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Department of Clinical Neurophysiology, Karolinska Hospital, Stockholm, Sweden.

1. A delayed reduction of tetanic tension during recovery from fatigue, produced by repeated tetanic contractions, has been found in isolated amphibian muscle fibres (Westerblad & Lännergren, 1986). We have now followed the recovery in a mammalian muscle preparation to see if a similar phenomenon may appear also in mammalian muscles. 2. Single motor units of the rat tibialis anterior muscle were fatigued by 200 ms-80 Hz stimulation trains, initially delivered every 4 s and then at decreasing intervals until tension was depressed to about 30% of the original. 3. After about 10 min of recovery tetanic tension was almost normalized. Tension then decreased in most units to a minimum, which occurred after 30-100 min of recovery. At this stage tension outputs ranged from 6 to 95% of controls. 4. The EMG amplitude was also followed and like tension it was first almost normalized and then depressed. Generally the EMG depression was bigger than the tension loss; the EMG amplitudes were also somewhat reduced in the units not displaying any clear force reduction. 5. Both tetanic tension and EMG amplitude recovered gradually from the depressed state and values similar to controls were eventually reached. 6. In conclusion, we have found a novel type of delayed tension reduction in mammalian muscle during recovery after fatiguing contractions. The exact cause of this reduction is presently unclear, but an important contribution from impaired excitation-contraction coupling is highly plausible.

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