Surface EMG to study the motor unit interspike intervals variability: electrode location, recording, and signal processing

Leonardo Abdala Elias
Escola de Engenharia El?trica e de Computa??o - UFG

Marcus Fraga Vieira
Laborat?rio de Biomec?nica - FEF/UFG

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     Last modified: February 28, 2007

Abstract
The electric signals of the peripheral skeletal muscles provide us important information about the motor units to which they belong, and indirectly, we can infer about the properties of the motor control system in humans. We can monitor the electromyographic signals (EMG) in two basic ways: using needle or wire electrodes (invasive methods), or using surface electrodes (non-invasive methods).
An important characteristic that must be verified in the motor control system in vertebrates (e.g., cats, humans, etc.) is their interspike intervals variability (Calvin and Stevens, 1968), and this can be verified through the electromyographic activity of the peripherals skeletal muscles (Sturm et al., 1997; Poliakov, et al., 1995), or through the direct detection of the action potentials in the motoneuron (Calvin and Stevens, 1968).
The aim of this work was to propose an experimental protocol for the evaluation of the motor unit interspike intervals variability of the Soleus muscle, using the surface electromyography (sEMG), supplying a fast and effective answer about an important physiologic mechanism thus.
Our protocol proposes the placement of the circular surface electrodes (Ag/AgCl; diameter ? 10 mm), with 2 cm of interelectrode distance in a bipolar differential detection. Those electrodes should be positioned 4 cm below the junction of the two heads of the Gastrocnemius muscle in a lateral portion, guaranteeing the placement of the electrodes in the Soleus muscle area, between the innervation zone and the tendon zone. A reference electrode was positioned in the wrist of the analyzed individuals.
The active sensors were pre-amplified with 20x gain and CMRR > 100 dB, and a 16-BIT ADC with 100x gain was used to digitalize the signal with 2000 S/s.
The electromyographic activity under the condition of an isometric volunteer contraction of low intensity (plantar flexion) was registered, and the data were stored in a personal computer, where they have been filtered through a band-pass filter (10-500 Hz, Butterworth, 4th order) for artifacts elimination.
A computer program, written in Matlab? language, detects the action potential picks belonging to the same motor unit, and calculates the interspike intervals, where we can infer about the statistics of these intervals, and visualize the variability of them.
With this protocol, the data concerning the interspike intervals variability of the Soleus muscle could be obtained, inferring about the behavior of the motor control system in humans using a non-invasive method thus.