Pii: S1388-2457(99)00150-9

Objectives: Acute poliomyelitis causes degeneration of anterior horn cells, followed by denervation. Reinnervation and muscle ®bre hypertrophy are mechanisms that compensate this loss of neurones. Concentric needle EMG (CNEMG) and macro EMG are two methods to assess the magnitude of initial involvement and the compensatory reinnervation. The aim of this study is to explore the difference between CNEMG and macro EMG describing the status of the motor unit in patients previously affected by polio. Methods: Macro and concentric needle EMG investigations were performed in 261 muscles in 121 patients with a remote history of polio. Results: CNEMG was abnormal in 211 muscles, macro EMG was abnormal in 246 muscles. The macro amplitude was 3±4 times `more abnormal' than CNEMG amplitude relative to the reference values. CNEMG duration was less abnormal and showed only weak correlation with macro amplitudes. The most likely explanation for the difference in magnitude of deviation from reference values for CNEMG and macro EMG, is a more pronounced `phase cancellation' between single ®bre action potentials in CNEMG. This is supported by simulation studies reported here. Conclusions: In conclusion macro EMG better re ̄ects the size of the motor unit than the CNEMG. For detection of concomitant disorders, CNEMG is the method of choice. q 1999 Elsevier Science Ireland Ltd. All rights reserved.