The reliability of monopolar and bipolar fine-wire electromyographic measurement of muscle fatigue

Bipolar intramuscular wire electrodes and spectral analysis of the electromyographic signal have been used to measure fatigue in muscles that cannot be studied with surface electrodes. Intramuscular electrodes can detect a greater range of frequencies from muscle, obtain a less distorted signal, and are therefore felt to be more sensitive to detecting fatigue. To determine the reliability and sensitivity of electrode placement (with a fixed distance) for assessing muscle fatigue, we placed three intramuscular electrodes in and two surface electrodes on the biceps brachii of 30 healthy male subjects. With these electrodes, we devised eight configurations that were analyzed separately for reliability. Subjects performed four, 30-s isometric fatiguing contractions divided between two testing sessions. Mean and median frequency of the power density spectrum were plotted against time. Linear regression was performed to obtain slopes, which were used as indicators of fatigue. The bipolar surface electrode configuration displayed mean and median frequency intrasession and mean frequency intersession reliability for slope. All four bipolar fine-wire configurations had mean and median frequency intrasession reliability (P less than or equal to 0.05). Only three of the four bipolar fine-wire configurations approached mean frequency intersession reliability, and none of the four displayed median frequency intersession reliability. The configuration with distal bipolar intramuscular electrodes placed 1 cm apart was the most reliable intramuscular technique. The bipolar fine-wire configurations studied showed a trend toward better reliability than monopolar fine-wire configurations. No intramuscular technique, however, was reliable enough for clinical use in the study of fatigue.