Estimation of the number of motor units in the human extensor digitorum brevis using MScanFit

Abstract

Objective: Our aim was to determine the number and size parameters of EDB motor units in healthy young adults using MScanFit, a novel approach to motor unit number estimation (MUNE). Since variability in MUNE is related to compound muscle action potential (CMAP) size, we employed a procedure to document the optimal EDB electromyographic (EMG) electrode position prior to recording MUNE, a neglected practice in MUNE.

Methods: Subjects were 21 adults 21-44 y. Maximum CMAPs were recorded from 9 sites in a 4 cm2 region centered over the EDB and the site with the largest amplitude was used in the MUNE experiment. For MUNE, the peroneal nerve was stimulated at the fibular head to produce a detailed EDB stimulus-response curve or "MScan". Motor unit number and size parameters underlying the MScan were simulated using the MScanFit mathematical model.

Results: In 19 persons, the optimal recording site was superior, superior and proximal, or superior and distal to the EDB mid-belly, whereas in 3 persons it was proximal to the mid-belly. Ranges of key MScanFit parameters were as follows: maximum CMAP amplitude (3.1-8.5 mV), mean SMUP amplitude (34.4-106.7 μV), mean normalized SMUP amplitude (%CMAP max, 0.95-2.3%), largest SMUP amplitude (82.7-348 μV), and MUNE (43-103). MUNE was not related to maximum CMAP amplitude (R2 = 0.09), but was related to mean SMUP amplitude (R2 = -0.19, P = 0.05).

Conclusion: The EDB CMAP was highly sensitive to electrode position, and the optimal position differed between subjects. Individual differences in EDB MUNE were not related to CMAP amplitude. Inter-subject variability of EDB MUNE (coefficient of variation) was much less than previously reported, possibly explained by better optimization of the EMG electrode and the unique approach of MScanFit MUNE.

Fig 1. CMAP mapping.

(A) Positions of the active EMG electrode used for mapping maximal CMAPs from 9 sites (site 0 to site 8). Site 0 is located at the approximate EDB mid-belly, and sites 1 to 8 are located at the designated target positions about the perimeter of a 2 cm x 2 cm square drawn on the skin. Also shown are 2 ink marks (one over the lateral malleolus and the other at the 5^th toe cleft) for determining the one-third distance to the EDB mid-belly (B) Overlay of CMAP waveforms in one participant. Single maximal CMAPs from sites 0 to 8, and repeat recordings from sites 0, 3 and 4, are displayed. In this case, site 4 was the optimal recording site.

Fig 2. Maximum CMAP waveforms, MScans, nerve conduction and MScanFit results in 3 subjects.

(A, D) Male 36 y. (B, E) Male 30 y. (C, F) Female 36 y. Maximum CMAPs evoked by nerve stimulation at the knee and ankle are overlayed in 30 ms windows in A, B, and C. (D-F) MScan recording (left) and corresponding modeled MScan (right) for each person. (G) Nerve conduction and MScanFit study results in the 3 subjects.

Fig 3. Correlations between MScanFit parameters in all subjects.

Data points for males are black symbols (N = 15) and females are white symbols (N = 6). (A) SMUP amplitude was positively correlated with maximal CMAP amplitude. (B) MUNE was negatively correlated with SMUP amplitude. (C) MUNE was not significantly correlated with maximum CMAP amplitude. Lines of best fit are shown only for correlations that were statistically significant. Circled data points correspond to the 3 subjects in Fig 2. Data points surrounded by a triangle are two subjects with similar CMAP maximum but different MUNE.