Understanding Seated Stability in Spinal Cord Injury Using an Asymmetric Two Arms Biomechanical Observer

Various pathologies and physical impairments diminish the capacity to maintain a seated balance, with spinal cord injury serving as a clinical example. Individuals with this condition often lose control of muscles below the injury level and commonly rely on a wheelchair for mobility. The impact of such injuries on seated postural control necessitates the development of new stabilization strategies in response to disturbances. These strategies differ significantly from those used by asymptomatic individuals. Particularly, they rely on upper limb movements as the primary means of control, given the absence of control from the trunk. Reconstructing the produced active joint torques at the shoulder and arm levels or the passive torque at the lumbosacral level is crucial to understand compensatory strategies and developing innovative monitoring techniques in rehabilitation exercises. The methodology starts from a nonlinear model with 5 Degrees of Freedom called “Trunk-2-Arms” (T2A) and proposes an observer based on quasi-LPV and LMI problem synthesis. The design of the observer uses a cascade of 3 models (trunk and the 2 arms) in order to reduce the design complexity. Therefore, local PI-observers are derived that allows to estimate the state and the human joint torques. The global estimation error convergence of the cascaded observer scheme is guaranteed using a separation principle and the Lyapunov theory. The methodology is validated through simulations and using real clinical data collected on 26 SCI people.