Intrinsic Constraints of Neural Origin: Assessment and Application to Rehabilitation Robotics

Abstract

Ideally, robots used for motor rehabilitation, in particular, during assessment, should minimally perturb the voluntary movements of a subject. In this paper, we show how a state-of-theart back-drivable robot, i.e., a robot that can be moved by the user with a low perceived mechanical impedance, when used for assessment can still perturb the voluntary movements of a subject. In particular, we show that, despite its low mechanical impedance, a robot may still not comply with the intrinsic kinematic constraints, which are of neural origin and are adopted by the human brain to solve redundancy in motor tasks. Specifically, the redundant task under consideration is the 2-D pointing task, which is performed by a subject with the sole use of the wrist [3 degree of freedom (DOF) kinematics]. Wrist orientations during pointing tasks are assessed in two different scenarios. In the first experiment, a lightweight handheld device is used, which introduces no loading effect. In the second experiment, similar pointing tasks are performed with the subject interacting with a state-of-the-art robot for wrist rehabilitation. In the first case, intrinsic kinematic constraints arise as 2-D surfaces embedded in the 3-D space of wrist configuration. Such surfaces are typically subject-dependent and reveal personal motor strategies. In the second case, a strong influence of the robot is remarked. In particular, 2-D surfaces still arise but are similar for all subjects and are referable to a mechanical origin (excessive loading by the robot). The assessment approach described in this paper, including both the experimental apparatus and dataanalysis method, can be used as a test for the degree of backdrivability of mechanisms and robots in relation to constraints of neural origin, thus allowing the design of robots that can actually cope with such constraints. The clinical potential impact is also discussed.

Motor adaptation during redundant tasks with the wrist

“…Anyway, absolute values of thickness remain in the range of physiological values (1 to 3 degrees [6], [10]), demonstrating that Donders' law still applies during the adaptation and washout processes. Nevertheless, the increasing of thickness with respect to baseline values show that the adaptation to an external perturbation during a redundant task increase the "biological noise" associated to this kind of motor strategy [12], [13].…”

“…Here, we propose an experimental setup and protocol to address these issues. In particular we will assess motor learning during adaptation, analyzing the kinematics indices proposed by [11] and the motor strategies studied in [6], [12]- [13].…”

An intrinsically compliant robotic orthosis for treadmill training

“…3. Actuators should be highly compliant with low mechanical impedance [10,17] in order to accommodate abrupt forces arising from clonus. The relationship between moment applied by the robotic orthosis and the joint angle is generally known as "mechanical impedance".…”

Ergonomic design of a wrist exoskeleton and its effects on natural motor strategies during redundant tasks

“…It was shown that excessive perceived mechanical impedance would alter motor strategies adopted by the brain during redundant tasks *This study was partially funded by the New Funding Initiative 2010 (NTU) and the Academic Research Fund (AcRF) Tier1 (RG 50/11), Ministry of Education, Singapore M. Esmaeili performed with a robot [4], despite the backdriveability of the robot itself.…”

“…those of a neural origin, should be considered alongside the more traditional "hard" (biomechanical) constraints. Building on the works [4], [5], this paper presents a preliminary study on motor strategy adaptation. These adaptations arise during kinematically redundant motor tasks while the subject wears exoskeletons of varying ergonomic levels.…”