The effect of endurance swimming training on AChE molecular forms was examined in 2 groups of functionally antagonist rat muscles, including ankle extensors and flexors, This exercise regimen, which entails predominant dynamic activity (i.e., involving extensive shortening) of both groups or muscles, resulted in marked selective G4 increases in all fast muscles. The G4 elevation exhibited by the ankle flexors was in sharp contrast to the G4 reduction reported in these same muscles following running training, during which their action is predominantly tonic. The results strengthen the conclusion that predominantly dynamic activity increases the G4 content of mature innervated fast muscles.
