Extraction and Analysis of the Single Motor Unit F-Wave of the Median Nerve

The matrix type multielectrodes have been proved to be useful for getting the information of motor unit (MU) properties (Monster et al., 1980; Reucher et al., 1987; Yamada et al., 1987; Masuda and Sadoyama, 1988; Kleine et al., 2000; Gazzoni et al., 2004). By this electromyography (EMG) technique, we can obtain many essential properties of MU that could not be obtained by conventional surface EMG and/or needle EMG. One of them is the waveform property of motor unit action potential (MUAP) which is enabled by extracting the single propagating MUAP. Another is the two-dimensional location of motor end-plates. There have been many applications of multichannel surface EMG for measuring voluntary muscle activities, but few for evoked EMG responses. However, recent studies have demonstrated the applications of the multichannel surface EMG for the measurement of muscle fiber conduction velocities (MFCVs) (Metani et al., 2005) , for the estimation of firing thresholds (Yamada, 2004), and for the motor unit number estimate (MUNE) (Blok et al., 2005; van Dijk et al., 2008). The F-wave represents a long latency response detected from a muscle following stimulation of a peripheral nerve at supramaximal intensity. The F-waves are produced by antidromic activation of motor neurons. The latency and the waveform of the F-wave change with each stimulus. The F-wave studies are widely used in clinical neurophysiology as a quantitative estimation method of the objective pathological changes. The F-waves that consist of a single MUAP occur also with submaximal stimulation. Low intensity stimulation could increase the probability of the single MU F-wave (Komori et al., 1991; Doherty et al., 1994; Stashuk et al., 1994; Yamada, 2004), and reduce the discomfort of the subjects. Therefore, different MUAPs can be readily analyzed by classifying F-waves. Many studies concerned with the single MU F-wave were performed for the estimation of the conduction velocity of nerve fibers (Doherty et al., 1994; Felice, 1998; Wang et al., 1999) and the MUNE (Stashuk et al., 1994; Hara et al., 2000). The conventional surface EMG technique was used in these studies, and there were few applications of multichannel surface EMG for F-wave studies. In a previous study, we investigated the classification and analysis of multichannel bipolar F-waves (Yamada et al., 2007). It is difficult to detect deep MUs from the bipolar EMG. So, in the present study, the thenar MUs F-waves were investigated by extracting single MU Fwaves from monopolar multichannel surface EMG signals. By increasing the number of