Relation between Discharge Rates on Needle Electromyography and Motor Unit Number Estimates in Leg Muscles

Abstract

Discharge rates of motor units on needle EMG depend on the number of motor units available for recruitment. We studied this dependence in extensor digitorum brevis (EDB) and tibialis anterior muscles (TA) in 21 healthy subjects, 24 patients with a neuropathy, and 6 patients with a myopathy. A strong non-linear correlation between the number of motor units and the discharge rate was found (EDB r=0.79, P<0.001; TA r=0.80, P<0.001). An increased discharge rate indicated a loss of motor units. The mean value of the maximal discharge rates was 11.5/s in the controls and 18.6/s in the patients with neuropathy in EDB muscles and 11.5/s in the controls, 12.5/s in the patients with neuropathy, and 11.2 in the patients with a myopathy in TA muscles, respectively. Maximal discharge rates also correlated with muscle strength, but did not depend on the gender. The correlation closely resembled the correlation previously found in hypothenar muscles. Thus, increased discharge rates may be used to estimate a loss of motor units in limb muscles.