Changes in shoulder torque timing and direction during withdrawal reflexes in the impaired upper limb of hemiparetic stroke subjects

Flexion withdrawal responses in the impaired/unimpaired upper limbs of hemiparetic stroke subjects were studied to determine the relationship between cutaneous afferent input and abnormal muscle synergies. A mildly noxious electrical stimulus was applied to the index finger while concurrently measuring relevant EMG and joint torque responses. The reflex response in the impaired arm was significantly altered. EMG and torque onsets were delayed at both the shoulder and elbow across all subjects along with torque reversals in two of the three shoulder torques (adduction-abduction in 4/8 subjects and shoulder extension-shoulder flexion in 6/8 subjects). Similar results have been observed during the characterization of spatial EMG profiles and during the execution of maximum voluntary torques. It follows that cutaneous and other sources of tonic sensory input may strongly influence muscle coactivation patterns in the impaired upper extremity of hemiparetic stroke subjects.