The aim of this study was to test the hypothesis that contractility of quadriceps femoris muscle during a 15-min period after a sustained maximum voluntary contraction for 1 min is determined by the interaction of posttetanic potentiation, metabolic fatigue, and nonmetabolic fatigue. Eleven healthy untrained men (age, 22.9±1.8 years; body weight, 77.5±5.2 kg) performed isometric 1-min maximum voluntary contraction at long (90° in knee joint) and short (135° in knee joint) muscle length at two different occasions. Contractility of quadriceps femoris muscle was monitored via the evoked contractions at 1, 10, 20, and 50 Hz and maximum voluntary contraction at short and long muscle length on both occasions. Force generating capacity was reduced immediately after 1-min maximum voluntary contraction at short and long muscle length, and then a bi-modal time-course of recovery was observed which consisted of (1) rapid recovery of all measured indexes at 3 min and (2) divergence in the changes of forces at low and high stimulation frequencies, as well as maximal voluntary contraction force at 7 and 15 min after exercising. The decline in force immediately after 1-min isometric load was caused by metabolic and nonmetabolic fatigue; however, factors related to the metabolic fatigue were prevalent. As the effect of metabolic fatigue was diminishing and posttetanic potentiation was still present, force generation capacity recovered at 3 minutes after exercising. Further dynamics of contractility can be explained by the fading influence of posttetanic potentiation and dominant effect of nonmetabolic fatigue.
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