Intramuscular EMG coherence in health and in neuromuscular disorders

The electromyogram (EMG) interference pattern (EMGIP) is the summed activities of many single fiber action potentials (SFAPs) as they propagate past the recording electrodes, and reflects their temporal and spatial relationships and their propagation speed. Comparing EMGIPs from two locations along the same muscle fibers should give insight into changes that were introduced between these two locations. These changes are expected to correlate with normal and abnormal muscle physiology. In this study, the coherence function was used as the comparison tool. This study was designed to measure the coherence between two EMGIPs recorded intramuscularly by two concentric electrodes separated by 1 cm along the same muscle fibers. The intent was to find coherence values in-and differences among-healthy and diseased muscles. There were 191 subjects: 33 healthy, 85 neuropathic, 28 myasthenic, 13 myotonic, and 32 myopathic. The two main features of this coherence analysis were the significant differences in coherence values between some of the groups in the frequency range of 0 to 70 Hz, and the marked lack of coherence between the two recording electrode EMGIPs in the frequency range above 300 Hz in all of the groups. Also, statistical analysis based on coherence values indicated two significantly distinct clusters in the frequency range of 0 to 70 Hz: the myopathy/myasthenic cluster, with relatively high coherence values, and the normal/neuropathy/myotonic cluster, with relatively low coherence values. Coherence-based classification was correct in 67% of subjects. On theoretical grounds, it was expected that increased dispersion of muscle fiber conduction velocities in myopathy will translate into high-frequency coherence differences between the groups, but this was not found. Instead, significant differences were found in the low-frequency range. It is postulated that the latter resulted from changes in the muscle volume conductor differences in packing of the muscle fibers and differences in fibrous tissue that resulted in differences of impedance within the muscles. This study emphasizes the need for further research into the role and possible utilization of the coherence function in electromyography.