Lecture 16 Dimitar Stefanov

Functional Neural Stimulation for Movement Restoration (FNS) FNS – activation of skeletal muscles in attempts to restore useful movement in upper and lower extremities of people with impairments of the central nervous system. Neural cell Muscle fibers excitation an electrical stimulus 1./ Unimpaired pathways to the muscle fibers:

Electric sourceMuscle fibers 2./ Impaired pathways to the muscle fibers: Examples of the FES: Cardiac pace makers Moe and Post (1962) – first application of the FES as a functional orthosis FES – solution of the problem of restoring of the human locomotion and manipulation. Best results – among people with incomplete paraplegia and people, moderately affected with hemiplegia. Best results to restoration of the grip and release functions. Successful FES control of two type grasping: lateral prehension (e.g. key grip) and palmar grip (e.g. three-jaw pinch). Elbow extension control – people with C4 lesions; Incensement of the movement range – people with C5/C6 lesions.

Three techniques for FES: 1.Surface stimulators - electrodes, placed on the skin surface over the muscle or nerve to be stimulated (transcutaneous electrodes); 2.Percutaneous stimulators – permanently implanted electrodes with wires chronically penetrating the skin which connect to to an external pulse generator (intramuscular electrodes or percutaneous electrodes); 3.Implantable stimulators – 1. both the electrodes and the biocompatible enclosure are permanently implanted in the body, near the excitable tissue. 2.A transmitting antenna on the skin surface delivers power and information to the multiple simulation sites. 3.Excitation with minimal energy; activation of deep located muscle and nerve tissues. Muscle and nerve physiology Electrical engineering Electrical safety Biomechanics Control systems FNS

Example for an implantable multichannel FNS system Functions of the wearable processor: Processing of biomechanical parameters (joint angles, foot contact); Generation of control signals; Maintain joint force against the muscle fatigue. Stimulation with transcutaneous electrodes: Widely used (no surgical intervention is required) Poor selectivelity and reachibility of deep located muscles Great values of the stimulation voltage is required.

Problems, which should be solved for efficient gait: Choice of simulation patterns and voltages; Development of stimulus for full knee extension during certain phases of the gait cycle; Coordinated control and graded contraction of different muscle groups. Selective simulation of nerve fibers Special electrodes for selective stimulation Tension control through starting with the slowest to the fastest twitch motor units; More physiologically based control; Very useful solution in case of upper-limb FES.

Electrode for selective stimulation. The tube is placed around a motor nerve. Slow twitch motor units provide less tension than fast twitch motor units but they do not fatigue as rapidly. Unimpaired motor control: Slow twitch fibers are recruited in activities which require low forces; fast twitch fibers are recruited for activities requiring high speed and/or high force. Slow twitch fibers are recruited for frequently occurring activities which require low forces.

FES strategy for efficient gait restoration with electrodes for selective stimulation (example): Slow twitch fibers could be recruited to maintain the postural stability during standing; Fast twitch fibers could be used for joint movements (to initiate and generate steps). FES strategy for efficient upper limb movement with electrodes for selective stimulation (example): Slow twitch fibers could be recruited to maintain the postural stability of the upper limb; Fast twitch fibers could be used for object lifting.

Response of the locomotor system to the FES Individual character (black box) Methods for parameters identification A./ gripping force/electrical stimulus

B./ Elbow flexion force/FES C./ Experimental parameterization of lower extremity for FES control

Development of the strategies for FES based on the modeling of the anatomical structure and location of muscles and tendons Knowing the location of the stimulated muscles and the average force, produced during their stimulation, an efficient FES strategy can be developed. (a) Anatomical location of the muscle to be stimulated; (b) Simple model of the Rectus femoris muscle; (c) mechanical model of the Rectus femoris muscle.

University and Medical Center in Ljubljana, Slovenia – peritoneal nerve stimulators, applied to over 2500 people; gait simulators (four channels), applied to over 100 people with spinal cord injuries; Case Western Reserve University, the Cleveland Metro Health Center, and Cleveland Veterans Affairs Medical Center – development of peritoneal and implanted systems for functional grasp, applied to 50 people with quadriplegia; Cleveland Metro Health Center and Cleveland Veterans Affairs Medical Center – complex gait for about 30 people with paraplegia. Some research institutions where significant FES research results are achieved:

Description of the FNS signal The current of the FNS depends on the proximity of the electrodes to the target muscle tissue. Typically 1-50 mA. Pulse repetition rate (frequency) pulse width (duration), amplitude. Stimulus strength is related to the charge density. charge density = current time/area range of mC cm 2

Pulsed waveform Monophasic or biphasic pulses Burst (carrier) signal – reduction of the potential pain Sufficient current in the target area for a length of time (100 – 600  S) Pulse amplitude – depends on the size of the electrode and the degree of current spread between the electrode and targets. Current generator: Produce regulated current between 0 and 60 mA; Voltage – from 0 to 180 V Maximum 50 pulses per second to prevent the fast fatigue Low duty-cycle – to maintain the charge balance and to minimize the risk of tissue damage Optimal stimulation of the concrete muscle group – very important condition for successful FNS. Requires knowledge of the response of each muscle to stimulation and knowledge of the muscle fatigue.

Systems for FNS: 1.Open loop control – time pattern stimulation. Sensitive to external disturbances, the muscle fatigue cannot be considered. 2.Closed loop control – (non-linear and adaptive controllers; fuzzy-controllers). 3.Problems: 1. It is difficult to be obtained meaningful physiological feedback from the stimulated muscle- joint system; time delay between the stimulation and force production; nonlinear characteristics between the muscle length and the muscle force. 2.It is difficult to be found precise model of the muscle recruitment for the concrete patient. Increased burst time is applied in case of muscle fatigue.

The time delay between muscle stimulation and muscle activation is called the neural dynamics. Non-linear relationship between input activation and generated joint torque (depends on the joint angle, the joint velocity and acceleration). Limb dynamics – depends on the mass and inertia characteristics of the limb. Block diagram of FNS model