Stimulation of the spinal cord may induce the growth and reorganization of neural pathways leading to the re-animation of paralyzed limbs. Growing evidence indicates that electrical spinal cord stimulation improves motor functions immediately via modulating the excitability of spinal circuitry in patients with spinal cord injury. Recently, a novel, non-invasive, well-tolerated and painless transcutaneous electrical stimulation strategy was demonstrated to be effective for improving lower limb motor function in healthy individuals and in patients with spinal cord injury. The investigators hypothesize that transcutaneous cervical electrical stimulation can enhance conscious motor control and functions of hand and arm via neuromodulation of spinal network.