Change of the relationship between superimposed M wave and preceding background activity during muscular fatigue

A superimposed M wave is the M wave during a sustained contraction. We estimated a relationship between the instantaneous frequencies (IFs) at specific times of the superimposed M wave and the mean power frequency (MPF) of the preceding background activity during muscular fatigue. The results showed that the relationship was classified into two (complex and simple) types. In the complex type, each behavior of the MPF-IF relationship showed a different manner. Three explicitly separated and the same inclination pattern appeared in the simple type. The decrease in the IFs was higher for the simple type than for the complex type. These types were possibly produced by the individual differences in the contribution of the fast twitch muscle fibers near the surface electrodes to the conduction velocity