The effects of preceded fatiguing on the relations between monopolar surface electromyogram and fatigue sensation

Relations between surface electromyogram (EMG) and fatigue sensation was compared between the first fatiguing contraction and the following contractions with insufficient rests. Six male subjects performed static contractions of the elbow flexors at 8 and 13% MVC with the forearm semipronated and at 13% MVC with the forearm supinated. Contractions were repeated 6 (13% MVC) or (8% MVC) times (C1, C2, C3, C4, C5, C6) with rests of 5 min (13% MVC) or 10 min (8% MVC). During the contractions the subjects reported values of rated fatigue sensation (VRFS) whose scale was from 0.0 (no fatigue sensation) to 4.0 (apparent pain and moderate severity) at every 30 sec. C1 ended between 3.5 and 4.0 of VRFS or at 18 min (13% MVC) or 20 min (8% MVC) of contracting time. C2-C5 ended when fatigue sensation became that at the end of C1 or 5 min had passed. Bipolar and monopolar surface EMG was recorded from 6 synergists of elbow flexors. Mean amplitude (AEMG) and relative power below 22Hz (RPWL20) were calculated. Though AEMGs were positively correlated with VRFSs in C1, the correlations were often obscure in C2-C6. Most AEMGs at the same VRFS were larger in C2-C6 than in C1. AEMGs at the start of C2-C6 were sometimes larger than those at the end of C1. RPWL20s of monopolar EMG positively correlated with VRFSs. The RPWL20 correlations were often reserved in all contractions. The increases of AEMGs in C2-C6 at the same VRFS cannot be explained by the migration of the activities among synergists, since the increases in AEMGs were seen in most synergists simultaneously. The compensation of the failure of muscle contractibility were not the only cause of the increases in the AEMGs during fatiguing contractions. The compression of EMG spectrum towards lower frequencies was usable to estimate the change of fatigue sensation.