A review of methods for achieving upper limb movement following spinal cord injury through hybrid muscle stimulation and robotic assistance.

Individuals with tetraplegia, typically attributed to spinal cord injuries (SCI) at the cervical level, experience significant health care costs and loss of independence due to their limited reaching and grasping capabilities. Neuromuscular electrical stimulation (NMES) is a promising intervention to restore arm and hand function because it activates a person's own paralyzed muscles; however, NMES sometimes lacks the accuracy and repeatability necessary to position the limb for functional tasks, and repeated muscle stimulation can lead to fatigue. Robotic devices have the potential to restore function when used as assistive devices to supplement or replace limited or lost function of the upper limb following SCI. Unfortunately, most robotic solutions are bulky or require significant power to operate, limiting their applicability to restore functional independence in a home environment. Combining NMES and robotic support systems into a single hybrid neuroprosthesis is compelling, since the robotic device can supplement the action of the muscles and improve repeatability and accuracy. Research groups have begun to explore applications of movement assistance for individuals with spinal cord injury using these technologies in concert. In this review, we present the state of the art in hybrid NMES-orthotic systems for upper limb movement restoration following spinal cord injury, and suggest areas for emphasis necessary to move the field forward. Currently, NMES-robotic systems use either surface or implanted electrodes to stimulate muscles, with rigid robotic supports holding the limb against gravity, or providing assistance in reaching movements. Usability of such systems outside of the lab or clinic is limited due to the complexity of both the mechanical components, stimulation systems, and human-machine interfaces. Assessment of system and participant performance is not reported in a standardized way. Future directions should address wearability through improvements in component technologies and user interfaces. Further, increased integration of the control action between NMES and robotic subsystems to reanimate the limb should be pursued. Standardized reporting of system performance and expanded clinical assessments of these systems are also needed. All of these advancements are critical to facilitate translation from lab to home.