The overall goal of the proposed research is to develop a new control system to maintain standing balance at various task-dependent, user-specified postures, and enable dynamically stable reciprocal stepping in persons paralyzed by spinal cord injuries (SCI) with an implanted motor system neuroprosthesis (NP) and prepare for ensuing home-going trials and clinical dissemination. A system that automatically modulate neural stimulation to maintain standing balance and generate successive reactive steps to achieve effective reciprocal gait will be translated to the Networked Neural Prosthesis System (NNPS). Each module of the NNPS contains internal sensors that would eliminate the need for external devices and cabling. The project will determine the feasibility of a safe and functional LE motor system NP that can be realized with a fully implantable system suitable for home and community use. Aim 1 will determine effective and efficient methods to estimate whole body center of mass (CoM) kinematics and evaluate suitability for real-time control with the NNPS. This will entail estimating CoM from external sensors attached to able-bodied volunteers and current recipients of standing and stepping NPs at the anticipated surgical location of modules of the NNPS. We will implement our existing biologically-inspired standing balance and stepping controllers with NNPS simulated signals in volunteers with SCI and evaluate their performance in response to internally generated and externally applied disturbances. Aim 2 will extend the control system to automatically generate successive reactive steps, and enhance inter-limb loading and dynamic balance during stance phases of gait. A system that enables forward progression of the CoM to maintain balance in the medio-lateral direction, optimizes forward propulsion, and ensures proper swing limb foot placement during successive reactive steps will be developed in simulation, and experimentally validated with able-bodied volunteers and recipients of implanted NPs. Our existing control systems for bipedal stance and reactive stepping will be adapted to maintain balance and achieve smooth translation of CoM during walking and recent development in the field of walking robots will be applied. We expect that walking with the extended biologically-inspired control system will be smoother, more efficient and more resilient to potential destabilizing influences than conventional pre-programmed stimulation. Aim 3 will create the resources required to translate the standing and walking controllers to the new NNPS platform in preparation for clinical implementation and ensuing home-going trials. Work will develop, document and verify operation of lower extremity-specific software and clinical/user interfaces to the NNPS. We will prepare and submit an Investigation Device Exemption to the USFDA for a new feasibility study of LE applications of the NNPS in preparation for ensuing clinical trials of safety and effectiveness.