Is the mean power frequency shift of the EMG a selective indicator of fatigue of the fast twitch motor units?

During repeated maximum isokinetic contractions parallel decreases in output and the mean power frequency (MPF) of the electromyogram (EMG) occur, followed by stable levels (the endurance level). This study aimed to investigate if hypoxia of the plantar flexors will result in a shift in MPF during the endurance level. Mechanical performance and simultaneous surface EMG activity of three plantar flexors (m. soleus, m. gastrocnemius medialis and m. gastrocnemius lateralis) were recorded during repeated isokinetic plantar flexions at 1.05 rad s-1 in 10 healthy women. The study consisted of two tests with 14 days in between: (1) a maximum test consisting of 125 maximum plantar flexions. After 100 contractions, using a tourniquet, a relative ischaemia (hypoxia) was induced. (2) a submaximum test performed at the level established between contractions 70-100 of the maximum test, consisting of 125 plantar flexions. During the maximum test mechanical performance output, the signal amplitude (RMS) of the EMG and the MPF decreased in parallel during the initial 50 contractions followed by stable levels during the subsequent 50 contractions. When hypoxia was induced decreases in mechanical performance and RMS appeared without significant decreases in MPF. The present study indicates that the MPF shift is mainly sensitive to biochemical changes in the type 2 fibres. Hypoxia mainly located to the type 1 fibres will not be detected using the MPF variable.