Reorganization of multi-muscle and joint withdrawal reflex during arm movements in post-stroke hemiparetic patients.

OBJECTIVES: To investigate the behavior of the nociceptive withdrawal reflex (NWR) in the upper limb during reaching and grasping movements in post-stroke hemiparetic patients. METHODS: Eight patients with chronic stroke and moderate motor deficits were included. An optoelectronic motion analysis system integrated with a surface EMG machine was used to record the kinematic and EMG data. The NWR was evoked through a painful electrical stimulation of the index finger during a movement which consisted of reaching out, picking up a cylinder, and returning it to the starting position. RESULTS: We found that: (i) the NWR is extensively rearranged in hemiparetic patients, who were found to present different kinematic and EMG reflex patterns with respect to controls; (ii) patients partially lose the ability to modulate the reflex in the different movement phases; (iii) the impairment of the reflex modulation occurs at single-muscle, single-joint and multi-joint level. CONCLUSIONS: Patients with chronic and mild-moderate post-stroke motor deficits lose the ability to modulate the NWR dynamically according to the movement variables at individual as well as at multi-muscle and joint levels. SIGNIFICANCE: The central nervous system is unable to use the NWR substrate dynamically and flexibly in order to select the muscle synergies needed to govern the spatio-temporal interaction among joints.