Effects of contraction level on the changes of surface electromyogram during fatiguing static contractions

Changes of the surface electromyogram (EMG) during a fatiguing static contraction were studied at contraction levels of 5, 10, 15, 20, 30, 40, and 50% of the maximum voluntary contraction (MVC). Seven subjects performed the static contractions of the elbow flexion and 6 subjects performed those of the planter flexion. Bipolar surface EMG was led from biceps brachii (BB) in the elbow flexion and from soleus in the planter flexion. Power spectra, mean amplitudes of EMG (AEMG) and durations of averaged waves classified by amplitude (AWCA) were calculated from EMG. The AWCA was calculated by averaging EMG waves separately for the 5 amplitude ranges of the trigger points. Relative power increased below 50 Hz in BB and 120 Hz in soleus during the contractions with time. Relative power in low frequencies (RPWL) increased with time even at the low contraction levels during one third part of the contraction from the start. But the RPWL decreased with time in some cases below and equal to 20% MVC. The increase in the RPWL was larger at 20-50% MVC than at 5-15% MVC contraction. The duration of AWCA increased with time at 30-50% MVC and decreased at 5-20% MVC. The duration, however, correlated positively with RPWL in part even at 5-20% MVC. The changes in the AWCA duration were attributed to the changes in conduction velocity (CV) of muscle action potential, since the relation between the changes in the duration by fatigue and the contraction levels was the same with the reported relation between changes in CV and the contraction levels.(ABSTRACT TRUNCATED AT 250 WORDS)