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 appro
ximately half. The enlargement of the M wave was sustained during stim
ulation 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 2
0 Hz stimulation was per-formed under ischaemic conditions, the M wave
first enlarged and then gradually declined. After 20 Hz stimulation w
as discontinued, the M wave increased in size; in the ischaemic experi
ments the release of the cuff produced a fur ther, rapid augmentation.
In both the ischaemic and non-ischaemic experiments, the amplitudes a
nd areas of the M waves during the recovery period became significantl
y larger than the resting values (range, 15-60% at the endplate zone).
4. The mean muscle fibre impulse conduction velocity decreased to les
s than half the resting value during 20 Hz stimulation, with or withou
t ischaemia, and then increased above the resting value during recover
y. 5. On the basis of previous experiments in animals, the augmentatio
n of the hi wave was attributed to enhanced electrogenic Na+-K+ pumpin
g, and the biceps brachii appeared to be an excellent preparation for
studying the time course of this enhancement.