Surface EMG of shoulder and back muscles and posture analysis in secretaries typing at visual display units

Objective: A study was carried out to investigate temporal changes of activ ation of shoulder and back muscles in workers at visual display units by me ans of surface EMG. Moreover, postural parameters were recorded to distingu ish fatigue-related from posture-related changes of the myoelectrical activ ity. Methods: Nine healthy female office workers typed texts spoken from ta pe during three I-h-long sessions. After the first and again after the seco nd hour there was a break of 15 min. Sixteen-channel surface EMG was bipola rly recorded from the erector spinae, trapezius, deltoid and sternocleidoma stoid muscles. Root mean square (RMS) and power spectrum median frequency o f the EMG were calculated. Sitting posture was assessed using an eight-chan nel movement analysis system with ultrasound markers. The position of the s eventh cervical spinous process and the left and the right acromion were an alysed synchronously with the EMG characteristics using regression analysis . Results: The normalised RMS of the left and right trapezius muscle increa sed, while the median frequency did not change. The increase of the normali sed RMS was significantly lower when the linear influence of posture was ex cluded. On average, the distance between C7 and the left and right acromion decreased within each working an hour. C7 became lower on average by 5.5 m m within an hour, whereas the acromions became lower by only 1.7 mm (left) and 3.3 mm (right). Conclusion: The increase in trapezius muscle activity w as partly related to a lifting of the shoulders to compensate a slight slum ping of the back. Another part of the EMG activity increase has to be attri buted to fatigue, to attention-related activity or to the combination of bo th. Therefore, training of the back muscles and a varied organisation of wo rk might have a preventive effect with respect to musculoskeletal complaint s in VDU workers.