M-response latencies changes as a tool for studying neuromuscular conduction during cyclic maximal voluntary...

M-response changes during cyclic maximal isometric voluntary forearm flexions were studied using a new method based on the following: 1) Monopolar stimulation of terminal motor point of the brachial biceps muscle by large surface electrodes; 2) Monopolar leading-off by a surface electrode placed on the middle between motor endplates and muscle fibers end; 3) Arm and forearm immobilization; 4) Averaging of M-response triggered by the stimulus during contraction; and 5) Latencies to the peak of negative (L1) and of final positive phase (L2) and peak-to-peak amplitude (PTPA) were measured, and peak-to-peak time (PTPT), which reflected muscle conduction velocity and leading-off electrode-to-end of fibers distance was calculated. Four cycles of 30-s contraction and 5-min rest were performed. Eight M-responses were evoked with a 1 Hz frequency starting from 5th s of the cycle. No significant changes of PTPA and L1 were found. Nevertheless, L2 significantly decreased in the third cycle by 6.4% and in the fourth-cycle by 7.6% of the first cycle value. PTPT decreased in the fourth cycle reaching 87.6% of the initial value. The advantage of the method and the possible causes of changes were discussed.