Development of the neuromechanics evaluation device (NED) for subject-specific lower limb modelling of spinal cord injury
Creators: name: Huang, Hsien-Yung
Publisher: name: Imperial College London
Year: 2018

description: Investigating human neuromechanics can be used to characterise motor impairments in neurologically affected individuals, and can give insight into how the brain controls movement. In particular, the lower limbs' neuromechanics is critical to assess the balance control and mobility. However, there are still few experimental results on the neuromechanics of the lower limb. This is especially true of the hip, in part due to the difficulty to quickly accelerate the heavy leg in a controlled manner. Furthermore, existing robotic interfaces for the lower limb typically constrain the joints motion and cannot provide the quick and smooth perturbations necessary to identify subject's biomechanics. In this context, this thesis presents: i) a new robotic interface that have developed to measure lower-limb neuromechanics, ii) a systematic investigation of the hip viscoelasticity carried out with this interface, and iii) a biomechanical model that provides subject-specific dynamic behaviour based on parameters from neuromechanics experiments. The Neuromechanics Evaluation Device (NED) is an endpoint-based cable-driven robotic interface that can be used to measure the hip, knee and ankle joint neuromechanics. Subjects can take an upright posture, which is important for subjects with weak motor condition, while the interface moves the limb's extremity over a large workspace without constraining the joint orientation. Rapid position displacements can be applied to the studied limb by the powerful actuator fixed to the ground. Mechanical evaluations showed that NED has a rigidity above 500N/m and viscosity below 50Ns/m. It is also able to produce fast perturbations (e.g. a displacement of 2cm within 230ms) without vibration, which can be used to identify lower-limb neuromechanics. The ability of NED to carry out various neuromechanics measurements is illustrated in two experiments carried out with healthy subjects. First, a measurement of the maximum voluntary torque at the hip joint yielded values in line with reported estimates in the literature. Second, a systematic investigation of the hip joint viscoelasticity was carried out with 10 subjects. In line with previous findings on the upper and lower limbs, hip stiffness was found to monotonically increase with the applied force, with a slight dependence on the hip angle. These experiments exemplify how NED can be efficiently used to characterise the lower-limb joint neuromechanics. The thesis further presents a model of lower limb neuromechanics integrating subject-specific parameters that can be identified through experiments with a robotic interface. The proposed model incorporates physiological parameters such as the torque-angle and the torque-angular velocity dependencies, as well as the joint viscoelasticity. The model was used to evaluate typical neuromechanics alterations caused by a spinal cord injury. In addition, the influence of various neuromechanical joint parameters to postural control was evaluated in simulations.