Biomechanical Modeling for Biologically Inspired Control of Neural Prostheses for Walking

Bipedal walking of humans can be described as a cyclic sequence of synergistic activities of both sensory and motor systems, where sensory systems provide necessary timing and spatial information for potentially required corrections of motor signals (muscle activity). Neural Prosthesis for Walking (NPW) is an assistive system which aims to augment muscle activity; thereby restore walking in individuals with paralysis. The controller for an NPW must provide external activation (i.e., bursts of electrical pulses to motor neurons of paralyzed muscles) which will ensure that the paralyzed extremity follows desired trajectory that is healthy like. The method suggested here is based on the following assumptions: 1) the control should be customized to the musculoskeletal properties of a potential user; and 2) it should mimic the operation of biological control. The method includes four steps: 1) collection of sensor data during walking of a healthy individual and estimation of trajectories in the form suitable for simulation; 2) estimation of muscle activations based on dynamic optimization applied to the model with parameters customized to the potential user; 3) application of classification and regression trees (CARTs) for determination of mapping between the recorded sensor inputs and simulation-determined muscle activities; and 4) transfer of the CART-determined map into a microcontroller which receives data from sensors mounted on the patient and outputs electrical stimulation to the electrodes positioned appropriately on the patient. The first 3 phases are off-line operations implemented on a Windows based host computer, while the last phase operates in real time (portable microcontroller-based stimulator). In the case presented here the sensors are accelerometers and force sensing resistors, and the stimulator supports up to four channels of stimulation. The results of this method were translated into a clinical study for a four-channel NPW assisting training of the walking of hemiplegic individuals.