NORMALITY AND STATIONARITY OF EMG SIGNALS OF ELBOW FLEXOR MUSCLES DURING RAMP AND STEP ISOMETRIC CONTRACTIONS

The purpose of this study was to test the stationarity and normality o f electromyographic (EMG) signals obtained while exerting isometric co ntractions: (a) when a steady force level is maintained (step contract ions); and (b) where the force level is increased linearly over time ( ramp contractions). Ramp elbow flexions were performed from 0 to 100% of the maximum voluntary contraction (MVC) in a 5-s period. For the st ep contractions, four force levels (20, 40, 60 and 80% MVC) were maint ained for a period of 3s each. EMG signals of the biceps brachii (BE) and brachioradialis (BR) muscles of 16 subjects were recorded with sur face electrodes and digitized at a sampling frequency of 2000 Hz. Test s of normality (Shapiro-Wilk test) and stationarity (reverse arrangeme nt test) were performed locally on short finite time records (512-ms w indows). Results show that, in general, EMC signals present a non-Gaus sian amplitude distribution and are stationary. Furthermore, the ampli tude distribution characteristics and the stationarity of the signal w ere not dependent on the muscle investigated, nor on the type of contr action or force level tested. The finding of local stationarity for bo th tasks is important, because it suggests that performing standard sp ectral analysis is applicable for both step and ramp contractions. It also allows a direct comparison between results obtained under both co nditions. (C) 1997 Elsevier Science Ltd.