Modeling of surface myoelectric signals - Part II: Model-based signal interpretation

Experimental electromyogram (EMG) data from the human biceps brachii were s imulated using the model described in [10] of this work, A multichannel lin ear electrode array, spanning the length of the biceps, was used to detect monopolar and bipolar signals, from which double differential signals were computed, during either voluntary or electrically elicited isometric contra ctions. For relatively low-level voluntary contractions (10%-30% of maximum force) individual firings of three to four-different motor units mere iden tified and their waveforms were closely approximated by the model. Motor un it parameters such as depth, size, fiber orientation and length, location o f innervation and tendonous zones, propagation velocity, and source width w ere estimated using the model. Two applications of the model are described. The first analyzes the effects of electrode rotation with respect to the m uscle cider direction and shows the possibility of conduction velocity (CV) over- and under-estimation. The second focuses on the myoelectric manifest ations of fatigue during a sustained electrically elicited contraction and the interrelationship between muscle fiber CV, spectral and amplitude varia bles, and the length of the depolarization zone, It is concluded that a) su rface EMG detection using an electrode array, when combined with a model of signal propagation, provides a useful method for understanding the physiol ogical and anatomical determinants of EMG waveform characteristics and b) t he model provides a way for the interpretation of fatigue plots.