Abstract
Musculoskeletal disorders of the neck and shoulders is a major problem for both the individual and the society. The preventive actions have, so fare, seldom been successful, and further research on the relation between muscular activity and disorders are needed. It has been hypothesised that, even for the lowest activity of a muscle, the same fibres may always be activated (the Cinderella hypothesis). Thus, the relative duration of muscular rest is of special interest to study. Moreover, during prolonged activity at low levels, there might be a rotation of the activation between motor units, which coincides with the occurrence of short periods of EMG silence (gaps).
Hence, whole-day electromyographic recordings (EMG), especially from the trapezius muscle, and characterisation of the low-level activity during real work, is most relevant in studies of neck and shoulder disorders. However, since the signal is in the μV range and with a wide frequency content (≈10 to 1,000 Hz), it is hard to acquire and certify high quality data. The quality may be improved by use of preamplifiers and ambulatory data loggers. Moreover, by calculating, and screening, the power spectra of the EMG, the mean power frequency and an artefact index, remaining possible artefacts may be detected. By inspecting the digitised (raw) EMG, these possible artefacts are then considered, and if appropriate, rejected, thus assuring a high quality of the data.
In a study of cleaners and office workers - where we considered the statistical, rather than the physiological, properties of muscular rest and gap frequency - the optimal criteria for gap duration and threshold level were practically the same as the ones earlier used. As a general measure of task load, muscular rest is more suitable than gap frequency, and normalisation to a submaximal, rather than a maximal, reference contraction may be preferable.
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