Effects of high-pass filtering on MUAP metrics

Previous observations suggest that elevated high-pass filter settings (1,600-3,200 HZ) can reveal greater motor unit action potential (MUAP) complexity (turns). We assessed the effect of high-pass filter settings (500, 1,000, 2,000 HZ) on MUAP metrics. MUAPs were recorded with a concentric needle and initially extracted by a decomposition software algorithm at 10 HZ-10 kHZ and further filtered offline at 500, 1,000, and 2,000 HZ. When reanalyzed by the decomposition software there were marked reductions in peak-peak amplitude, area, area-to-amplitude ratio, and duration at the 500 HZ filter with lesser subsequent reductions at higher filter settings. In contrast, turn and phase counts did not change significantly. Individual MUAPs tracked across filter settings showed rare increases in turn count at the 500 HZ setting but a subsequent decrease in counts with higher filter settings. We conclude that the routine use of elevated high-pass filters, as in quantitative EMG analysis, does not enhance turn count.