Participants will recieve 60 min of active cervical transcutaneous spinal cord stimulation with intermittent bouts of active Cerebellar theta burst stimulation 3x/week for 8 weeks, alongside functional task practice.

Participant will recieve 60 min of active cervical transcutaneous spinal cord stimulation only 3x/week for 8 weeks, alongside functional task practice.

Participants will recieve 60 min of sham cervical transcutaneous spinal cord stimulation with intermittent bouts of sham Cerebellar theta burst stimulation 3x/week for 8 weeks, alongside functional task practice.

Theta burst stimulation (TBS) is a pattern of repetitive transcranial magnetic stimulation that will be delivered over the lateral hemisphere of the cerebellum. Sham TBS will be delivered using a sham coil over the cerebellum.

Feasibility, which will be assessed through adherence, recruitment, retention, and adverse event rates across all groups. We will specifically document session numbers, dates and times, as well as the frequency and severity of skin irritations, abnormal blood pressure responses (e.g., autonomic dysreflexia), cardiac responses (e.g., tachycardia, bradycardia) and any symptoms. Participant safety will be monitored throughout the intervention via regular skin integrity checks and cardiovascular recordings (blood pressure and heart rate) throughout each session.

Change in upper limb strength from pre- to post-intervention using the Graded Redefined Assessment of Strength, Sensibility, and Prehension (GRASSP) strength sub-score. The GRASSP evaluates three domains: strength manual muscle testing of key upper limb muscles), sensibility (light touch and pinprick discrimination), and prehension, which includes both a qualitative analysis of grasp patterns and a performance-based component (GRASSP-Prehension Performance) that assesses functional use of the hand during object manipulation tasks. Strength will be the primary dependent measure from this measure.

Change in functional connectivity strength of the cortical sensorimotor network from pre- to post- intervention using resting-state functional MRI (fMRI). We will use a Philips Ingenia Elition 3.0T MRI scanner with a 32-channel sensitivity head coil to scan brain and a separate 20 channel dStream head/neck coil for cervical spinal cord. We will collect T1 and resting state functional MRI scans of both brain and cervical spinal cord at baseline and post-intervention. Resting state functional MRI will be acquired to characterize functional reorganization of the brain and/or spinal cord driven by cerebellar TBS + cervical tSCS (or tSCS alone) alongside functional task practice.

Brain and spinal cord functional connectivity: We will characterize properties of the sensorimotor network, particularly global efficiency, which represents the overall capacity that the network has to transfer information (i.e., quantifies the extent to which nodes of the network are integrated).

Arm and hand sensorimotor control will be indexed using Kinarm standard tests. Specifically, by the task score and path: length ratio during a four target visually guided reaching task using the Kinarm End-Point Lab (BKIN Technologies Ltd., Kingston, ON, Canada). These metrics take into account the spatial and temporal components of reaching, providing nuanced information regarding motor control strategies. We will also characterize proprioceptive sense by using an arm position matching task on the Kinarm and quantifying absolute matching error.

Neurological evaluations of participants with SCI will be performed using the ISNCSCI exam. The level and severity of damage to motor and sensory pathways will be determined by a trained team member using the standard ISNCSCI examination (2019 revision). In brief, this examination includes the assessment of motor function of key muscles in the upper and lower extremities using an established scale (muscle power graded as 0-5; for a total of 20 muscles in the four limbs) and sensory evaluation to light touch and pin prick in 28 dermatomes of the body. Sensory scores for each dermatome are assigned as 0=absent, 1=abnormal, and 2=normal. With this coding system, participants who perceive a pinprick as minimally sharp touch are assigned the same score as those who perceive it as almost normal. Perception of light touch stimuli is graded similarly.