Changes in Motor Unit Activity and Metabolism in Human Skeletal Muscle during and after Repeated Eccentric and Concentric Contractions

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Abstract
The electromyographic (EMG), muscle glycogen and blood lactate changes in quadriceps muscle group during repeated 40 maximal eccentric and concentric contractions were studied and the recovery in EMG, muscle glycogen and serum creatine kinase values during a 4 day period following the work test was investigated. The subjects were normal males and the test order (eccentric or concentric) was randomly selected. The results indicated first, that the EMG parameters (I[integrated]EMG, AMUP [average motor unit potential]), muscle glycogen and blood lactate changed in a similar manner during both fatigue loads. Despite the high tension work no selective depletion of glycogen was observed in the slow or fast twitch muscle fibers in either type of work. The restoring of muscle glycogen occurred in a similar manner after both fatigue loads and no significant differences were present between eccentric and concentric works in the serum creatine kinase levels for a 2 day period. The eccentric work was associated with muscle soreness, which was strongest during the 2nd day after the termination of the work test. The recovery of the EMG parameters were also delayed in eccentric fatigue. After concentric fatigue EMG-activity returned to normal values within 2 days.