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 muscular load in studies of work related musculoskeletal disorders, especially for the trapezius muscle. The occurrences of periods of EMG silence (gaps), the time below a predetermined threshold level (muscular rest) and various percentiles of the amplitude distribution (APDF) are commonly used summary measures. However, the effects of the criteria used to calculate these measures (e.g., gap duration, threshold level, normalisation method) on the sensitivity of these measures to accurately differentiate work loads is not well known.Bilateral trapezius EMG was recorded, for a full workday, for 58 subjects following both maximal (MVE) and submaximal (RVE) reference contractions. Gap frequency, muscular rest, and percentiles were derived for eight fundamental work tasks. The calculations were performed using different gap duration criteria, threshold levels and normalisation methods.A gap duration of less than 1/2 s, and threshold level approximately 0.3% MVE for gap frequency, and approximately 0.5% MVE for muscular rest, were the criteria that optimised sensitivity to task differences. Minimal sensitivity to tasks and a high sensitivity to individuals was obtained using gap frequency with a threshold level of approximately 1% MVE. Normalisation to RVE, rather than MVE, improved sensitivity to differences between tasks, and reduced undesirable variability. Muscular rest was more sensitive to task differences than APDF percentiles.     

Determination of esophageal probe insertion length based on standing and sitting height

The present study derives simple formulas for the prediction of optimal insertion length of an esophageal temperature-sensitive probe from the measurements of either standing or sitting height. The formulas assume that the optimal site for an esophageal temperature probe is in the region of the esophagus bounded by the left ventricle and aorta, corresponding to the level of the eighth and ninth thoracic vertebrae (T8 and T9, respectively). An esophageal probe was constructed of polyethylene tubing containing 1-cm segments of alternating radiopaque and nonradiopaque tubing in the distal 20 cm of the probe. The probe was inserted through a nostril into the esophagus of 20 subjects (12 males and 8 females) of various heights (range 163-194.6 cm) and weights (range 52.2-100.8 kg), and lateral chest radiograms were obtained for determination of the insertion length of the probe (L) required to situate the probe in the retrocardiac esophagus. Analysis of the radiograms demonstrated that, at the level of the intervertebral disc between T8 and T9, the probe was below the tracheal bifurcation and close to the left ventricle. The distance from the nasal flare to this level showed a good correlation with the subject's stretched stature (r2 = 0.71) and sitting height (r2 = 0.86). The following equations were derived to predict the placement of the esophageal probe at the T8/T9 level based on standing height: L (CM) = 0.228 x (standing height) - 0.194, and sitting height: L (cm) = 0.479 x (sitting height) - 4.44.