Performance adaptive training control strategy for recovering wrist movements in stroke patients: a preliminary, feasibility study

Abstract

Background: In the last two decades robot training in neuromotor rehabilitation was mainly focused on shoulder-elbow movements. Few devices were designed and clinically tested for training coordinated movements of the wrist, which are crucial for achieving even the basic level of motor competence that is necessary for carrying out ADLs (activities of daily life). Moreover, most systems of robot therapy use point-to-point reaching movements which tend to emphasize the pathological tendency of stroke patients to break down goal-directed movements into a number of jerky sub-movements. For this reason we designed a wrist robot with a range of motion comparable to that of normal subjects and implemented a self-adapting training protocol for tracking smoothly moving targets in order to facilitate the emergence of smoothness in the motor control patterns and maximize the recovery of the normal RoM (range of motion) of the different DoFs (degrees of Freedom).

Methods: The IIT-wrist robot is a 3 DoFs light exoskeleton device, with direct-drive of each DoF and a human-like range of motion for Flexion/Extension (FE), Abduction/Adduction (AA) and Pronation/Supination (PS). Subjects were asked to track a variable-frequency oscillating target using only one wrist DoF at time, in such a way to carry out a progressive splinting therapy. The RoM of each DoF was angularly scanned in a staircase-like fashion, from the "easier" to the "more difficult" angular position. An Adaptive Controller evaluated online performance parameters and modulated both the assistance and the difficulty of the task in order to facilitate smoother and more precise motor command patterns.

Results: Three stroke subjects volunteered to participate in a preliminary test session aimed at verify the acceptability of the device and the feasibility of the designed protocol. All of them were able to perform the required task. The wrist active RoM of motion was evaluated for each patient at the beginning and at the end of the test therapy session and the results suggest a positive trend.

Conclusion: The positive outcomes of the preliminary tests motivate the planning of a clinical trial and provide experimental evidence for defining appropriate inclusion/exclusion criteria.

Figure 1

3DoF Wrist Device. It has 3 DOFs: F/E, P/S, Ad/Ab. One motor is used for F/E and P/S; two motors for Ad/Ab.

Figure 2

Controller diagram. The "assist-as-needed" force parabolic term continuously inputs torque τ_m when errors are present during the tracking task. The input torque to the robot/hand system is the sum of different contributions of a viscous field τ_v, a gravity τ_G and inertia τ_I compensation. τ_H is the torque applied by the subjects wrist.

Figure 3

Virtual reality environment in the therapy session. A) Experimental set-up in the P/S case: the dolphin chasing the ball. The two bars on the left of the screen display two performance indicators. B) F/E excercise; D) Ab/Ad excercise; D) P/S exercise.

Figure 4

Course of the target frequency when the offset position steps through the ROM. At the beginning of each step the frequency is reset to its minimum value (0.1 Hz); the maximum possible value is 1 Hz. Subject S3.

Figure 5

complementary analysis between assistive torque and maximum frequency reached during tracking (subject S3). (A) Left panel: Maximal target frequency reached for the different DOFs during the 40s steps, identified by the starting position in the ROM with respect to the neutral position. Right panel: Mean value of the assistive torque (in 10-3Nm) during the corresponding steps.(B) Mean tracking speed, for the different DOFs, in the different 40s steps, identified by the starting position in the ROM with respect to the neutral position. Gray and black curves correspond to the opposing parts of the movements (F vs. E, Ad vs. Ab, P vs. S). (C) For each value of the offset rotation and each DOF, the graphs show the ROM of the robot (shaded band) and the ROM of subject S3 (black curves). X-axis identified the spammed ROM for the exercised Dof; positive and negative value are referred respectively to F/E, Ab/Ad and P/S while zero is the neutral position. Y-axis is the amplitude oscillation reached by the target (shaded band) and by the subject.