Properties of spike potentials detected by a surface electrode in intact human muscle

A clearly discriminable train of spikes was detected by ordinary silver disc electrodes fastened to the skin surface overlying m. vastus medialis during voluntary contraction in man. Some properties of these surface spikes were obtained. (1) Motor unit potentials detected by the inserted electrode loaded in the muscle closely under the fascia were found to be synchronized with the surface spikes. (2) The conduction velocity was around 3.5 m/s. (3) A starting point of the excitation, i.e., ,end-plate," was located at a point one-third of the observed length from the distal end in this particular case. (4) The conduction velocity showed a linear relationship with muscular temperature and Q10 approximately equal to 2.0 in the range of 17-31 degrees C. (5) The amplitude of the surface spikes decreased monotonously with increasing the distance between the source and electrode. (6) Wave forms and threshold values were highly reproducible. (7) Similar surface spikes have been found in six other muscles in the subject S.M. and in m. vastus medialis in five other subjects.