Electromyographic insertional activity in normal limb muscles.

A mechanical electrode inserter was used to allow reproducible electromyographic (emg) electrode insertions. The electrical activity provoked by the mechanical insertion was recorded and quantitatively analyzed in the anterior tibialis, the extensor hallucis longus, and the extensor digitorum brevis of unimpaired human subjects. No statistically significant differences were found among these muscles in the total duration of sharp spike activity or in the duration of sharp spike activity following cessation of needle electrode movement. There was a statistically significant reduction in the total duration of mechanically-provoked electrical activity in the extensor digitorum brevis when compared to the anterior tibialis using the technique employed in this study. There was a statistically significant increase in time following the onset of needle electrode movement until the initiation of sharp spike activity in the extensor hallucis longus as compared to the anterior tibialis. Abnormalities are present in the extensor digitorum brevis muscle of clinically unimpaired subjects. Endplate spikes can be recorded as positive-shaped potentials in the endplate zones of muscle. Nonreproducible positive-shaped potentials of unknown etiology are seen to occur singly or in pairs following insertional activity in normal muscle. When examining muscle for the early evidence of provokable positive sharp wave and fibrillation potentials, care must be taken not to mistake normal characteristics of insertional activity for pathological abnormalities.