Motor unit discharge pattern and conduction velocity in patients with upper motor neuron syndrome

Motor unit properties were analyzed in patients with upper motor neuron syndrome (UMNS). Multi-channel surface electromyographic (EMG) signals were recorded for 300 s from the biceps brachii muscle of seven male subacute patients (time from lesion, mean ± SE, 4.9 ± 1.0 months). In three patients, both arms were investigated, leading to 10 recorded muscles. Patients were analyzed in rest-like condition with motor units activated due to pathological muscle overactivity. For a total of 12 motor units, the complete discharge pattern was extracted from EMG decomposition. Interpulse interval variability was 7.8 ± 0.9%. At minimum discharge rate (6.4 ± 0.4 pulses per second, pps), conduction velocity was smaller than at maximum discharge rate (12.0 ± 0.9 pps) in all motor units (3.60 ± 0.21 m/s vs. 3.84 ± 0.20 m/s). Conduction velocity changed by 1.35 ± 0.48% (different from zero, P < 0.01) for each increase of 1 pps in discharge rate. It was concluded that conduction velocity of low-threshold motor units in subacute patients with UMNS had similar values as reported in healthy subjects and was positively correlated to instantaneous discharge rate (velocity recovery function of muscle fibers).