Submitted on April 29, 2006
Accepted on October 24, 2006
Sensitivity of the cross-correlation between simulated surface EMGs for two muscles to detect motor unit synchronization
Kevin G. Keenan1, Dario Farina2, Francois Meyer3, Roberto Merletti4, and Roger M. Enoka5*
1 Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York, United States
2 Center for Sensory-Motor Interaction, Aalborg University, Aalborg, Denmark
3 Electrical and Computer Engineering, University of Colorado, United States
4 Biomedical Engineering Center, Politecnico di Torino, Torino, Italy
5 Department of Integrative Physiology, University of Colorado, Boulder, Colorado, United States
* To whom correspondence should be addressed. E-mail: roger.enoka{at}colorado.edu.
The study evaluated the use of cross-correlation analysis between simulated surface electromyograms (EMGs) of two muscles to quantify motor unit synchronization. The volume conductor simulated a cylindrical limb with two muscles and bone, fat, and skin. Models of two motor neuron pools were used to simulate 120 s of surface EMG that was detected over both muscles. Short-term synchrony was established using a phenomenological model that aligned the discharge times of selected motor units within and across muscles to simulate physiological levels of motor unit synchrony. The correlation between pairs of surface EMGs was estimated as the maximum of the normalized cross-correlation function. After imposing four levels of motor unit synchrony across muscles, five parameters were varied concurrently in the two muscles to examine their influence on the correlation between the surface EMGs: 1) excitation level (5, 10, 15, and 50% of maximum); 2) muscle size (350 and 500 motor units); 3) fat thickness (1 and 4 mm); 4) skin conductivity (0.1 and 1 S/m); and 5) mean motor unit conduction velocity (2.5 and 4 m/s). Despite a constant high level of motor unit synchronization among pairs of motor units across the two muscles, the cross-correlation index ranged from 0.08 to 0.56 with variation in the five parameters. These results indicate that cross correlation of the surface EMGs from two muscles provides a limited measure of the level of synchronization between motor units in the two muscles.
