Normal and myopathic propagation of surface motor unit action potentials

With the aid of a computer-assisted multichannel EMG system, the propagation of motor unit action potentials is analysed during isometric voluntary muscular contraction under normal and myopathic conditions. A linear array of 30 surface electrodes is fixed above the biceps brachii muscle, parallel to the longitudinal muscle axis and centered over the end-plate zone. The EMG is simultaneously recorded on all channels and displays the propagation of surface potentials on both sides of the innervation band. The mean muscle fibre conduction velocity is computed by a cross-correlation technique. Five adult patients in a late stage of progressive muscular dystrophy show a highly altered propagation behaviour of motor unit action potentials without a time shift between potentials of adjacent electrodes. A mathematical model is described which predicts such a behaviour and suggests that it must be due to a pathological longitudinal spread of end-plates in this patient group. In 5 boys with Duchenne muscular dystrophy and only moderate impairment of the biceps muscle the mean muscle fibre velocity was reduced (2.81 +/- 0.34 m/sec) compared to 17 healthy subjects (4.42 +/- 0.37 m/sec). Each myopathic patient examined could be separated clearly from the group of healthy subjects on the basis of the surface EMG analysis.