EMG Biofeedback for physiotherapy students

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EMG BIOFEEDBACK

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Feedback
• Feedback is a response to a process or activity
• According to Weiner, 1948, ‘feedback is a method of controlling a
system by reinserting into it the results of its past performance’
• Physiological feedback is a self-regulatory biological system in
which the output or response affects the input either positively or
negatively

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Feedback
• Physiological feedback refers to intrinsic information.
• These include kinesthetic, visual, cutaneous, vestibular, and
auditory signals.
• Feedback from some measuring instrument refers to extrinsic
information
• Also referred to as biofeedback.

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everydayforms of biofeedback

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Positive Feedback
• Change in one direction
causes further change in the
same direction Pressure
on cervix
Oxytocin
release
Uterine
contraction

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Negative Feedback
• Change in one direction causes further change in the
opposite direction
Body Temperature
Rises
Body Sweats more
Body Temperature
drops

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In Biofeedback…
1. The information is detected,
2. provided in an understandable way to the patient who can then,
at their own initiation,
3. use the information to achieve a measure of control over the
same process.
i.e. engaging the patient in a ”closed loop” learning, using
feedback until enough development of his motor skills occurs, so
that an “open loop“ movements can be achieved.

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Physiologic
signal
Response by
patient
Physiologic
signal
Response by
patient
Detection
and
feedback
Open Loop
Closed Loop

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youmust have
accurate feedback, of course

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Requirements
• To be realistic and successful, three main elements are needed in
order to enhance motor learning:
1. Relevant
2. Accurate
3. Speed of information
• Most EMG biofeedback devices nowadays introduce 50 to 100
milli sec. delays before the signal can reach the ears or eyes of
the patient

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EMG Biofeedback
• EMG does not measure muscle contraction directly.
• EMG measures electrical activity associated with muscle
contraction.
• Electrical activity of muscle measured in micro volts (1 volt =
1,000,000 µV)

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EMG Biofeedback
Advantages:
• Can be integrated with other therapeutic interventions
• An enhancer of the therapy
• Reduce patient’s reliance on the therapist
• Gain control without reliance on the therapist, and once
gained,
• Maintain control without either the therapist or the
machine.

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Working
A biofeedback loop.
Here the forearm
muscles are monitored to
provide a visual readout
to the user.
The feedback loop is
completed when the user
alters her muscle tension
to adjust the readout.

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Working
Two active electrodes
One reference electrode

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Differential Amplifier
• A differential amplifier is used as the first stage of
processing.
• Its advantage is that the elements which are common in
both the signal inputs are subtracted by this differential
amplifier.
• Ambient noise is subtracted out at this very first stage.

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Working - CMRR
• Ability of the differential amplifier to eliminate the common noise
between active electrodes is called the common mode rejection
ratio (CMRR)
• Two signals are fed to a differential amplifier which subtracts the
signal of one active electrode from the other active electrode.
• Differential amplifier also uses reference electrode to compare the
signals of the two active electrodes.
• This in effect cancels out or rejects any components that the two
signals coming from the active electrodes have in common thus
amplifying the difference between the signals.

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Working - filtering
• EMG signals are in the range of 0 – 500 Hz
• Frequencies above 500 Hz is filtered out
• The dominant energy of the EMG signal is in the range of
50 – 150 Hz
• Motion artifacts in the range of 0 – 20 Hz
• Frequencies below 20 Hz are filtered by using a high pass
filter

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Working – Rectification & Integration
Raw EMG activity in muscle is an alternating voltage -
direction or polarity is constantly reversing
Rectification is the summation of electrical activity. To
determine the overall increase and decrease in electrical
activity, deflection toward the negative pole must be flipped
upward toward the positive pole
The signal may then be integrated by measuring the area
under the curve for a specified period. Integration forms the
basis for quantification of EMG activity.

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Cues
Visual cues
1. Meter read outs
2. Flashing lights
3. Oscilloscope
4. Computer screens
Auditory cues
1. Changing tones
2. Clicks

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Features of the BF Devices
• Gain settings, low & high
• Sound
• Threshold

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Gain settings
• On low gain settings the machine will require a greater
signal before the output changes - in other words, it is
less sensitive. Used for muscle recruitment.
• On higher gain settings, a small amount of EMG activity
will be easily seen by the patient. Used for relaxation.

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Sound
• Most EMG biofeedback devices offer an audible feedback
in addition to the visual information
• the change in sound is such that the frequency of the
‘beeps’ increases with increased EMG activity.

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Threshold
• Incorporation of a threshold system enables the patient
targets to be set.
• The audible signal can be adjusted so that it will only be
heard if the patient achieves a preset activity level.
• To achieve an increase in muscle activity, the threshold
can be set so that the audible feedback only comes on
when 50% of the scale has been achieved.

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Equipment

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Electrode Placement
• Silver Chloride construction
• judicious electrode placement
• electrode spacing
• Noise/ movement artefact
• active electrodes should be
placed in parallel with the
dominant muscle fibres

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Uses
• Facilitate muscle contractions
• Promote increased motor recruitment
• Regain neuromuscular control
• Decrease muscle spasm
• Promote relaxation

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Indications
• Stroke
• Spinal cord injury
• Improving muscle action
• Posture control
• Balance and mobility
• Trunk muscle re education
• Respiratory muscle control
• Incontinence

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Contraindications
• If the patient is prohibited from moving the joint or
isometric contractions, then BF should NOT be used
• Unhealed tendon grafts
• Avulsed tendons
• Third degree tears of muscle fibers
• Unstable fracture
• Injury to joint structure, ligaments, capsule, or articulating
surface

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Technique for Motor Recruitment
• Position pt., Explain to pt., Work in quiet area.
• Apply sensors to an area to demonstrate the desired A-V signal and activity.
• Adjust to the most sensitive level that picks up any MUAP that pt. can
produce
• Instruct pt. to try to produce an A-V signal
• As voluntary muscle activity improves, the A-V signal will increase. Adjust
the gain to decrease the sensitivity, so pt. must work harder to recruit more
muscle units.
• As pt. masters in one position, change positions
• Document all parameters. Tx duration 30-60 min
• Clean up

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Inhibition of Spasticity
• Goal: decrease undesirable muscle activity that may be interfering with
functional movement.
• Eval. spasticity, Explain to pt., Apply sensors and demonstrate desired
activity.
• Find the least sensitive setting that produces minimal A-V feedback
• Have pt relax, use techniques and have pt. lower the A-V signal.
• As the signal decreases, lower the shaping controls (gain), to a more
sensitive level. As pt. relaxes better, continue to increase sensitivity
• Change positions

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Pressure Biofeedback
• A simple device to provide
feedback to ensure quality, and
precision in exercise performance
and testing.
• Monitors position of the low back
and provides feedback when the
abdominal muscles are not actively
or effectively protecting the spine.

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References
1. http://en.wikipedia.org/wiki/Biofeedback
2. http://bme2.aut.ac.ir/~towhidkhah/MotorControl/Resources/EMG.pdf
3. http://www.electrotherapy.org/modalities/biofeed.htm
4. Low & Reed, Electrotherapy Explained, principle and Practice, 4th edition, 2009, Elsevier,
5. Jagmohan Singh, Textbook of Electrotherapy,1st Edition, 2005, Jaypee Publications.