DETERMINANTS OF MEAN MOTOR UNIT SIZE - IMPACT ON ESTIMATES OF MOTOR UNIT NUMBER

The purpose of this study was to compare two fundamentally different m ethods of deriving the average surface-detected motor unit action pote ntial (S-MUAP) size from which to calculate a motor unit number estima te (MUNE), namely: (1) the simple arithmetic average of S-MUAP paramet er values; and (2) a computer-derived datapoint by datapoint average w aveform which takes account of differences in S-MUAP shapes and durati ons. Multiple point stimulation was used to collect representative sam ples of between 11 and 20 S-MUAPs (mean 15 +/- 2 SD) from the median-i nnervated thenar muscles of 20 healthy control subjects between 20 and 76 years of age (mean 48 +/- 19 SD). The average S-MUAP size based on peak-to-peak amplitude, negative peak amplitude, and negative peak ar ea measurements was calculated using the two different methods. The me an S-MUAP sizes based on the average waveform were significantly lower in all cases than those based on the simple average of S-MUAP paramet er values. Differences tended to be greatest for MUNEs based on peak-t o-peak amplitude (35%), less for negative peak amplitude (20%), and le ast for negative peak area (16%). (C) 1993 John Wile & Sons, Inc.