The role of the biarticular agonist and cocontracting antagonist pair in isometric muscle fatigue.

Isometric knee extensions until exhaustion at 30%, 50%, and 70% of maximum voluntary contraction were performed by 18 healthy subjects. During muscle fatigue, surface electromyographic activity was recorded from the knee-extensors vastus lateralis, vastus medialis, and rectus femoris, and the coactive antagonistic biceps femoris. The electromyographic parameter median frequency (MF) served as a measure of fatigue. Coefficients of regression of the MF fatigue changes were analyzed statistically. MF fatigue occurred within the coactive biceps femoris and was significantly more pronounced than in the quadriceps. When the MF fatigue shifts of the coactive biceps femoris were compared with each of the three investigated parts of the quadriceps separately, MF fatigue shifts were similar in shape for the biarticular coactive biceps femoris and the biarticular rectus femoris, but differed significantly between the biceps femoris and the two monoarticular muscles, vastus medialis and vastus lateralis. As both the biarticular agonist and coactive antagonist muscles fatigued at a higher rate than the two monoarticular muscles, it seems likely that this biarticular agonist/antagonist pair determines the time to the limit of endurance.