Position sense and force control: assessment in unimanual and bimanual tasks

Human-environment interactions are common natural occurrences affecting every action. The environment includes objects whose manipulation requires careful somatosensory integration. For successful manipulation, the nervous system must be able to represent and predict the geometrical and mechanical features of sensory stimuli arising from the interaction with objects. These interactions involve sensory perturbations that must be predicted and compensated by the nervous system. Despite the importance of somatosensory integration, a comprehensive understanding of how the unimpaired sensory-motor system integrates information on force and position remains elusive. Over the last decades, the evolution of technology has allowed researchers to develop highly controllable settings for evaluating sensory-motor integration and delivering haptic feedback. However, most of the existing haptic setups consist of systems with limited workspace and reduced-force capabilities. Recent advancements in exoskeleton devices provide a framework for developing haptic setups adequate to cover the full-human range of motion and offer a wide range of force and torque. Moreover, considering the prevalence of real-life activities involving two hands, bimanual control should be implemented and integrated in virtual reality and haptic interfaces. The aim of my thesis project was to understand proprioception and force control in unimanual tasks and extend from that to bimanual and multi-joint tasks. To do this, I developed six setups (three unimanual and three bimanual), which progressively increased the complexity of the technologies employed and the human movements examined, to investigate motor strategies in unimpaired subjects. In the first unimanual setup, I enrolled 36 subjects to study with a planar manipulandum how subjects control the contact force exerted by their dominant arm in predictable (known arm position) and unpredictable (unknown arm position) environments. I was surprised to observe that contact forces can be precisely controlled with variable contact impedance, that is, without a persistent relation between applied force and resulting motion. In the second unimanual task, I investigated how proprioception of wrist position is affected by different types of kinesthetic perturbations of multi-joint arm movements. I enrolled 18 healthy subjects employing a 3-DoFs wrist device. Results evidenced important findings that should also be considered in the clinical evaluation of neurological patients: testing patients’ proprioception in a configuration that is close to the joints’ physiological workspace limits may increase mechanoreceptors excitation and provide a fine measurement of sensory acuity. Finally, in the third unimanual setup, since the proprioception involving the concurrent evaluation of proximal and distal multi-joint (more than a single DoF) upper limbs movements remained an open question, I evaluated 18 healthy participants wearing a robotic exoskeleton. Even in this application, results have relevance to common clinical practice: standard proprioceptive tests are manually dispensed by the therapists, the use of similar wearable technologies that contemplate a multi‐joint and 3D-space evaluation could drastically improve measurement accuracy and reliability. Regarding the bimanual studies, in the first one, I evaluated 12 young participants controlling position and force while orienting an object with both hands. To approximate a scenario common to daily living activities, I designed an instrumented stand-alone device and implemented a coupled task oriented to assess bimanual proprioception. Results showed how much the perception of one's body in space affects the proprioceptive acuity for targets near to or far from the body. Proper changes in the evaluation protocol suggest the possible use in the clinical practice of such low-cost instrumentation. The same device was employed even in the second setup. In this case, it was opportunely fixed to make the task decoupled and used to evaluate the bimanual coordination in isometric force control. Compared to other studies, investigating the sole fingers’ contribution, here I considered the full arm by involving both proximal and distal muscles. Two populations were evaluated: young and elderly subjects. The inclusion of elderly subjects introduces insights about the deterioration of human abilities including higher asymmetry, lower accuracy, and more variable performance. Even this setup, appropriately modulated, may be adopted by therapists to evaluate neurological patients. Finally, with the third setup, I designed a task in which subjects performed multi-joint upper limb reaching movements in 3D-space while manipulating a virtual object with variable compliances, i.e., that should handle with less/more care. I re-programmed a bimanual robotic exoskeleton to provide several forms of haptic feedback. I tested the potentiality and the system stability on 15 healthy subjects of this new technology to evaluate motor strategies in the presence of simulated objects capable of reproducing more or less deformable materials. This last application provides a fully-customized environment that should be introduced even in rehabilitative applications requiring the bimanual control of concurrent position and force sense while haptic feedback is provided. Accurately assessing proprioceptive deficits can complement regular therapy to better predict the recovery path. Moreover, bimanual haptic interfaces could provide solutions to clinical evaluation or motor recovery treatment of patients with neurological damages, increasing the efficiency of training and reducing the amount of individual attention needed from the clinician. My outcomes on healthy subjects denote the potentialities of the designed and/or implemented device, tasks, and haptic interface. In particular, they denote a starting point for fully customized environments which could have implications for several assessment or rehabilitative interventions in patients with neurological diseases.