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

To improve quantitative assessment of motor unit recruitment by standard concentric needle electromyography (CNEMG), hypothenar muscles of 22 healthy subjects, 18 with denervation, and 10 with a myopathy were studied. Discharge 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% limit 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 rate 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 myopathic muscles. Measurement of the discharge rate is a simple and reliable procedure. If the discharge rate is high in a hypothenar muscle, loss of motor units can be inferred. Moreover, the discharge rate value gives an estimate of the number of motor units in that muscle. Thus, we suggest that maximal discharge rate be included in electromyographic reports.