Conduction velocity in human muscle : an EMG study in fatigue and neuromuscular disorders

The muscle fiber conduction velocity (MFCV) is the speed of the depolarization wave along the sarcolemma. This is a step in the cascade that causes the muscle to contract. This study describes two methods for the determination of the MFCV, an invasive and a surface EMG method. Each method has its specific advantages and limitations. The invasive EMG method is performed in resting muscle, usually biceps brachii. The muscle is stimulated by a monopolar needle electrode, with a fixed frequency of 1 Hz, outside the end-plate region. A concentric needle electrode is inserted at some distance in the activated fiber bundle, and after optimal positioning a polyphasic potential is obtained. The MFCV is determined from the latencies of the potentials and the distance between the stimulation electrode and the recording electrode. The surface EMG method is performed in voluntarily activated muscle. Two bipolar surface electrodes are placed in a linear array, parallel to the muscle fibers, nearly halfway between the endplate zone and the distal tendon. The MFCV is calculated from the latency shift between the two EMG signals. Other EMG parameters are also measured. The most studied parameters are: (1) the MFCV, based on the time shift between the two signals by means of the cross-correlationmethod; (2) the power spectrum of the signal; the position of this spectrum is reflected by the value of the median frequency (Fmed); (3) the energy content of the signal, by means of the integrated EMG (IEMG).