Motor unit potential recruitment reference values in common upper and lower extremity muscles

To define reference values for motor unit (MU) recruitment during needle EMG of six commonly examined muscles at low to moderate contraction.

Needle examination was performed for each muscle in a total of 111 subjects without neuromuscular disorders. Fastest firing rates and recruitment ratios (RRs) were calculated in at least 5 sites within each muscle. Upper limits of normal based on 97th percentile for fastest MU firing rates and RRs were calculated for each muscle. The means of fastest firing rates were compared among muscles using the Friedman and Wilcoxon signed rank tests.

The upper limits of normal were 12-15 Hz for fastest firing rates and were slightly higher in the deltoid and triceps than the other muscles.

Firing rates >15 Hz recorded at multiple sites within a single muscle exceed the 97th percentile of normal subjects and may suggest reduced MU recruitment. In some muscles, rates >12 Hz might support mildly reduced recruitment. Recruitment ratios varied depending on number of firing MUs, whereas the fastest firing MU rate did not.

The determination of reference values for fastest MU firing rates can help to identify mild reduction in recruitment with more accuracy.