Review
Control of neural prostheses for grasping and reaching

https://doi.org/10.1016/S1350-4533(02)00187-XGet rights and content

Abstract

In recent years several neural prostheses have been developed and tested as orthoses or as therapeutic systems for hemiplegic and tetraplegic subjects aiming to improve the upper extremities function. The use of neural prostheses demonstrated that the targeted group of subjects could significantly benefit from functional electrical stimulation that is integrated in goal directed movements. In this paper the control for neural prostheses is explained using available systems that apply either surface or implantable interfaces to sensory-motor systems. Further more, a new strategy that has been tested for control of reaching and grasping within a neural prosthesis especially designed for neurorehabilitation is described. This, so-called, coordination strategy was based on mimicking the output space model of natural control determined in reach/grasp/release movements of healthy humans.

Introduction

Reaching, grasping, and releasing (RGR) functions belong to so-called goal-directed movements. A goal-directed movement can be defined as a planned tuning of the body segments’ position, ultimately leading to the accomplishment of a task [1], [2], [3]. This movement is a highly complex, perceptually driven dynamical process [4] that comprises two essentials: (1) planning, and (2) execution. An injury or disease of the central nervous system (CNS) diminishes the ability to execute effectively goal-directed movements. Humans with CNS lesion are anxious to regain ability to reach and grasp; thus, return to independent life style in the easiest, simplest and fastest possible manner. Motor Neural Prostheses (NPs) are devices that could restore the RGR functions, that is, contribute to execution of goal-directed movements.

During the last 40 years several Functional Electrical Stimulation (FES) based NPs for grasping were developed [5]. Most of these devices use surface stimulation, although there are effective implantable systems. NPs were frequently combined with mechanical bracing and/or surgical procedures in order to constraint more proximal joints. Recently, the evidence was gained that NPs, in addition to orthotic, have substantial therapeutic effects, that is, they promote the long-term recovery of functions [6]. This evidence was summarized from studies [6], [7], [8] where NPs were applied in hemiplegic subjects.

This paper reviews the existing control paradigms used in NPs for reaching and grasping, especially considering possible therapeutic applications where NP has to be integrated into the preserved natural mechanisms of control.

Section snippets

Synergistic control of grasping

The simplest synergy for opening and closing of the hand is the tenodesis. The tenodesis uses the intrinsic anatomic feature of the human arm/hand complex. When the wrist extends, the finger flexors passively pull the fingers to flex, and vice versa, when the wrist flexes, the finger extensors passively pull fingers to extend; thus, the wrist flexion/extension generates weak grasping.

The Bionic Glove [9] is a NP that enhances the tenodesis in patients who have preserved voluntary control of

Synergistic control of reaching

Methods to control the whole arm, that is, to assist manipulation (reaching) in humans lacking voluntary shoulder, elbow, and wrist movements control have received a great deal of attention [23], [24], [25], [26].

Miller et al. [27] suggested the stimulation of the Triceps Brachii in order to restore elbow extension in C5 tetraplegic subjects who had preserved shoulder movement and elbow flexion. The control system used a look-up table, and included three input signals (flexion/extension and

Discussion

Neural prostheses are currently being applied to control paretic or paralysed upper extremities in humans after CNS injury or disease with limited success. There are several reasons impeding the efficacy of the application of NPs. One of the major limitations is the difficulty to drive complex movements in a way that is controllable volitionally by the user. The body of knowledge how to control a single muscle and a single joint is remarkable, yet the acceptable control of multi-segmental

Acknowledgements

This work was partly supported by the Danish National Research Foundation, Denmark. I would like to acknowledge Dr. Francisco Sepulveda for valuable comments and suggestions.

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