Motor unit properties of biceps brachii during dynamic contractions in chronic stroke patients

The aim of this study was to investigate motor unit (MU) characteristics of the biceps brachii during sinusoidal contractions in chronic stroke patients using high-density surface electromyography. Ten sinusoidal elbow flexion and extension movements were performed both passively and actively by 18 stroke patients and 20 healthy subjects. Motor unit action potentials (MUAPs) were extracted, and their root-mean-square value (RMSMUAP) was calculated. RMSMUAP was significantly larger in stroke than in healthy subjects. In both groups RMSMUAP was smaller during the stretch phase of passive movement than during active movement. The larger MUAPs indicate enlarged MUs, possibly as a result of reinnervation. The lower RMSMUAP values during passive stretch than during active movement indicates that the stretch reflex mainly activates smaller MUs, while a larger part of the MU pool can be recruited voluntarily. RMSMUAP may have added value for monitoring changes in peripheral MU properties after stroke