Objective - of the proposed research is to determine whether a novel hybrid neuroprosthesis (HNP) combining functional neuromuscular stimulation (FNS) and incorporating an advanced sensor based controlled variable impedance knee mechanism (VIKM) for walking in persons with paraplegia from spinal cord injury (SCI) can: (1) reduce impact forces during loading, improve forward momentum, reduce the fluctuations in vertical trunk motion, and improve foot-ground clearance during walking, and (2) restore stair descent and stand-to-sit functions by controlling the resistance to knee flexion while lowering of the body. Research Plan and Methodology - Three phases of the study are: 1) design, development, and validation of a closed loop control system using our novel variable impedance knee mechanism (VIKM) for modulation of stance phase knee flexion during gait, 2) evaluation of the functional benefits of incorporating knee flexion during stance phase of gait, and 3) evaluation of the ability of the VIKM to control stair descent and stand-to-sit maneuver. Five individuals with thoracic paraplegia from SCI will be recruited to test the VIKM as part of a hybrid neuroprosthesis (HNP). Subjects will be implanted with an eight channel pulse generator and intramuscular electrodes to activate iliopsoas/sartorius for hip/knee flexion, hamstrings for hip extension and quadriceps for knee extension and gastrocnemius for plantar flexion. These are key muscles for standing up, for initiating the swing and for forward propulsion of the body. The ankle joint will be spring loaded to return the foot to neutral during swing after actively plantar flexing for push-off. A prototype VIKM will be incorporated in previously designed exoskeleton and combined with implanted FNS systems to yield a novel HNP. A closed loop controller to modulate knee during gait will be based on feedback from sensors measuring foot-ground contact, hip and knee angles, thigh and leg velocity, and acceleration. A microcontroller will modulate damper current to control the resistance of the VIKM based on phase of the gait cycle. The finite state controller will control the VIKM and modulate stimulation to the paralyzed muscles. It will be first evaluated in two able-bodied individuals to ensure that it can accurately identify phases of gait (controller validation) and provide adequate resistance against knee collapse during stance (VIKM validation). The VIKM is expected to lessen the effect of impact and absorb shock during initial loading, reduce body center of mass displacement and improve toe clearance in early swing by regulating knee flexion. During stair descent and stand-to-sit maneuver the emphasis will be on lowering of the body and weight acceptance when the VIKM will perform most of the work normally performed by the eccentrically contracting muscles impossible to control with FNS. The controller will be optimized to reduce the need for upper extremity support during lowering and landing on the step below and sitting surface. Walking with the VIKM-based HNP will be compared to walking with FNS-only and conventional orthosis. Statistical analysis of the data will be carried out using a within subject experimental design to test effect of VIKM. Impact force at initial contact, average vertical motion of body center of mass, and stance phase knee flexion will be measured. A one-way repeated measure ANOVA will be used to test for significant differences and the Tukey honestly significant difference multiple comparison tests will be used to determine 95% confidence intervals.