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Bipolar surface EMG (sEMG) signals of the trapezius muscles bilaterally were recorded continuously with a frequency of 800 Hz during full-shift field-work by a four-channel portable data logger. After recordings of 60 forest machine operators in Finland, Norway and Sweden, we discovered erroneous data. In short of any available procedure to handle these data, a method was developed to automatically discard erroneous data in the raw data reading files (Discarding Erroneous EPOchs (DESEPO) method. The DESEPO method automatically identifies, discards and adjusts the use of signal disturbances in order to achieve the best possible data use. An epoch is a 0.1 s period of raw sEMG signals and makes the basis for the RMS calculations. If erroneous signals constitute more than 30% of the epoch signals, this classifies for discharge of the present epoch. Non-valid epochs have been discarded, as well as all the subsequent epochs. The valid data for further analyses using the automatic detection resulted in an increase of acceptable data from an average of 2.15–6.5 h per day. The combination of long-term full-shift recordings and automatic data reduction procedures made it possible to use large amount of data otherwise discarded for further analyses.

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Aim The purpose of this paper was to describe and evaluate different aspects of muscle activity patterns associated with musculoskeletal discomfort/pain. Method Surface electromyography (sEMG) of the right upper trapezius and the right extensor digitorum muscles was conducted continuously during one working day in 19 male forest machine operators driving harvesters, 20 driving forwarders and 20 researchers at the Forest Research Institute. Perceived discomfort/pain in the right side of the neck and the right forearm was rated morning, noon and afternoon with Borg’s CR-10 scale. Static, median and peak levels of muscle activity were analyzed and the number and total duration of EMG gaps (muscular rest) were calculated. Sustained low-level muscle activity (SULMA) was defined as continuous muscle activity above 0.5% of the maximal EMG activity quantified into 10 periods of predetermined duration intervals from 1.6 to 5 s up to above 20 min. The number of SULMA periods is presented within each interval and as cumulative periods above the already determined levels. The operators handled control levers seated in a fixed position while the researchers performed mainly PC work and other varied tasks. Results A positive correlation was found between discomfort/pain in the right upper trapezius muscle region in the afternoon and cumulative SULMA periods above 10 min duration, and a negative correlation to cumulative SULMA periods also including the short durations. No specified patterns were found for discomfort/pain in the right extensor digitorum or for the other EMG measurements. All EMG measurements distinguished to some extent between the occupational groups, especially between machine operators driving harvesters and researchers. Conclusions Number of SULMA periods longer than 10 min per hour was positively correlated, and predominantly short periods were negatively correlated, to complaints in the neck region. This seems promising in order to find duration limits for sustained low-level muscle activity as a risk factor for musculoskeletal disorders.

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The purpose of this prospective cohort study was to evaluate relationships between sustained low-level muscle activity (SULMA) in the neck and pain after 1 year among machine operators of forest harvesters (n = 19), forwarders (n = 20) and forest researchers (n = 20). Surface electromyography of the right upper trapezius muscle was measured during one working day. Continuous muscle activity (SULMA periods) were analysed in predetermined time intervals. Neck pain was assessed by the Borg's category-ratio scale and the Standardized Nordic Questionnaire (dichotomised into pain duration ≤30 or >30 d). Harvesters reported significantly more pain than researchers. A higher number of long SULMA periods >8 min duration increased the risk of neck pain >30 d during the successive year (odds ratio 3.0, 95% CI 1.2–7.8). Perceived personal economy above average was associated with less pain, while other potential confounders or intermediate variables were not significant. Low-level trapezius muscle activity in periods longer than 8 min may constitute a risk for neck pain.

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The present cross-sectional study was performed to analyze potential risk factors for upper extremity disorders in two groups of forest machine operators driving harvesting vehicles and performing equal tasks in France (n=18) and Norway (n=19). This comparative design implied similar work tasks, but potentially different external work demands. Previous studies have suggested higher levels of neck and shoulder complaints among Norwegian operators compared to those of the French. This may be related to different external work demands and/or individual motor performance. Surface electromyography (EMG) of the right upper trapezius (RUT) and extensor digitorum (RED) muscles were measured continuously during 1 working day (7.5–8 h per operator) and video of body postures was recorded inside the cabin (1 h per operator). A questionnaire on external work demand factors and psychosocial stressors was used together with the collection of symptom data. A physical examination was performed on all workers. Borg's CR-10 scale of intensity of discomfort/pain was rated four times throughout the test day. The French operators reported less complaint (p<0.01) in the right neck compared to their Norwegian colleagues. Furthermore, the French had on average two to three times longer lunch breaks during 5 work days, less continuous hand intensive use of the control lever with more frequent short breaks during the test day compared to the Norwegian operators. However, the average static load level on the RUT muscle for the whole working day was significantly higher among the French (0.7% EMGmax) in comparison with the Norwegian operators (0.3% EMGmax). No difference was found in number of periods with sustained low-level muscle activity neither in the RUT nor the RED muscle. In conclusion, our results support that the higher prevalence of discomfort/pain among the Norwegian forest machine operators may be related to organizational factors.

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The goal of this cross-sectional study was to investigate the impact of different physical work station designs, expressed in two different brands of forest vehicles, on the muscle activity patterns in the neck and upper extremities among the vehicle operators. Surface electromyography (sEMG) was continuously recorded bilaterally on the trapezius (TM) and the extensor digitorum muscles (EDM) during one working day among operators driving Timberjack and Valmet vehicles, either as harvesters (n = 7 and 6, respectively) or forwarders (n = 9 and 9, respectively). Both the construction of the crane in relation to the chassis and the design of the control levers vary between the Tim-berjack and Valmet vehicles, which demand different ergonomic performance by the operators. The operators mostly handle control levers in the harvesters or forwarders, the latter with a more varied work load, in a fixed, seated working posture in the cabin for long hours with little rest. The sustained low- level muscle activity was quantified by periods with muscle activity above 0.5 percent EMGmax into 10 predetermined duration intervals from 1.6 to 5 s up to above 20 min (SULMA periods). These SULMA periods were analyzed both for number in the different intervals and cumulated periods above the predefined values. Amplitude and frequency parameters were analyzed and the number and total duration of muscle rest periods were calculated. The operators driving Valmet harvesters had a significantly higher number of long cumulated SULMA periods above 10 min in the left TM, and showed a higher level of static muscle activity and less total duration of muscle rest in TM bilaterally. The operators driving Timberjack forwarders had a significantly higher number of SULMA periods between 10 and 20 min in the right TM. No difference was found between the operators in the EDM activity pattern. The results of our study showed that operators driving Valmet harvesters had more sustained low-level activity in the neck than those driving Timberjack, including a higher number of long cumulated SULMA periods, higher static level, and less muscle rest. Despite a small sample, the results in muscle activity pattern raise the question of needs for improvements of the forest vehicle workstation design.