Stefan Thorn

Work related perceived stress and muscle activity during standardized computer work among female computer users

The current study investigated the associations between work-related perceived stress and surface electromyographic (sEMG) parameters (muscle activity and muscle rest) during standardized simulated computer work (typing, editing, precision, and Stroop tasks). It was part of the European case-control study, NEW (Neuromuscular assessment in the Elderly Worker). The present cross-sectional study was based on a questionnaire survey and sEMG measurements among Danish and Swedish female computer users aged 45 or older (n=49). The results show associations between work-related perceived stress and trapezius muscle activity and rest during standardized simulated computer work, and provide partial empirical support for the hypothesized pathway of stress induced muscle activity in the association between an adverse psychosocial work environment and musculoskeletal symptoms in the neck and shoulder.

A comparison of muscular activity during single and double mouse clicks

Work-related musculoskeletal disorders (WMSDs) in the neck/shoulder region and the upper extremities are a common problem among computer workers. Occurrences of motor unit (MU) double discharges with very short inter-firing intervals (doublets) have been hypothesised as a potential additional risk for overuse of already exhausted fibres during long-term stereotyped activity. Doublets are reported to be present during double-click mouse work tasks. A few comparative studies have been carried out on overall muscle activities for short-term tasks with single types of actions, but none on occurrences of doublets during double versus single clicks. The main purpose of this study was to compare muscle activity levels of single and double mouse clicks during a long-term combined mouse/keyboard work task. Four muscles were studied: left and right upper trapezius, right extensor digitorum communis (EDC) and right flexor carpi ulnaris. Additionally, MU activity was analysed through intramuscular electromyography in the EDC muscle for a selection of subjects. The results indicate that double clicking produces neither higher median or 90th percentile levels in the trapezius and EDC muscles, nor a higher disposition for MU doublets, than does single clicking. Especially for the 90th percentile levels, the indications are rather the opposite (in the EDC significantly higher during single clicks in 8 of 11 subjects, P<0.05). Although it cannot be concluded from the present study that double clicks are harmless, there were no signs that double clicks during computer work generally constitute a larger risk factor for WMSDs than do single clicks.

Mechanisms for work related disorders among computer workers

Work related musculoskeletal disorders are common among computer workers, especially in the neck/shoulder region and the upper extremities. The relation between physical and psychosocial work load and generation of pain is still unclear. In this paper we describe five models. According to the often addressed Cinderella hypothesis, the pain is due to an overuse of low threshold muscle fibres. In a series of studies including intramuscular electromyography from the trapezius muscle, we have found several motor units that were active throughout coarse arm movements, during prolonged computer work tasks, and in both voluntary and stress induced contractions. Furthermore we have seen that motor unit statistics varies significantly between repeated measurements in one individual, hence the method would be inappropriate for group comparisons. Finally, we discuss a model based on a general model, literature studies, and own research.

Heavy vehicle frontal sled crash test analysis - chest deflection response in the Hybrid III dummy

The aim of this study was to analyse the Hybrid III dummy chest loading in heavy vehicle frontal crashes. In total, eight truck front-to-trailer-type sled tests were performed. The Hybrid III, in driver position, was equipped with the chest deflection sensor system RibEye and its standard potentiometer sensor was compared with the RibEye deflection data. The chest impact point was established by using Fuji film impression, as well as from video data. Chest-to-steering wheel rim contact occurred in all tests. Differences in chest loading were found between the Hybrid III dummy reference test and the load case identified in this study, predominantly impact velocity, direction, distribution and location of loading. The steering wheel rim contact is the major contributor to chest deflection. The chest deformation consists of anterior/posterior compression and upward deflection of the sternal plate. The Hybrid III standard chest deflection sensor is not reliable in this load case due to the modes of sternal displacement and the single-point measurement location. Combining the RibEye system with global dummy kinematic data and accurate contact detection is required to fully assess chest deflection. New biomechanical data are needed to adapt the injury risk assessment to the identified load case.

Improving Hybrid III Injury Assessment in Steering Wheel Rim to Chest Impacts Using Responses from Finite Element Hybrid III and Human Body Model

Objective: The main aim of this study was to improve the quality of injury risk assessments in steering wheel rim to chest impacts when using the Hybrid III crash test dummy in frontal heavy goods vehicle (HGV) collision tests. Correction factors for chest injury criteria were calculated as the model chest injury parameter ratios between finite element (FE) Hybrid III, evaluated in relevant load cases, and the Total Human Model for Safety (THUMS). This is proposed to be used to compensate Hybrid III measurements in crash tests where steering wheel rim to chest impacts occur. Methods: The study was conducted in an FE environment using an FE-Hybrid III model and the THUMS. Two impactor shapes were used, a circular hub and a long, thin horizontal bar. Chest impacts at velocities ranging from 3.0 to 6.0 m/s were simulated at 3 impact height levels. A ratio between FE-Hybrid III and THUMS chest injury parameters, maximum chest compression C max, and maximum viscous criterion VC max, were calculated for the different chest impact conditions to form a set of correction factors. The definition of the correction factor is based on the assumption that the response from a circular hub impact to the middle of the chest is well characterized and that injury risk measures are independent of impact height. The current limits for these chest injury criteria were used as a basis to develop correction factors that compensate for the limitations in biofidelity of the Hybrid III in steering wheel rim to chest impacts. Results: The hub and bar impactors produced considerably higher C max and VC max responses in the THUMS compared to the FE-Hybrid III. The correction factor for the responses of the FE-Hybrid III showed that the criteria responses for the bar impactor were consistently overestimated. Ratios based on Hybrid III and THUMS responses provided correction factors for the Hybrid III responses ranging from 0.84 to 0.93. These factors can be used to estimate C max and VC max values when the Hybrid III is used in crash tests for which steering wheel rim to chest interaction occurs. Conclusions: For the FE-Hybrid III, bar impacts caused higher chest deflection compared to hub impacts, although the contrary results were obtained with the more humanlike THUMS. Correction factors were developed that can be used to correct the Hybrid III chest responses. Higher injury criteria capping limits for steering wheel impacts are acceptable. Supplemental materials are available for this article. Go to the publishers online edition of Traffic Injury Prevention to view the supplemental file.