Innervation-zone width in disease and its changes with fatigue

This study was designed to measure the average size of the motor unit functional innervation zone (FINZONE) in healthy and in diseased muscles, intramuscularly, in fatigue-inducing maximal voluntary contractions. The bicipital EMG interference patterns of 188 subjects (32 healthy, 83 neuropathic, 28 myasthenic, 13 myotonic, and 32 myopathic) were recorded with coaxial needle electrodes. From them, the FINZONE size criterion was computed, repeatedly, each 5.84 s to complete fatigue. The data display showed that the FINZONE size diminished with fatigue. Over all groups, this decrement was significant at p = 0.00001. Separately, only the normals and the neuropathic groups reached significance. Measured by the FINZONE size, the five groups divided into two significantly different clusters: (a) the {myopathic/myasthenic} cluster with large FINZONE sizes and (b) the {normal/neuropathic/myotonic} cluster with smaller ones. Several explanations for these findings were entertained. Also, as significant group differences were found, these results may prove helpful, complementing other variables, in the diagnosis of neuromuscular disorders. Both issues (FINZONE diagnostic value and FINZONE size decrement with fatigue) deserve additional study.