An analytical approach to investigate arm free standing of a paraplegic subject while using a new HKAFO orthosis

The aim of this study is to develop a theoretical model to investigate arm free standing of paraplegic individuals using a new designed Hip-Knee-Ankle-Foot Orthosis (HKAFO). To this end, a serial five-link chain with two degrees of freedom, representing the subject and orthosis has been considered in sagittal plane, and the equations of motion were derived. Due to great importance of anthropometry data and passive joint moments to achieve arm free standing for paraplegic subjects, a careful study has been done in the literature to find appropriate models to estimate physical properties of human body and also to determine passive joint moments. In both cases, different models were compared and those which show better agreement with experimental results have been selected. Afterwards, the equilibrium points of the system were found and linearized form of the equations of motion is examined to find dynamically stable equilibrium points. In addition, in order to achieve stable standing, COG stability constraint is also enforced. On the other hand, with a closer look at the interaction between the contributing joint torques, we are able to interpret stable standing of the paraplegic physically and more importantly find the maximum thigh angle with which the equilibrium state can be achieved. The results of our experiments verify the model and the feasibility of arm free standing for the paraplegia.