The spatial relationships between the needle and individual muscle fibers play the most important role in determining the waveform. The rise time helps estimate the distance between the recording tip of the electrode and the discharging motor unit. The resistance and capacitance of the intervening tissue act as a high-frequency filter. The measurement of the rise time confirms the suitability of the recorded potential for quantitative analysis. Another quantitative analysis of the EMG is the power spectrum analysis, which is influenced by number of motor units firing near the electrode and 'tissue filter' effect. The aim of this study was to investigate the relationship of the power spectrum and rise time. To examine this relation the extensor digitorum communis muscle was examined in 10 healthy subjects with a very weak voluntary contraction. The rise time and power spectrum was obtained from 10 MUAPs in each subject with a monopolar needle electrode. The mean power frequency as well as median power frequency of the MUAP were increased with decreasing rise time of MUAP. The study thus indicates that the amount of high frequencies in the power spectrum is greater in a MUAP with a shorter rise time. We concluded that power spectrum analysis may be used a factor to determine if a MUAP is acceptable for inclusion in analysis.
