Motor unit firing patterns in light manual work

Work related musculoskeletal disorders (WMSDs) in the shoulder/neck area and the upper extremities are a common problem among computer operators, especially women. A frequent type of WMSDs in the shoulder/neck area, which also may exist in the upper extremities, is chronic muscle pain (myalgia). The mechanisms for the development (etiology) of myalgia in the shoulder/neck area and in the upper extremities, in spite of the usually low muscle load levels in these muscles, are not fully understood. This thesis has explored current knowledge of WMSDs among computer users and has investigated especially the validity of the so-called Cinderella hypothesis, which may explain the etiology of myalgia among computer users. The Cinderella hypothesis postulates that some groups of muscle fibres belonging to low threshold motor units (MUs) will remain active as long as the overall muscle activity exceeds a certain threshold level. In the context of monotonous workloads, e.g. computer work, this recruitment behaviour would cause an overuse of selective muscle fibres leading to muscle tissue damages, which in turn induce myalgia. This thesis reports the results of three experimental studies of muscular activity in healthy human subjects. The subjects carried out short term and long term work tasks mimicking real life computer workloads. Intramuscular electromyography (EMG) measurements were made in a shoulder muscle (m. trapezius) and a finger/wrist muscle (m. extensor digitorum communis, EDC). By decomposition of the intramuscular EMG signals into short segments and classification of the segments into specific MU groups, MU firing patterns could be analysed for each test subject carrying out various work tasks such as in computer work. The results show that there is a possibility for low threshold MUs to be continuously active in the trapezius muscle and the EDC muscle during long term work tasks such as in computer work. These results support the Cinderella hypothesis on a selective overuse of specific low threshold MUs in the context of computer work. Only some of the studied low threshold MUs showed this continuously active behaviour however, while others were active intermittently, which might be a result of MU substitution. Finally, still open and important questions include how long a continuous activity or which firing pattern is needed for the muscle cells to become damaged and how the pain generation process leading to myalgia functions.