Robot-aided assessment of wrist proprioception in view of robotic rehabilitation

Opening a door or a jar, writing, and squeezing a lemon are actions that need intact wrist function to be correctly performed. Indeed, the wrist is a joint particularly relevant in fine manipulative tasks, during which accuracy of movement and force exertion is mandatory to accomplish the action properly. In this context, a crucial role is played by sensory afferent information, in particular by proprioception, the sense responsible for awareness of human trunk and limb position, movement, force, and sense of effort. Individuals deprived of proprioceptive afferent information still maintain gross motor functions, despite exhibiting significant deficits during fine actions. Since numerous neurological diseases and orthopedic injuries are known to impair sensory functions, in my doctoral research activity I focused on investigating the role of proprioception using robotic methodologies and technologies. Among the advantages brought by robotic technologies, there is increased temporal and spatial accuracy in the assessment, the opportunity to synchronize other devices to broaden the set of available measurements, and the possibility to develop smart algorithms for rehabilitative training purposes. However, a challenge still open in the field is to develop rehabilitative protocols able to maximize the effectiveness of the intervention, minimizing the temporal duration and the effort required. To achieve this goal, we need to deepen our knowledge of human sensorimotor functions, particularly how sensory integration is carried out and how, consequently, motor commands are generated. For these reasons, my first research objective was to investigate how able-bodied subjects exploit both intrinsic and augmented proprioceptive afferent information. Additionally, the second objective of my research work was to investigate the potential of human wrist robotic rehabilitation in two specific clinical populations of subjects, i.e. orthopedic patients and persons with Multiple Sclerosis (MS). Concerning the former, since the knowledge about the potential of robotics in orthopedic rehabilitation was limited, my goal was to compare the effectiveness of traditional wrist rehabilitation with that of a rehabilitative program entirely performed using a robotic device. The MS population, instead, has been extensively studied particularly with regard to the treatment of motor incoordination through upper-limb robotics. In my work, I focused on the impact of adaptive strength training to prevent muscle weakness in MS and increase cross-education, a phenomenon whereby training one limb results in increased strength in the opposite limb. The whole research project involved the use of a wrist manipulandum with three Degrees of Freedom (DoFs) and a range of motion comparable with that of the human wrist. The robot allows either completely active or assisted wrist extension/flexion (EF), radial/ulnar deviation (RUD), pronation/supination (PS), and coordinated rotations involving multiple DoFs. The device is equipped with high-resolution encoders to measure angular displacement along its three DoFs and with motors, to deliver torques to manipulate the wrist joint, compensate for weight and guarantee low inertia. The robot can be synchronized with external devices useful to investigate wrist sensorimotor function, like the electromyographic system to assess muscle activity. The main findings of my research point out that the sensitivity of proprioceptive afferent information can be influenced by both physiological and external factors. In particular, the wrist position sense of able-bodied subjects was found to be symmetric between hands, but anisotropic between different directions in the EF/RUD space. Among the external factors that can influence proprioception, I focused on the effects of short-term dynamic fatigue, which was able to modify the subject's sensitivity not immediately after the end of the task, but some time later (about four minutes). Another factor that I found to affect the human position sense is task learning. By learning, the proprioceptive space could undergo a task-specific recalibration. Conversely, being the only afference to carry information strictly related to the joint space, the proprioceptive feedback turned out to be the crucial source of information for optimally learning a task. These experimental results highlighted the importance of accurately assessing and exploiting the sensory afferents coming from the wrist joints and muscles to improve the (re)learning process. Additionally, protocols of robotic rehabilitation were found to be helpful and versatile tools to explore wrist sensorimotor functions and treat a wide set of different dysfunctions in both orthopedic and neurologic conditions. After a wrist traumatic event, the benefit of wrist robotic therapy was comparable with that of traditional manual therapy, thus confirming the potential of robotic rehabilitation also for this unexplored clinical population. After a 4-week-long training, MS subjects increased the strength of their forearm muscles, in both the trained and untrained limbs. Even though the clinical outcome measures did not reveal a significant improvement, the subjects’ verbal feedback was enthusiastic and satisfied with the impact the trial had had on their lives. Finally, my research work investigated the use of protocols of robotic rehabilitation as helpful and versatile tools to explore wrist sensorimotor functions and treat dysfunctions arising from both orthopedic and neurologic conditions. After a wrist traumatic event, the benefit of wrist robotic therapy was comparable with that of traditional manual therapy, thus confirming the potential of robot-based rehabilitation also for this unexplored clinical population. After a 4-week-long training, MS subjects increased the strength of their forearm muscles, in both the trained and untrained limbs. Even though other clinical measures did not reveal a significant improvement, the subjects’ verbal feedback was enthusiastic and satisfied with the impact the trial had had on their lives.