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Department of Biomedical Sciences, McMaster University, Hamilton, Ontario, Canada.

1. The effects of repeated excitation on the compound action potential, or M wave, of mammalian muscle fibres have been investigated in the human biceps brachii. 2. During continuous indirect stimulation at 10 and 20 Hz the mean voltage-time area of the M wave doubled within the first minute, while the mean peak-to-peak amplitude increased by approximately half. The enlargement of the M wave was sustained during stimulation at 10 Hz but not at 20 Hz. Stimulation at 3 Hz caused a small increase which was significant for M wave amplitude only. 3. When the 20 Hz stimulation was performed under ischaemic conditions, the M wave first enlarged and then gradually declined. After 20 Hz stimulation was discontinued, the M wave increased in size; in the ischaemic experiments the release of the cuff produced a further, rapid augmentation. In both the ischaemic and non-ischaemic experiments, the amplitudes and areas of the M waves during the recovery period became significantly larger than the resting values (range, 15-60% at the endplate zone). 4. The mean muscle fibre impulse conduction velocity decreased to less than half the resting value during 20 Hz stimulation, with or without ischaemia, and then increased above the resting value during recovery. 5. On the basis of previous experiments in animals, the augmentation of the M wave was attributed to enhanced electrogenic Na(+)-K+ pumping, and the biceps brachii appeared to be an excellent preparation for studying the time course of this enhancement.