Myoelectric signal conduction velocity and spectral parameters: influence of force and time

Reports on measurement of muscle fiber conduction velocity in humans are scarce. Inferences on the behavior of conduction velocity have been drawn from the behavior of myoelectric spectral parameters. The present report contains information on conduction velocity and spectral parameters studied at various muscle contraction levels and during and after sustained contractions. The following results have been obtained from measurements on the tibialis anterior muscle. Conduction velocity demonstrated a positive correlation with limb circumference and with muscle force output. Thus we conclude that the diameters of the muscle fibers of high-threshold motor units are, on an average, larger than those of low-threshold motor units. The study of a sustained contraction and on the recovery after such a contraction revealed that conduction velocity consistently decreased during a strong contraction as did various myoelectric spectral parameters. However, the spectral parameters decreased approximately twice as much as did the conduction velocity, and we conclude that factors other than the conduction velocity along the muscle fibers affect the myoelectric signal during a high-level contraction. These other factors appertain to changes in the firing statistics of individual motor units as well as the correlation between the firings of different motor units.