Longitudinal variations of characteristic frequencies of skeletal muscle fibre potentials detected by a bipolar electrode or multi-electrode.

We aimed to reveal reasons for longitudinal variations of characteristic frequencies of electromyographic signals detected by surface longitudinal multi-electrodes. Since the terminal phases were reduced in bipolar recordings, we tested whether the frequency variations reflected the effects of the excitation origin and extinction as in monopolar recordings. A precise and fast convolution method to calculate the signals detected by a multi electrode was suggested. The contribution of different electrode poles was introduced in the impulse response. When a longitudinal multi-electrode with an even number of poles was positioned above the end-plate of asymmetrical fibres, the signal mainly reflected the processes of the excitation extinction This increased the signal mean and median frequencies Although the effects of origin and extinction of the excitation were significantly reduced in the spatially filtered signals, the frequency variations along the fibre reflected these intrinsic features of any skeletal muscle fibre of finite length.