Work related musculoskeletal disorders (WMSDs) in the shoulder/neck area are a common and increasing problem among European computer workers, especially women. Long-term low-level workloads with low degree of muscle rest are a potential risk factor for developing WMSDs. The purpose of the present study of female computer users (age 45-65 years) in Denmark and Sweden was to investigate if subjects with self-reported neck/shoulder complaints (cases, N = 35) show less trapezius muscle relative rest time (RRT) than controls (N = 44) when performing standardised short-term computer work tasks in controlled laboratory conditions. Surface electromyography (EMG) signals were recorded bilaterally from the upper trapezius muscles during a type, edit, precision and colour word stress task. Besides RRT, 10th percentile RMS values were calculated. On the average, 15 of the cases and 18 of the controls showed analysable EMG files per task. For the colour word stress task, the results showed lower RRT values and higher 10th percentile RMS amplitude levels among cases compared to controls. No such signs could be found for the other tasks performed. The present results indicate an increased motor response to a psychological stressor among subjects with self-reported neck/shoulder complaints.

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Abstract-Motor unit (MU) properties of the biceps brachii andFugl-Meyer score were assessed in stroke patients and healthy controls during passive and active elbow flexion and extension contractions. The level of motor recovery as assessed with the Fugl-Meyer score was correlated with the ratio of the size of the motor unit action potentials (MUAPs) at the affected side and the unaffected side. This ratio may reflect the extent to which reinnervation has occurred on the affected side. The RMS value of the MUAPs recruited during the stretch phase of the passive contractions was lower than the RMS value of MUAPs recruited during active contractions. This may indicate that only smaller MUs are affected by increased sensitivity to muscle stretch while a larger part of the MU pool can be recruited voluntarily.

The purpose of this study was to investigate whether differences in the peripheral and control properties of the neuromuscular system due to long-term preferential use, related to side dominance, affect postural muscles, such as the upper trapezius. Therefore, fatigability properties of the upper trapezius muscles of the dominant and non-dominant side were assessed. Surface EMG signals were detected from the upper trapezius muscles of both sides of nine right- and five left-handed subjects with adhesive linear electrode arrays consisting of eight contact points. Static constant force contractions with the arms 90 degrees abducted were performed by the subjects while holding hand loads of 0 kg, 0.5 kg, and 1 kg. Surface EMG spectral and amplitude variables were computed from the recorded signals. EMG spectral variable rate of change (indicating fatigue) showed a statistically significant difference between the two sides, with the dominant side less fatigable than the non-dominant one. The observed differences held for both the right- and left-handed subject group. A possible explanation for the results is that long preferential use of one side with respect to the other leads to changes in muscle fiber membrane and control properties, in agreement with previous results on limb muscles.