Dimitar Stefanov

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Lecture 16

• Dimitar Stefanov

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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.
1./ Unimpaired pathways to the muscle fibers
Muscle fibers
excitation an electrical stimulus
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2./ Impaired pathways to the muscle fibers
Electric source
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.

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

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• 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.

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.

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• 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.

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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.
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• 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.

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Response of the locomotor system to the FES
Individual character (black box)
Methods for parameters identification
A./ gripping force/electrical stimulus
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B./ Elbow flexion force/FES
C./ Experimental parameterization of lower
extremity for FES control
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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.
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Some research institutions where significant FES
research results are achieved
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.
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Description of the FNS signal
Pulse repetition rate (frequency) pulse width
(duration), amplitude.
Stimulus strength is related to the charge
density.
charge density current time/area
range of 10-300 mC cm2
The current of the FNS depends on the proximity
of the electrodes to the target muscle
tissue. Typically 1-50 mA.
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• 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 mS)
• 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.
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• Systems for FNS
• Open loop control time pattern stimulation.
  Sensitive to external disturbances, the muscle
  fatigue cannot be considered.
• Closed loop control (non-linear and adaptive
  controllers fuzzy-controllers).
• Problems
• 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.
• 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.
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Block diagram of FNS model
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.