but not among controllers and forklift-truck drivers. A health interview revealed that muscuao-skeletal complaints were most often located in the upper extremity in the cases of the assemblers/welders and the cash-register operators. It is proposed that the SCK increase during work is a result of a high local muscular load due to fatigue and energy depletion of muscle cells producing a greater efflux of muscle enzymes. The evaluation of SCK changes during work may be an important tool in occupational health for early detection of work tasks producing local muscular strain.

Health problems among operators of plastic welding machines and exposure to radiofrequency electromagnetic fields

SummaryTo study possible medical effects of high radiofrequency radiation (RF), 113 Swedish men and women were studied by means of a structured interview and rating of subjective symptoms. A test session was included in order to examine coordination and muscular function of the hands. A neurological test concerning two-point discrimination (2-PD) was also done. As referents, 23 women, sewing machine operators and assembly workers, were chosen, interviewed and likewise tested. Exposure measurements were taken of the RF fields around the welding machines. The present Swedish ceiling value of 250 W/m2 for the equivalent power density was exceeded in more than 50% of the machines. The highest leakage fields, both for electric and magnetic fields, were found near machines used in factories for ready-made clothing, which gave a high exposure to the hands. Irritative eye symptoms were reported by 23% of the men and 40% of the women. A group of 27 persons was selected for a clinical eye examination and checked by photographs, and nine persons had modest conjunctivitis. A high prevalence of numbness in hands, especially among women, was found. A significantly impaired 2-PD was found in the exposed women as compared to the referent group. The pregnancy outcome for 305 female plastic welders during 1974–1984 did not show any significant differences with the Swedish average concerning malformation or prenatal mortality.

Exposure to welding fumes and chronic renal diseases, a negative case-referent study

SummaryIt was observed that some patients with glomerulonephritis or interstitial nephritis were welders. A hypothesis that welding fumes were an occupational hazard was put forward and initiated the following case-referent study. 143 cases were defined as living males, age 25 to 65 years, with renal disease which caused renal biopsy from 1976–1982. Three population-based referents for each case were selected from the national register of the total population matched by sex, age and community. Exposure was assessd by means of a questionnaire. Only 7% of the cases and 12% of the referents did not answer the questionnaire. There were no differences in exposure rates between cases and referents, even if exposure restrictions were applied. The power of the case-referent study was high.