INTRAMUSCULAR AND SURFACE EMG POWER SPECTRUM FROM DYNAMIC AND STATIC CONTRACTIONS

During sustained static contractions an increase in the root mean squa re (rms) amplitude and a decrease in mean power frequency (MPF), or me dian power frequency (MF) of the electromyographic (EMG) signal are in dicators for the development of muscle fatigue. However, when studying dynamic contractions the interpretation of these variables has been q uestioned. Therefore, the purpose was to compare the EMG variables rec orded from a non-fatigued muscle during a slow low level dynamic contr action to those during a static contraction of similar force level. Su rface and intramuscular EMG registrations were obtained from the brach ial biceps muscle during: (a) a static isotonic contraction, (b) a dyn amic contraction and (c) a static anisotonic contraction. During contr actions (a) and (b) the recruitment pattern was analysed using the pre cision decomposition method. No differences in rms, MPF or MF between the dynamic and static contractions or between the concentric and ecce ntric 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 dynam ic contraction at low force level does not influence the power spectru m. We suggest that in occupational studies a possible muscle fatigue d evelopment with time can be estimated using EMG recordings from the wo rk tasks.