Cervical transcutaneous spinal cord stimulation (tSCS) is a non-invasive electrical stimulation technique that can used to activate the nerve fibers at multiple spinal segments of the cervical level. These nerve fibers branch from (via sensory fibers) and to (via motor fibers) the upperlimb muscles. The tSCS stimulation technique has recently attracted considerable interest in motor rehabilitation of upper-limb function in spinal cord injured (SCI) individuals. The efficacy of cervical tSCS in upper-limb motor recovery was suggested to depend on the spatiotemporal activation of sensory fibers rather than motor fibers. To better understand the neural targets of cervical tSCS, I investigated the mechanisms of sensory and motor fiber activation using both experimental and computational simulations. In my first study, I developed a computational model to analyze activations of different groups of motor and sensory fibers during cervical tSCS. First, I developed a 3D geometrical model of the cervical spinal cord and stimulation electrodes, and used it to estimate the electric potential distributions along the trajectory of the sensory (proprioceptive and cutaneous) and motor fibers at the C7 spinal segment. The cathode electrode was placed at the C7 vertebral level, while the anode was on the anterior neck. Dedicated motor and sensory fiber compartment models were used to simulate the activation of motor, proprioceptive sensory and cutaneous sensory fibers. The minimum stimulation intensity necessary to activate the nerve fibers were estimated and compared across the different groups of fibers. My results showed that groups of proprioceptive and cutaneous sensory fibers were co-activated at lower stimulation intensities compared with motor fibers. These findings suggested a sizable contribution of sensory proprioceptive and cutaneous fibers underlying the rehabilitation mechanisms of upper-limb motor recovery after SCI. In my second study, I examined the excitability and selectivity of cervical spinal segments using different cervical tSCS cathode and anode electrode configurations in an experimental investigation including ten able-bodied individuals. Excitability and selectivity were examined by comparing the evoked responses recorded from electromyography (EMG) signals of six proximal arm and distal hand muscles while the cathode was placed at three positions (C6, C7, and T1) and the anode was fixed on the anterior neck. Additionally, EMG evoked responses were also compared across four anode configurations (neck, shoulders, iliac crests, and back). My results showed that distal hand muscles were preferentially activated when the cathode was placed over the C7 and T1 vertebral levels compared with the C6 cathode position. For all cathode positions, proximal arm muscles were activated at lower stimulation intensities compared with distal hand muscles. Moreover, all muscles were more effectively activated when the anode was placed over the anterior neck compared with the other anode configurations (shoulders, iliac crests, and back). Therefore, these results suggested that placing the cathode at C7 and T1 vertebral levels while the anode is fixed on the anterior neck can increase the activation of sensory fibers, thus improving rehabilitation outcomes. In my third study, I used computational simulations to examine the excitability of caudal spinal segments (C7 and C8 level) using cervical tSCS electrode configurations consistent with my experimental study. First, I updated the 3D geometrical model to include the C8 spinal segment, and used it to estimate the electric potential distribution along the trajectories of sensory proprioceptive and motor fibers at both C7 and C8 spinal segments. The cathode electrode was simulated at three positions:​ C6, C7, and T1 vertebral levels;​ and in three sizes:​ 5.0 x 5.0 (L), 3.5 x 3.5 (M);​ and 2.5 x 2.5 (S) cm², while the anode was fixed on the anterior neck. Dedicated motor and sensory compartment models were used to simulate motor and sensory proprioceptive fibers, respectively. The minimum stimulation intensity necessary to activate the fibers were compared between motor and sensory fibers and across the different cathode configurations. My results showed that sensory fibers at C7 and C8 spinal segments were activated at lower stimulation intensities when the cathode was placed at C7 or T1 vertebral level compared with the C6 cathode position. Moreover, proprioceptive sensory fibers were activated at lower stimulation intensities when smaller cathode sizes were used. Importantly, for all cathode configurations, motor fibers were consistently recruited at higher stimulation intensities compared with sensory fibers. These results corroborate the experimental results obtained in my experimental study, further suggesting that using small electrodes may optimize the activation of hand muscle sensory fibers during cervical tSCS. From the results I obtained in these three studies, it can be concluded that the activation of proprioceptive and cutaneous sensory fibers may underly the cervical tSCS rehabilitation mechanisms. Moreover, sensory fibers may be more effectively activated by placing the cathode electrode at C7 or T1 vertebral levels, while the anode is fixed on the anterior neck. Overall, these outcomes contribute for better understanding the neural targets activated during non-invasive cervical tSCS, which can be applied to achieve more effective rehabilitation protocols.