Stimulation map for control of functional grasp based on multi-channel EMG recordings

Abstract

Transcutaneous activation of muscles with electrical stimulation has limited selectivity in recruiting paralyzed muscles in stroke patients. However, the selectivity could be increased by the application of smaller electrodes and their appropriate positioning on the skin. We developed a method for selecting the appropriate positions of the stimulating electrodes based on electromyography (EMG). The EMG activity maps were estimated from signals recorded with two electrode arrays and two 24-channel wearable amplifiers positioned on the nonparetic and paretic forearms. The areas where the difference between the EMG maps obtained from the nonparetic and paretic arms was significant were identified as the stimulation sites. The stimulation was applied through array electrodes with magnetic holders and two wearable stimulators with four output channels each. The measures of functionality included joint angles measured with goniometers (hand opening) and grasp force measured with a multi-contact dynamometer (grasping). The stimulation protocol comprised co-activation of flexors and extensors to stabilize the wrist joint and prevent pronation/supination.

Introduction

Electrical stimulation of paretic and paralyzed upper limbs was introduced many years ago, yet the clinical evidence is still not sufficiently convincing to make this technique widely used [1], [2]. Functional electrical therapy (FET), i.e., intensive functional exercise combined with surface electrical stimulation of the paretic arm, promoted greater recovery of motor control and function in acute stroke patients [3] compared with the same treatment applied in chronic stroke patients [4]. However, in both acute and chronic stroke, the difficulty of determining the most effective positions for the stimulating electrodes remains a major challenge [5].

Selective activation of the muscles that control each finger and the wrist with electrical stimulation is a challenge that was initially addressed by Nathan [6]. The technology that has allowed fabrication of array electrodes for stimulation and the development of advanced electronic stimulators has provided a basis for determining how selective stimulation of the forearm muscles can be achieved [e.g., [7], [8], [9], [10]]. These studies led to two major conclusions: 1) small electrodes that are positioned appropriately can improve the selectivity of stimulation [11], [12], [13], [14] and 2) stimulation delivered asynchronously through several small electrodes at a lower frequency (approximately 10 pulses per second) rather than a single large electrode at a high frequency (approximately 30 pulses per second) allows prolonged stimulation that results in fused contraction [15], [16], [17]. A remaining challenge is how to easily and quickly select the number of electrodes and their relative positions with respect to the excitable tissue to produce adequate prehension and a safe and strong grasp with minimal wrist interference.

The hypothesis that we introduce is that by comparing EMG maps recorded while the patient performs the target function (hand opening, hand closing, holding an object for various types of grasp) using the nonparetic and paretic arm, one can determine the positions (regions) over the peripheral sensory-motor systems that can be stimulated. Thus, by mimicking the activation map of the nonparetic arm, we can replicate on the paretic side the complex and hardly predictable neural interplay that is unique to each individual. The quasi-normal activation of neural systems is likely to influence the motor control system and contribute to the development of synergies that can facilitate more effective and efficient movements and possibly re-train the brain so the patient no longer depends on the FES (carryover effect). Our hypothesis testing was facilitated by the availability of a practical and easy-to-use multi-channel recording system (Smarting [18]) and new array electrodes with quick contacts [19]. The results we present here come from the experimental work in clinical tests with stroke patients. The variables we used to quantitatively assess the achieved function (hand opening and grasping) were the differences in the achieved joint angles and grasping forces between the non-paretic (without stimulation) and paretic hands when stimulated.