SURFACE-POTENTIALS GENERATED BY SYNCHRONOUS ACTIVATION OF DIFFERENT FRACTIONS OF THE MOTOR POOL

Surface electromyogram potentials were recorded from the abductor poll icis brevis, abductor digiti minimi, and extensor digitorum brevis mus cles in response to finely graded nerve stimuli. Successive potentials were subtracted to obtain intermediate potentials 2%, 5%, 10%, 20%, a nd 50% of the maximal compound muscle action potential (mCMAP). The av erage latency of the onset and negative and positive peaks, and the av erage duration of the negative phase and whole potential were similar for all degrees of fractionation, although smaller fractionation was a ssociated with increasing variability. An initial positivity occurred with some of the smaller fractions of the CMAP, particularly those wit h the lowest stimulus threshold, Submaximal CMAPs closely resembled th e mCMAP once their amplitude was greater than 5-10% of the mCMAP. Our results support the common practice of expressing H-reflex amplitudes as a percentage of the M wave and may partly explain why reflexes ''sc ale'' in response to tonic activation. (C) 1996 John Wiley & Sons, Inc .