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.