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Muscle Activity Patterns in the Neck and Upper Extremities Among Machine Operators in Different Forest Vehicles

Abstract 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.