Sensitivity for detecting fibrillation potentials: a comparison between concentric and monopolar needle electrodes. The sensitivity of monopolar and concentric electrodes for detecting fibrillation potentials (FP) has never been formally compared. We studied 35 muscles with FP, sampling 20 sites each with concentric and monopolar needles. The concentric needle identified 0.88 +/- 3.44 (mean +/- standard deviation) more sites with spontaneous activity. Although statistically significant (Wilcoxon signed rank test P less than .03), this difference in sensitivity did not appreciably affect diagnostic interpretation. Subjects described the concentric needles as more painful. Needle insertions in 25 other muscles demonstrated that needle movement generated the majority of FP. We suggest that the increased tissue injury caused by concentric needles may account for both their increased sensitivity and discomfort. 