Muscle fiber conduction velocity related to stimulation rate

Muscle fiber conduction velocity in human biceps brachii muscle, produced by voluntary contraction and by contraction owing to microstimulation of a single motor unit, was measured with surface array electrodes. The conduction velocity of the fibers in the motor unit was calculated from the conduction time of the motor unit action potential along the electrode array and the electrode separation. With voluntary contraction, a conduction velocity of 4.25 +/- 0.43 m/sec (mean +/- S.D., n = 68) was obtained. In recording the surface EMG, the mean firing rate of the motor unit was 15.8 imp/sec (range 6-24 imp/sec). Significantly slower conduction velocity of 3.69 +/- 0.33 m/sec (mean +/- S.D., n = 56) was found after microstimulation (P less than 0.001). The higher the stimulation rate the higher was the conduction velocity. With increasing stimulus rates of 5, 10, 20 and 40 c/sec, the mean and S.D. of the conduction velocity were 3.74 +/- 0.33 m/sec (2.1% increase in the mean value to 1 c/sec stimulus rate), 4.16 +/- 0.37 m/sec (13.6%), 4.35 +/- 0.54 m/sec (18.8%) and 4.80 +/- 0.49 m/sec (31.1%), respectively. The firing rate for voluntary contraction was in the same range of the one obtained with 10-20 c/sec electrical stimulation, conduction velocity was the same in the two conditions. We conclude that measurement of muscle fiber conduction velocity should also be standardized with muscle fiber firing rate.