EMG filling analysis, a new method for the assessment of recruitment of motor units with needle EMG

Objectives

The progression of recruitment of motor unit potentials (MUPs) during increasing voluntary contraction can provide important information about the motor units (MUs) innervating a muscle. Here, we described a method to quantitate the recruitment level of the intramuscular electromyographic (iEMG) signal during an increasing force level.

Methods

Concentric needle EMG signals were recorded from the tibialis anterior of healthy subjects as force was gradually increased from 0 to maximum force. The iEMG filling process was analyzed by measuring the EMG filling factor (FF), calculated from the mean rectified iEMG and the root mean square iEMG.

Results

(1) The iEMG activity at low contraction forces was “discrete” (FF<0.3) for all participants. (2) The iEMG activity at maximal effort was “full” (FF>0.5) for 83 % of the participants, whereas it was “incompletely-reduced” (0.3<FF< 0.5) for 17 % of the participants. (3) The FF increased rapidly for forces up to 20 % MVC, and then levelled off for higher forces: thus, the FF curve had a typical exponential shape.

Conclusions

The iEMG filling method can be considered of general applicability since the FF increased over a wide range in all healthy participants.

Significance

The EMG filling analysis may have potential to detect scenarios of MU loss and remodelling in neurogenic and motor neuron diseases.