Use of a Novel Robotic Interface to Study Finger Motor Control

Abstract

Stroke is the leading cause of permanent adult disability in the U.S., frequently resulting in chronic motor impairments. Rehabilitation of the upper limb, particularly the hand, is especially important as arm and hand deficits post-stroke limit the performance of activities of daily living and, subsequently, functional independence. Hand rehabilitation is challenging due to the complexity of motor control of the hand. New instrumentation is needed to facilitate examination of the hand. Thus, a novel actuated exoskeleton for the index finger, the FingerBot, was developed to permit the study of finger kinetics and kinematics under a variety of conditions. Two such novel environments, one applying a spring-like extension torque proportional to angular displacement at each finger joint and another applying a constant extension torque at each joint, were compared in 10 stroke survivors with the FingerBot. Subjects attempted to reach targets located throughout the finger workspace. The constant extension torque assistance resulted in a greater workspace area (p < 0.02) and a larger active range of motion for the metacarpophalangeal joint (p < 0.01) than the spring-like assistance. Additionally, accuracy in terms of reaching the target was greater with the constant extension assistance as compared to no assistance. The FingerBot can be a valuable tool in assessing various hand rehabilitation paradigms following stroke.

Appendix

The Jacobian matrix (J) relating the three joint torques $\left( {{\varvec{\tau}} = \left[ {\begin{array}{*{20}c} {{\varvec{\tau}}_{\mathbf{MCP}} } \\ {{\varvec{\tau}}_{\mathbf{PIP}} } \\ {{\varvec{\tau}}_{\mathbf{DIP}} } \\ \end{array} } \right]} \right)$ to the forces and moment $\left( {{\mathbf{f}} = \left[ {\begin{array}{*{20}c} {{\mathbf{F}}_{\mathbf{x}} } \\ {{\mathbf{F}}_{\mathbf{y}} } \\ {{\varvec{\tau}}_{\mathbf{z}} } \\ \end{array} } \right]} \right)$ at the fingertip is given by Eq. A.1:

$$ J = \left[ {\begin{array}{*{20}c} { -\; l_{\text{P}} \sin \left( {\vartheta_{\text{P}} } \right) - l_{\text{M}} \sin \left( {\vartheta_{\text{P}} + \vartheta_{\text{M}} } \right) -\; l_{\text{D}} \sin \left( {\theta_{\text{P}} + \theta_{\text{M}} + \theta_{\text{D}} } \right)} & { -\; l_{\text{M}} \sin \left( {\vartheta_{\text{P}} + \vartheta_{\text{M}} } \right) -\; l_{\text{D}} \sin \left( {\theta_{\text{P}} + \theta_{\text{M}} + \theta_{\text{D}} } \right)} & { - l_{\text{D}} \sin \left( {\theta_{\text{P}} + \theta_{\text{M}} + \theta_{\text{D}} } \right)} \\ {l_{\text{P}} \cos \left( {\vartheta_{\text{P}} } \right) + l_{\text{M}} \cos \left( {\vartheta_{\text{P}} + \vartheta_{\text{M}} } \right) + l_{\text{D}} \cos \left( {\theta_{\text{P}} + \theta_{\text{M}} + \theta_{\text{D}} } \right)} & {l_{\text{M}} \cos \left( {\vartheta_{\text{P}} + \vartheta_{\text{M}} } \right) +\; l_{\text{D}} \cos \left( {\theta_{\text{P}} + \theta_{\text{M}} + \theta_{\text{D}} } \right)} & {l_{\text{D}} \cos \left( {\theta_{\text{P}} + \theta_{\text{M}} + \theta_{\text{D}} } \right)} \\ 1 & 1 & 1 \\ \end{array} } \right] $$

(A.1)

where, l _P is the length of proximal finger segment, l _M is the length of middle finger segment, l _D is the length of distal finger segment, θ_P is the MCP flexion/extension angle, θ_M is the PIP flexion/extension angle, and θ_D is the DIP flexion/extension angle.

The joint torques are related to the fingertip forces/moment by:

$$ {\varvec{\tau}} = J^{\text{T}} {{f}} $$

(A.2)

Changes in the angle with respect to the fingertip of an applied force affects the joint torques generated. The changes are not uniform across the joints. For example, altering the force direction from 80° with respect to the long axis of the distal segment to 100° has minimal effect on the PIP and DIP torques, but has a significant effect on the MCP torque (Fig. A.1).

Figure A.1

The torques resulting at each of the three finger joints from an input force of 5 N directed at the fingertip. Finger posture, in terms of (MCP, PIP, DIP) flexion joint angles, is (45°, 45°, 20°). Force direction refers to degrees between the input force vector and the vector described by the distal phalanx (0° denotes a force directed along the long axis of the distal phalanx toward the DIP joint). Positive torque signifies an extension torque