The possible causes of the transient enlargement of muscle compound ac
tion potentials during repetitive stimulation (''pseudofacilitation'')
are considered. The phenomenon cannot be due to mechanical artefact,
while hypersynchronization of the muscle fiber action potentials, the
usual explanation, can only make a minor contribution. A more convinci
ng explanation, for which there is now experimental evidence, is that
the muscle fibers undergo hyperpolarization, due to the intramuscular
release of norepinephrine and consequent stimulation of the electrogen
ic Na+,K+-pump. Defective phosphorylation of the Na+,K+-pump is a poss
ible cause of the transient weakness and myotonia in myotonic dystroph
y. (C) 1994 John Wiley & Sons, Inc.