Sensitivity of trapezius electromyography to differences between work tasks - influence of gap definition and normalisation methods

Surface electromyography (EMG) has been used extensively to estimate muscul ar load in studies of work related musculoskeletal disorders, especially fo r the trapezius muscle. The occurrences of periods of EMG silence (gaps), t he time below a predetermined threshold level (muscular rest) and Various p ercentiles of the amplitude distribution (APDF) are commonly used summary m easures. However, the effects of the criteria used to calculate these measu res (e.g., gap duration, threshold level, normalisation method) on the sens itivity of these measures to accurately differentiate work loads is not wel l known. Bilateral trapezius EMG was recorded, for a full workday, for 58 subjects f ollowing both maximal (MVE) and submaximal (RVE) reference contractions. Ga p frequency, muscular rest, and percentiles were derived for eight fundamen tal work tasks. The calculations were performed using different gap duratio n criteria, threshold levels and normalisation methods. A gap duration of less than 1/2 s, and threshold level approximate to 0.3% MVE for gap frequency, and approximate to 0.5% MVE for muscular rest, were the criteria that optimised sensitivity to task differences. Minimal sensit ivity to tasks and a high sensitivity to individuals was obtained using gap frequency with a threshold level of approximate to 1% MVE. Normalisation t o RVE, rather than MVE, improved sensitivity to differences between tasks, and reduced undesirable variability. Muscular rest was more sensitive to ta sk differences than APDF percentiles. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.