Depth and intensity of equivalent current dipoles estimated through an inverse analysis of surface electromyograms using the image method

The depth and intensity of equivalent current dipoles that can create the surface potentials of active motor units in human skeletal muscles are estimated through an inverse analysis of surface electromyographic (EMG) potentials in an attempt to measure detailed muscular activity non-invasively. The inverse analysis is conducted by repetition of forward analyses. In the study, the image method is used for forward analysis, because it is the simplest potential calculation method for electric currents in a semi-infinite volume conductor. Using this method, surface EMG potentials are calculated for current sources assumed to be located in a muscle. An inverse analysis is then carried out by searching for the depth and intensity of such current sources that would minimise the sum of squares difference between measured and calculated surface EMG potentials. The inverse analysis is applied to surface EMG potentials measured from the biceps brachii of three healthy subjects. As a result, the individual current sources are estimated to be 2.7 +/- 1.6 mm deep and 0.5 +/- 0.9 nAm in intensity, whereas the total current intensity for individual motor units is 2.4 +/- 2.9 nAm.