Motor unit properties of biceps brachii in chronic stroke patients assessed with high-density surface EMG

The aim of this study was to investigate motor unit (MU) characteristics of the biceps brachii in post-stroke patients, using high-density surface electromyography (sEMG). Eighteen chronic hemiparetic stroke patients took part. The Fugl-Meyer score for the upper extremity was assessed. Subjects performed an isometric step contraction consisting of force levels from 5 to 50% MVC while sEMG of the biceps brachii was recorded with a two dimensional 16-channel electrode array. This was repeated for both sides. Motor unit action potentials (MUAPs) were extracted from the EMG signals, and their root-mean-square value (RMSMUAP, reflecting MU size) and mean frequency of the power spectrum (FMEANMUAP, reflecting recruitment threshold) were calculated. FMEANMUAP was smaller on the affected than on the unaffected side, indicating an increased contribution of low-threshold motor units, possibly related to degeneration of high-threshold motor units. The ratio of RMSMUAP on the affected side divided by that on the unaffected side correlated significantly with the Fugl-Meyer score. This ratio may reflect the extent to which reinnervation has occurred on the affected side.