Muscular activity in light manual work - with reference to the development of muscle pain among computer users

Doctoral thesis, 2005

Work related musculoskeletal disorders (WMSDs) in the neck/shoulder area and the upper extremities are common among computer users, despite the relatively low muscle load levels involved. Muscle pain (myalgia), primarily experienced in the neck/shoulder area, is a frequent type of WMSD. The Cinderella hypothesis postulates that groups of muscle fibres (motor units, MUs) are continuously active during monotonous workloads, leading to MU overuse and myalgia. The main scope of this thesis was to investigate whether muscular activity patterns during computer work are consistent with the Cinderella hypothesis on selective MU overuse. In three studies, MU activity patterns were registered by intramuscular wire electromyography (EMG) in the upper trapezius (N=8 and N=4, respectively) muscle and the extensor digitorum communis (EDC) muscle (N=8) during prolonged (25-60 minutes) and short-term (20 seconds) work tasks characterising computer work. Surface EMG (SEMG) techniques were chosen to measure the overall activity in the muscles studied. In a study of 79 elderly female computer users with vs. without neck/shoulder complaints (cases/non-cases), SEMG was recorded bilaterally in the upper trapezius muscles during a type, edit, precision and colour word stress computer task (task length 2-5 minutes). Median and 10th percentile SEMG root mean square (RMS) levels and relative muscle rest time (RRT) was compared in cases vs. non-cases. For 11 subjects performing 60 minutes of computer work, median and 90th percentile SEMG RMS levels were compared for the upper trapezius, EDC and flexor carpi ulnaris (FCU) muscles during single vs. double mouse clicks. In addition, EDC MU activity was detected in three of the subjects during 50 consecutive mouse clicks. Continuously active MUs were found both in the upper trapezius and in the EDC muscles during the prolonged and short-term low-level work tasks investigated, although a majority of the MUs were intermittently active. During the stress task (but not during the typing, editing or precision mouse work tasks), decreased trapezius muscle rest was indicated for cases compared to non-cases. These findings give support to the Cinderella hypothesis on an MU activity-based development of myalgia in the context of computer work and indicate that stressful working conditions increase the risk for muscular overuse. No support was found for double clicking a computer mouse constituting a higher risk for MU double firings (doublets) and WMSDs than single clicking. Further investigations are needed of the underlying mechanisms of MU substitution and inter-individual MU activity differences, and of the causal pathways between continuous and sustained MU activity, selective MU overuse and chronic pain.