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

Abstract 

Motor unit properties were analyzed in patients with upper motor neuron syndrome (UMNS). Multi-channel surface electromyographic (EMG) signals were recorded for 300s 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±0.9pps) in all motor units (3.60±0.21m/s vs. 3.84±0.20m/s). Conduction velocity changed by 1.35±0.48% (different from zero, P<0.01) for each increase of 1pps 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).

Keywords: Motor unit, Upper motor neuron syndrome, Multi-channel EMG, Conduction velocity