Co-contraction and Corticoreticular Excitability in Spinal Cord Injury

This study is to elucidate whether hyperexcitability of the cortico-reticulospinal tract leads to abnormal co-contraction and spasticity of elbow flexors in persons with cervical cervical spinal cord injury. By measuring the supplementary motor area (SMA) regulation of the StartReact response, it is possible to quantify whether the cortico-reticular pathway is excessively active or weakened in these persons. Participants will 1. Be assessed for upper limb function 2. Receive motor evoked potential of tested biceps muscle 3. Receive a standard StartReact paradigm to measure the reticulospinal excitability. 4. Received a conditioning-test paradigm to assess corticoreticular excitability. 5. Conduct a sustained isometric elbow extension task

Inclusion Criteria: * Diagnosis of cervical spinal cord injury, traumatic or non-traumatic, with motor level of C3-C8 * Chronic phase of injury (≥180 days post-onset) * ASIA Impairment Scale (AIS) grade C or D * Age between 20 and 65 years old * Presence of spasticity in the biceps brachii (modified Ashworth scale score ≥1) Exclusion Criteria: * Presence of a pacemaker or other metal implants in the head * A personal or family history of epilepsy or seizures. * History of adverse effects from prior electrical stimulation. * Cognitive or communicative impairment that precludes the ability to provide informed consent or follow study instructions.