Intramuscular and surface EMG power spectrum from dynamic and static contractions

During sustained static contractions an increase in the root mean square (rms) amplitude and a decrease in mean power frequency (MPF), or median power frequency (MF) of the electromyographic (EMG) signal are indicators for the development of muscle fatigue. However, when studying dynamic contractions the interpretation of these variables has been questioned. Therefore, the purpose was to compare the EMG variables recorded from a non-fatigued muscle during a slow low level dynamic contraction to those during a static contraction of similar force level. Surface and intramuscular EMG registrations were obtained from the brachial biceps muscle during: (a) a static isotonic contraction, (b) a dynamic contraction and (c) a static anisotonic contraction. During contractions (a) and (b) the recruitment pattern was analysed using the precision decomposition method. No differences in rms, MPF or MF between the dynamic and static contractions or between the concentric and eccentric phase of the dynamic contraction were found. Furthermore 60% of the identified motor units were active both in the concentric and the eccentric phase. This indicates that motor control during a slow dynamic contraction at low force level does not influence the power spectrum. We suggest that in occupational studies a possible muscle fatigue development with time can be estimated using EMG recordings from the work tasks.