Chap' 4

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Chap. 4 SENSORY FEEDBACK FOR LOWER LIMB
PROSTHESES
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Overview
An internal model of the human body is used by
the central nervous system to decide the adequate
motor commands needed to execute movements.
Presence of lesions, such as limb amputation,
induces a mismatch between the output predicted
by the internal model and the movement actually
executed by the body. Thus, a re-organisation of
the motor strategies is needed that induces an
update of the internal model. In prosthetic
subjects rehabilitation can induce the update of
the internal model. If the subject is provided
with some kind of artificial sensory reafference,
it is likely to assume that the process of
updating the internal model can be improved. The
aim of our research is to develop a system
especially designed to provide sensory
biofeedback to lower limb amputee subjects.

• Biofeedback techniques for rehabilitation of
  lower limb prostheses
• Role of sensory feedback for the reconstruction
  of the internal model
• Artificial feedback
• Auditory, visual, and tactile biofeedbacks
• Portable device for sensory substitution of the
  ground pressure information in prosthetic foot

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1. Introduction

• Internal model by the CNS system for the
  adequate motor commands
• Limb amputation ? a mismatch between the output
  by the internal model and the movement
• Reorganization of the motor strategies is needed
  to readapt and update the internal model.
• Artificial sensory information
• Biofeedback can help prosthetic subject.
• Lower limb amputee is lack of pressure
  information by the prosthetic foot on the ground.
• Artificial biofeedback
• - induces the location, and the amount of foot
  pressure.
• - should transmit this information to nervous
  system or sensory substitution.
• The artificial feedback with the control of
  grasping in upper limb prostheses

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2. Theories of Movement Control

• The whole body is involved during the
  locomotion.
• CNS system has to control the movement and the
  balance.
• A motor act implies a progressive modification
  of posture.
• Every posture modification implies a balance
  problem.
• In other words, CNS has to control movement ?
  posture ? balance
• Some body segments must be kept stationary
  during movement execution.
• - For example, a constant head inclination for
  the clear vision of the external world.

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2.1. Coordination between posture and movement

• Movement a series of successive postures due
  to a progressive modification of the static
  balance between forces exerted by the agonist
  muscles.
• Balance point
• Postural control ? produce the movement
• ? be ready to move
• Anticipatory characteristics of postural control
  during movement
• Action-Perception cycle

To decide the motor program
To provide the relevant command
Programmer
Peripheral receptors
Comparator
Regulator
To provide the relevant command ? Comparison
between the actual and the desired position ?
Identification of the movement completeness and
activation of the succeeding motor act ?
Modificationof the trajectory in case of
inadequacy
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2.1. Coordination between posture and movement

• The coordination of a motor act requires the
  control of a great number of DOF.
• Reduction DOF to use synergies
• Synergy
• - The cooordination among different body segments
  
• - A specific configuration of muscle activation
  for a specific posture
• - By sensory input by external disturbance
• - Or by internal commands for voluntary movements
• - Not strictly pre-programmed but adapted
• - Dependent of the support condition
• - Some basic synergies are innate, and other are
  acquired.

New environments
Old Synergy
New Synergy
Learning process

• Motor strategy the use of specific synergy or a
  sequence of synergies
• Choice of the motor strategy higher level
  function by nervous system

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2.2. Internal model

• The internal model matches better.
• Motor commands based on the knowledge of the
  geometry, kinematics and dynamics of the body
  segments of the initial and current state of the
  system
• Internal model a synthesis of the experience
• Brain Dual-mode for movement control
• ? Continuous servo-mechanism
• - Close-loop by sensory re-afference and
  pre-programmed motor acts
• - Sensory re-afference build up and update the
  internal model, and choose the most appropriate
  synergies.
• ? Simulation of the movement to predict the
  consequences and therefore to choose the better
  motor strategy
• - Use of internal maps, i.e. pool of neurons
  representing external world characteristics
• - Information from sensory feedback from the
  periphery
• - Sensory re-afference provides information on
  the actual state of movement execution.

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2.2. Internal model

• With changes in environment (i.e. microgravity)
  or the presence of nervous and muscle-skeleton
  apparatus, the internal model becomes inadequate.
• - Lack of correspondence
• The internal model matches better.
• Reorganization of the motor strategies and
  synergies to accomplish a movement.
• Functional re-education in lower limb
  amputations
• Rehabilitation program
• - Aware of new condition
• - Learn how to use his prosthesis and residual
  functionality
• - Update of the internal model during
  rehabilitation

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3. Natural Feedback

• Sensory receptors neural structures to provide
  information from the external world and from
  within the body
• Sensory systems
• - Exteroreceptive sensitive to external stimuli
• (vision, audition, skin sensation, and chemical
  senses)
• - Proprioceptive information about the body
  position in space and the relative position of
  body segments to one another
• - Interoceptive internal events in the body such
  as unconsciousness
• Somatic sensory system
• - All three involved.
• - Tactile sensations by mechanical stimulation
  applied to the body surface
• - Proprioceptive sensations by mechanical
  displacements of muscles and joints
• - Perceived at the periphery, and processed and
  relayed to higher level of the nervous system
  through spinal cord or the medulla.

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3.1. Tactile sensation

• Mechanoreceptors
• - Slowly adapting mechanoreceptors continuously
  responding
• - Rapidly adapting mechanoreceptors respond only
  at the onset of the stimulus
• Two rapidly adapting mechanoreceptors
• - Meissner corpuscle in the paillae of the
  corium of the hand and foot, front of the
  forearm, the skin of the lips and the mucous
  membrane of the tip of the tongue
• - Pacini corpuscle in subcutaneous positions at
  palm of the hand and the sole of the foot
• Two slowly adapting mechanoreceptors
• - Merkel receptors in the papillae and
  epithelium of the skin of man and animals,
  especially the skin area with no hair
• - Ruffini corpuscle subcutaneous tissue of human
  finger

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3.2. Preprioceptive sensation

• Preprioception
• - Sense of balance, primarily by the vestibular
  apparatus
• - Sense of stationary position of the limbs
• - Sense of limb movement (kinethesia)
• Sense of the limb position and movement by
  peripheral receptors
• Peripheral receptors
• - Mechanoreceptors located in the joint capsules
• - Cutaneous mechanoreceptors
• - Mechanoreceptors located in the muscle and
  transducing muscle stretch
• - A receptor in each spindle responds to an
  increase of muscle length and transmits to the
  spinal cord.
• - The Glogi tendon organs stretch receptor
  responding to an increase in tension rather than
  length, measuring forces, and sending impuses to
  the spinal cord.

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4. Artificial Feedback

• Natural prosthesis
• - Replaces the peripheral part of the damaged
  system and connected to CNS.
• - Blind subjects with an array of microelectrodes
  implanted in cerebral cortex.

• Substitutive prosthesis
• - Uses a different sensory system for
  non-producible system.
• - Navigation system with the obstacle avoidance
  for the blind.
• - Auditory and tactile signals to the subject.

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4. Artificial Feedback

• For an amputee
• - Foot pressure might be the only information
  that can be reasonably measured.
• Natural and substitutive solutions to create
  artificial feedback for the blind

Nervous pathway
Pressure sensors
CNS
Natural prosthesis
Visual, Auditory, Tactile
Substitutive prosthesis
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5. Center of Pressure

• Maintaining balance is the most important for
  lower limb amputees, especially at the beginning
  of rehabilitation.
• Balance when the ground projection of the body
  CM lies inside the support surface.
• CM position
• - Not directly detectable.
• - Must be reconstructed through the internal
  model using muscle-tendon receptors and the
  cutaneous receptors of the foot sole.
• ? Provides CNS with signals and can be used to
  obtain the position of CP.

5.1. Instrumentation for CP evaluation

• CP the point of application of the vertical
  component of the resulting ground reaction force
• Need to measure the intensity of ground reaction
  forces on the foot surface

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5.1.1. Force plates

• Measure the forces and the moments during the
  stance phase
• Vertical, anterior-posterior, medial-lateral
  components
• Piezoelectric when stressed, electric charge
  generated
• Resistive sensors
• Optical systems
• Deformable metals
• Strain-gauge
• KISTLER force plates
• - Quartz transducers
• - No power supply required.
• - Special charge amplifier and low noise coaxial
  cables required to convert the charge
  proportional to the applied load.
• - More sensitive and greater force range than
  strain-gauge type
• - Drift
• AMTI force plates
• - Stain-gauges (load cells)
• - No drift

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5.1.2. Sensorized insoles

• Few sensors placed in specific zone or in a
  matrix form
• Matrix sensors 60 - 960 sensors
• F-Scan system
• - Flexible and trimmable sensor with 960 sensing
  locations
• - High-speed data communication
• - peak force vs. time, pressure vs. time, peak
  pressure vs. time, and pressure profile
• - Advantage Successive gait cycle
• - Disadvantage Movement hindrance due to cables
• Data communications
• - Cable
• - Data logger A/D converter carried with the
  subjects
• - Telemetry only a limited number of data
  transferred

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5.1.3. Telemetric acquisition of CP

• On-line computing of the CP position during
  movement is essential.
• Recently developed prototype (Instituto
  Superiore di Sanita, Italy)
• - Matrix of 64 pressure sensitive sensors
• - Analog signal processing by a small sized
  resistive circuit
• - CP position in x-y coord and the vertical GRF
  displayed in real-time.
• - Analog output from the insole directly control
  biofeedback devices
• - Insole output connected to a small A/D
  converter at a frequency of 60Hz
• - By telemetry, digital output transmitted in 7
  bytes records to the serial port of PC.
• - Applied for the visual and auditory biofeedback

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5.2. Normal trajectory of CP during walking

• Gait cycle
• - Stance phase
• - Swing phase
• Normal Gait
• ? CP lies on the medial-posterior heel,
• ? moves through the mid-foot region,
• ? continues towards the forefoot, crossing the
  metatarsal heads
• ? terminates in the region of the great and 2nd
  toe.
• Area of CP trajectories during successive steps

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5.2. Normal trajectory of CP during walking

• Area of CP trajectories during successive steps

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6. Visual and Auditory Feedback
6.1. Visual feedback

• Visual biofeedback showing CP position on a
  screen
• Left and right foot images with reference areas
  
• Points outside the reference area in different
  color
• - information on the deviation from the desired
  performance
• Display of both CP trajectory and the movement
  in visual feedback
• - difficulty in watching the screen and
  simultaneously correcting his mistakes during the
  initial phase of re-education
• Visual representation of CP movement is more
  useful to the physiotherapists
• On-line trajectory of CP position can better
  understand how to instruct the subject.

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6.2. Acoustic feedback

• Acoustic feedback by using sounds to be aware of
  the CP distance from the reference area.
• Different sound for deviations from the
  reference area
• Acoustic biofeedback simple tones or recorded
  sounds
• - Complex messages confuses the subject.
• Avoid instantaneous correction only if the
  subject leaves the reference area for at least
  50ms.
• Advantage
• - does not employ the sensory pathways for
  movement control
• - The simple message makes the task easier.
• Disadvantages
• - too noisy

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7. Tactile and Proprioceptive Biofeedback

• Tactile and proprioceptive feedback for
  artificial limbs
• a. Direct neural stimulation
• b. Transcutaneous stimulation
• c. Mechanical vibrators
• Only a few studies in lower limb prostheses.
• Kawamura et al.
• - Surface electrocutaneous signals to provide
  foot sole pressure information
• - Four independent tape switches on the sole of
  the prosthetic foot drive four surface electrodes
  on the thigh in above-knee amputees.
• - Detection of prosthesis knee angle
• Direct peripheral nerve stimulation by Cliiper
  et al.
• - Four Pt-Ir electrodes inserted in the sciatic
  nerve of patients
• - The frequency of electrical stimulation was
  modulated by the bending moment through the
  strain gauges on the pylon of the prosthesis.

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7. Tactile and Proprioceptive Biofeedback

• Sabolich and Ortega (1994)
• - Noninvasive system with transcutaneous
  electrical stimulation to sense organs at the
  socket-limb interface.
• - Two pressure transducers in the sole of the
  artificial foot
• - Send tingling signals to the amputees residual
  limb.
• - Some benefits in weight distributions, step
  length, and stance time

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8. A Portable Device for Tactile Stimulation
8.1. The system

• Mechanical tactile stimulation for artificial
  sensory feedback
• Components Two FSR sensors, a circuit, and two
  small eccentric DC motors
• FSR sensors ideal for touch control,
  inexpensive, thin (gt0.15mm),
• durable (107 actuations) and environmentally
  resistant
• The box (circuit and power supply) 220g

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8.1. The system

• Two eccentric motors from the mobile phone
• For BK amputees, two vibrators on the thigh in
  anterior and posterior positions to maintain the
  spatial correspondence of with three sensors.
• Four-sensors-four-corresponding-stimulators
• Limitations
• - Spatial discrimination ability in tactile
  stimulation
• - Constant conscious effort from the subject

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8.2. Rehabilitation protocol

• Sensory feedback on the pressure by artificial
  foot on any surface
• (automobile clutch, brake, or bicycle pedals)
• Rehabilitation protocol
• - Step I. Orthostatic exercises to obtain static
  balance
• - Step II. Walk inside the parallel bars
• - Step III. Walk with some auxilliary devices
  such as crutches
• - Step IV. Walk without any auxilliary devices
• Experiments
• - Motion analysis with ELITE system
• - F-Scan and Video-controller
• Main goal of the experiment
• - to quantify the advantage of the tactile
  sensory feedback in reducing the duration of the
  rehabilitation process in amputees

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9. Conclusions

• The sensation of the ground pressure of
  artificial foot
• - important in control and coordination of
  walking, running, and doing other activities
• CNS needs sensory re-afference in order to
  update and maintain the internal model for
  appropriate strategies
• Process of updating the internal model for an
  amputee with some kind of artificial sensory
  information