[Unit potential analysis in the diagnosis of myopathies: development of quantitative EMG parameters]

In standard concentric needle electromyography the typical triphasic potential of a motor unit (MUP) consists of the spike component and the positive initial and terminal parts. Whereas the main spike is generated by a few muscle fibers located close to the tip of an electrode, the initial and terminal parts represent the sum of activities of a large proportion of fibers within the motor unit. The size of the initial and terminal part is assessed by measuring the duration of the MUP. In addition to the duration we calculated the mean absolute voltage outside the spike component: The signal was digitized and several individual MUPs of the same motor unit were averaged. The point of maximal negative rise was identified. Then the mean absolute voltage between -5 ms and +5 ms around this point, excluding the interval from -1 ms to +1 ms, was calculated. This parameter is exactly defined and unambiguous. The biceps brachii muscles of 30 healthy volunteers were examined in order to establish normal values. In 24 patients suffering from various myopathies 18 had significant (p less than 0.01) reduced initial and terminal parts, whereas only 13 patients showed decreased potential duration (according to Buchthal 1957).