An EMG index: Its values in health and disease and its changes with fatigue

This study was designed to measure an EMG index which relates to the width of the motor unit innervation zone under the assumptions made in Lindstrom's model. This index was measured in healthy and in diseased muscles, intramuscularly, in fatigue inducing maximal voluntary contractions. The bicipital EMG interference patterns of 188 subjects in five diagnostic groups (32 healthy, 83 neuropathic, 28 myasthenic, 13 myotonic and 32 myopathic) were recorded with coaxial needle electrodes. From them, our index was computed repeatedly, in periods of 5.84 s each, to complete fatigue. The data display showed that our index decreased with fatigue. This decrement was found to be significant at P=0.00001 when all cases' data was combined. Separately, however, only the normals and the neuropathic groups reached significance at P<0.05. Measured by our index, the five groups divided into two significantly different clusters: (1) The myopathic/myasthenic cluster with large index values, and (2) the normal/neuropathic/myotonic cluster with smaller ones. Several explanations to these findings are entertained. Also, as significant group differences were found, it is safe to anticipate that this index may prove helpful, complementing other EMG indices, in the diagnosis of neuromuscular disorders. The suggested index and its decrement with fatigue deserve additional study.