Effect of small motor unit potentials on the motor unit number estimate

Small surface motor unit potentials (S‐MUPs) may have a negative influence on the variability of the motor unit number estimate (MUNE). According to published consensus criteria S‐MUPs with a negative peak amplitude smaller than 10 μV should be omitted. The effect of omitting small S‐MUPs on the MUNE was evaluated using a simulation model. The model incorporated a healthy and amyotrophic lateral sclerosis (ALS) distribution formed with real S‐MUPs. Using a random drawing process the MUNE was calculated with and without small S‐MUPs. In the healthy population 27% of all S‐MUPs were small. MUNE determined without these S‐MUPs was marginally less variable. However, MUNE values dropped about 24% at a sample size of 20. In ALS, only 12% of the total population of 130 S‐MUPs were small. MUNE dropped about 12% without the small S‐MUPs. By omitting small S‐MUPs the differences between the healthy and ALS distributions become smaller. Therefore, incorporating small S‐MUPs in the estimate is suggested. © 2008 Wiley Periodicals, Inc. Muscle Nerve, 2008