Relation between maximum discharge rates on electromyography and motor unit number estimates

To improve quantitative assessment of motor unit recruitment by standard co ncentric needle electromyography (CNEMG), hypothenar muscles of 22 healthy subjects, 18 with denervation, and 10 with a myopathy were studied. Dischar ge rates of motor units were measured in CNEMG recordings comprising action potentials of. at most, 4 motor units. Motor unit number estimation (MUNE) was done using the manual incremental method. In controls, the upper 95% l imit of the discharge rate was 16.2/s. In all subjects, a strong nonlinear correlation between the number of motor units and the maximal discharge rat e was found (r = 0.88, P < 0.0001), Increased discharge rates were found in all but one of the paretic muscles with denervation, but in none of the my opathic muscles. Measurement of the discharge rate is a simple and reliable procedure. If the discharge rate is high in a hypothenar muscle, loss of m otor units can be inferred. Moreover, the discharge rate value gives an est imate of the number of motor units in that muscle. Thus, we suggest that ma ximal discharge rate be included in electromyographic reports. (C) 2000 Joh n Wiley & Sons, Inc.