Electromyogram power spectrum and features of the superimposed maximal M-wave during voluntary isometric actions in humans at different activation levels

The frequency characteristics of the electromyogram (EMG) power spectrum, s uch as the median or the mean power frequency. as well as the duration of t he muscle compound action potential response to a single supramaximal elect rical stimulus (maximal M-wave) may both be related to the conduction veloc ity (CV) of the muscle fibre. To investigate this further. we studied in te n male subjects: the EMG of the vastus lateralis. vastus medialis and rectu s femoris muscles during maximal isometric knee extensions at 40%, 60%, 80% and 100% of maximal voluntary contraction and also the maximal M-wave, eli cited by a single supramaximal stimulus to the femoral nerve. of the same m uscles at rest or superimposed on the same levels of voluntary contraction. The EMG was recorded during the constant force phase of the voluntary cont ractions, the duration of which was 2.5-4 s, with a 1.5 min pause between c ontractions. The average EMG (aEMG) and the median frequency (MF) were then calculated. The results indicated that as aEMG increased with increase in force, MF remained unchanged. However, while the amplitude of the M-wave wa s not affected, the duration of the M-wave was shorter as the force level i ncreased. The duration of the M-wave may be affected by recruitment of fast er motor units. by increased firing rate of the active units and by changes in the muscle fibre length. The shorter duration of the M-wave observed at higher force levels was not, however. accompanied by a corresponding incre ase in MF. The MF could not therefore be used as a parameter to reflect the changes in voluntary muscle activation and CV. When MF was calculated by o verlapping the fast Fourier transform (FFT) windows (0.4 s long window move d data point by data point to the right) for I s periods of the isometric p lateau phase at each force level, the difference between the lowest and the highest MF was quite substantial. These variations suggest that FFT window placement, i.e. which part of the signal is chosen for the analyses, may p lay an important role even in isometric situations.