Ambulatory Monitoring and Analysis of Surface Electromyographic Signals in Ergonomic Field Studies - Methods and Applications

Doctoral thesis, 2004

Factors contributing to and the mechanisms behind the development of work-related musculoskeletal disorders (WMSDs) are still not fully known. Increased knowledge in this area is necessary in order to recommend appropriate actions to counteract the increasing costs to society and reduce the individual suffering related to these problems. Although specific hypotheses regarding the development of WMSDs are best tested in carefully constructed laboratory studies, field studies performed at workplaces are of great importance in providing knowledge of the working conditions where the problems arise. The information gathered can be used to formulate new hypotheses. The first part of this thesis concerns the development of a system for ambulatory monitoring and analysis of surface electromyographic (sEMG) signals. The developed system records the myoelectric activity by means of bipolar surface electrodes and stores the average rectified value (ARV) and mean power frequency (MPF) per second on a memory card for off-line analysis. Efforts have been made to render the system easy to carry and use, thus making it suitable for field studies. The possibility of biofeedback based on changes in ARV (including the amount of muscle rest) and/or MPF of the EMG signal was considered during the design phase; thus these variables have to be computed and presented in real-time. The second part of this thesis presents studies in which the developed system has been used. The first three studies mainly deal with methodological developments where 1) long-term trapezius activity recorded at the work-place has been demonstrated; 2) sEMG-signals provided by the system have been integrated with video analysis; 3) sEMG-signals recorded alongside direct measurements of back and upper arm positions enable comparisons of trapezius activity and upper limb movements. The remaining five studies report findings from two periods, three years apart, of data collected from supermarket cashiers regarding the activity of the trapezius muscle and its correlation with psychophysiological stress responses and neck and shoulder pain. It was found that cashiers reporting neck/shoulder pain had higher muscle activity and a shorter period of muscle rest than their pain-free work-mates. A significant correlation was also found between muscle activity and self-reported negative experiences at the workplace. After the introduction of a job rotation model allowing the cashiers to combine work at the checkout desk with tasks in the supermarket departments, a decrease in blood pressure and shoulder muscle activity in the non-dominant side was accompanied by self-reports indicating positive experiences of the changed work situation. A comparison between work on the checkout desk and tasks in the various departments revealed a more static activation of the shoulder in cashier work. It is concluded that ambulatory monitoring of sEMG-signals is a valuable tool in ergonomic field studies, and that the developed system provides reliable information that makes practical evaluation of muscle activation patterns possible.