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The influence of the frequency of discharge and the duration of firing on the parameters of human single muscle fiber (SMF) potentials was studied. Thirty-five SMFs were investigated. We found that when the frequency increased, the depolarized area (b) of the intracellular potentials lengthened due to the increase in the velocity of spreading (v) of the potentials. The lengthened b caused a decrease in amplitude of the SMF potentials. During prolonged activity at a constant mean frequency the following phases of the changes were observed: (i) v decreased, b shortened, and the amplitude increased; (ii) v decreased further and the time for depolarization and repolarization (TDR) increased, b lengthened, and the amplitude decreased; (iii) v became stable, TDR increased further, and the amplitude decreased additionally; and (iv) in three cases a last phase was observed, where v and TDR increased, b lengthened, and the amplitude decreased still more. With different muscle fibers, the consecutive phases had different time intervals. |
