Dependence of conduction velocity on spike interval during voluntary muscular contraction in human motor units

The dependence of the conduction velocity of the action potential on the spike interval from a preceding potential was studied using human single motor units during voluntary muscle contraction. The spike potential was recorded from the surface of the skin overlying M. vastus medialis by use of surface electrodes (diameter 5 mm). The results were as follows: Conduction velocity increased with a decrease in spike interval. The relation between conduction velocity and spike interval can be expressed as log v = kt + a, where v is conduction velocity (m X s-1), t is spike interval (ms) and k and a are constants. All data can be expressed by the above formula with a highly significant correlation. After arterial occlusion, there was little or no relation between conduction velocity and spike interval. This lack of correlation between these parameters was not restored to the initial correlation within a period of 15 min. Effects similar to occlusion could be seen in the result of prolonged isometric contraction. However the lack of correlation between these parameters was restored to the initial correlation within a 15 min recovery period. At low muscular temperature, the relation could not be described by a logarithmic regression. As the spike interval became shorter, the conduction velocity decreased. After cessation of cooling, the relation was restored to the initial correlation within 30 min. The mechanisms of these changes are still uncertain but some possible factors can be considered, for example, composition of the extracellular fluid, pH, temperature and permiability of the excitable membranes of the muscle.