CONDUCTION VELOCITY IN HUMAN MUSCLE An EMG study in fatigue and neuromuscular disorders

Muscle fiber conduction velocity (MFCV) is studied in 51 healthy volunteers using an invasive (IMFCV) and a surface (S-MFCV) EMG technique in the biceps brachii, in order to acquire normal values and to study reproducibility and effects of gender and age. In the invasive technique, muscle fibers are directly stimulated with needle electrodes, and the latencies of the evoked potentials are measured by a concentric needle electrode. Surface EMG is used to determine the S-MFCV, the median frequency of the power density spectrum and the integrated EMG (IEMG). A clear correlation between the mean MFCV of both methods is found. The invasive method gives information particularly about the variability of the MFCV, since the fibers are measured irrespective of innervation and recruitment order. The surface method is especially suited for examination of contraction-induced MFCV changes. Its main advantages are, beside the noninvasive character, the easy repeatability and the possibility to measure at different force levels or during continuous contractions. The S-MFCV increases with age. It is hypothesized that this is an effect of hypertrophy in the muscle fibers of the remaining MU's, and therefore a consequence of the physiological motor unit loss during aging. We found a positive correlation between the neuromuscular efficiency (the quotient of force and IEMG) and the mean S-MFCV. It is suggested that this is a reflection of the higher neuromuscular efficiency of type II motor units.