The authors are working on the personalisation of an innovative low-cost, lightweight, and easy-to-use active orthosis to facilitate over-ground walking with crutches by individuals with spinal cord injury (SCI) who possess remaining hip function. Personalisation will involve selection of the best knee motor control strategy for each subject, using predictive walking simulations that combine OpenSim patient-specific models with GPOPS-II optimal control predictions. In the present work, we describe a direct collocation optimal control framework to obtain a dynamically consistent walking motion that reproduces experimental measurements.