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Biofeedback
Biofeedback
• A therapeutic procedure which uses
electronic
or electromechanical
instruments to accurately measure,
process, and feedback reinforcing
information via auditory or visual signals
• Used to help patient develop greater
voluntary control of either neuromuscular
relaxation or
muscle re-education
following injury
Role of Biofeedback
• Feedback includes information related
to the sensations associated with
movement itself as well as information
related to the result of the action relative
to some goal or objective
Role of Biofeedback
• Feedback refers to intrinsic information
inherent to movement including
kinesthetic, visual, cutaneous, vestibular,
and auditory signals collectively termed as
response produced feedback
• Also refers to extrinsic information or some
knowledge of results presented verbally,
mechanically, or electronically to indicate
the outcome of some movement
performance
Role of Biofeedback
• Feedback is ongoing, in a temporal
sense, occuring before, during, and
after any motor or movement task
• Feedback from some measuring
instrument which provides moment to
moment information about a biologic
function is referred to as biofeedback
Role of Biofeedback
• Provides patient with a chance to make
correct small changes in performance
which are immediately noted and
rewarded so that eventually larger
changes or improvements in
performance can be accomplished
• Goal is to train patient to perceive
changes without the use of a measuring
instrument so that they can practice by
Biofeedback Instrumentation
• Designed to monitor some physiologic
event, objectively quantify these
monitorings, and interpret the
measurements
• Biofeedback units cannot directly measure
a physiologic event but they record some
aspect which is highly correlated with the
physiologic event
Biofeedback Instrumentation
• Biofeedback reading gives an indication of
a physiological process but should not be
confused with the physiological process
itself
Biofeedback Instruments
Peripheral skin temperatures measures the extent
of vasoconstriction or vasodilation
 Finger phototransmission units
(photoplethysmograph) measures vasoconstriction
and vasodilation
 Units that record skin conductance activity
indicating
sweat gland activity
 Units that measure electromyographic activity
(EMG) indicating amount of electrical activity
during muscle contraction

EMG Biofeedback
• Nerve fiber conducts an
impulse to the neuromuscular junction where
acetylcholine binds to
receptor sites on the
sarcolemma inducing a depolarization of the
muscle fiber which creates movement of ions and
thus an electrochemical gradient around the
muscle fiber
• Changes in potential difference or voltage
associated with depolarization can be detected
Measuring Electrical Activity
• EMG does not measure muscle
contraction directly
• EMG measures electrical activity
associated with muscle contraction
• Electrical activity of muscle measured in
microvolts (1 volt=1,000,000 µV)
• EMG readings can be compared only
when the same equipment is used for
all readings
Measuring Electrical Activity



EMG biofeedback unit receives small
amounts of electrical energy generated
during muscle contraction through
electrode
Separates or filters electrical energy from
other extraneous electrical activity on skin
and amplifies the EMG electrical energy
Amplified EMG activity is then converted to
some type of information which has
meaning to the patient
Anatomy of EMG Biofeedback
EMG Electrodes
• Skin surface electrodes most common while
indwelling electrodes are not practical
• Electrodes made of stainless steel or
nickel- platted brass recessed in a
plastic holder
• Some electrodes permanently attach to
cable wires while others may snap
onto the wire
• Some units include a set of three
electrodes preplaced on a velcro band
EMG Electrodes
• Size of electrodes ranges between 4mm
in diameter for recording small muscle
activity and 12.5 mm for larger muscles
• Increasing the size of the electrode will
not cause an increase in the amplitude
of the signal
• Electrodes may be disposable or nondisposable and require some type of
conducting gel
EMG Electrode Placement
• Skin must be appropriately prepared by
scrubbing with an alcohol-soaked prep
pad
• Electrodes should be placed as near to
the muscle being monitored as possible
• Electrodes should be parallel to the
direction of the muscle fibers
• Spacing of the electrodes is critical to
reduce extraneous electrical activity
(noise)
Separation and Amplification of
EMG Activity
• Electrode arrangement
must eliminate noise
• 2 active electrodes and
1 reference electrode
• Active electrodes
pick up electrical
activity from motor
units firing in the
muscles beneath
the electrodes
Separation and Amplification of
EMG Activity
• Magnitude of the
small voltages
detected by each
active electrode will
differ with respect to
the reference
electrode creating
two separate signals
Separation and Amplification of
EMG Activity
• Two signals are fed to
a differential amplifier
which subtracts the
signal from one active
electrode from the
other active electrode
• Differential amplifier
uses reference
electrode to compare
the signals of the two
active electrodes
Separation and Amplification of
EMG Activity
• 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
Separation and Amplification of
EMG Activity
• Ability of the differential
amplifier to eliminate
the common noise
between
active
electrodes is called
the common mode
rejection ratio
(CMRR)
Separation and Amplification of
EMG Activity
• External noise can be
reduced by using
filters which make
amplifier
sensitive
to some incoming
frequencies and less
sensitive to others
• Amplifier will pick up
signals only at
bandwidth produced
by electrical
activity
Converting EMG Activity to
Meaningful Information
• After amplification and filtering the EMG signal
indicates true electrical or raw EMG activity in
muscle
• Raw EMG is an alternating voltage - direction or
polarity is constantly reversing
Converting EMG Activity to
Meaningful Information
• To determine the overall increase and decrease in
electrical activity deflection toward the negative
pole must be flipped upward toward the positive
pole
• This summation of electrical activity is referred to
as rectification
Converting EMG Activity to
Meaningful Information
• Once rectified EMG signal can be smoothed to
eliminate the peaks and valleys or high frequency
fluctuations which are produced with a changing
electrical signal
Converting EMG Activity to
Meaningful Information
• Once smoothed the signal may be integrated by
measuring the area under the curve for a
specified period of time
• Integration forms the basis for quantification of
EMG activity.
Converting EMG Activity to
Meaningful Information
• Biofeedback units generally provide
either visual or auditory feedback
relative to the quantity of electrical
activity
• Visual feedback uses lights, bars, or
analogue or digital meters
• Auditory feedback uses increasing or
decreasing tones, buzzing, beeping or
clicking.
Setting Sensitivity
• Signal sensitivity or signal gain may be
set
• Sensitivity may be set at 1 µV, 10 µV, or
100 µV
• A high signal gain means the
biofeedback unit is sensitive enough to
detect the smallest amounts of electrical
activity
High sensitivity levels should be used
Setting Sensitivity
• Low sensitivity gain should be used in
muscle-reeducation during which the
athlete may produce several hundred µV
of EMG activity
• When adjusting the sensitivity range it
should be set at the lowest level that does
not elicit feedback at rest
Clinical Applications
• Muscle Re-education
Involves regaining neuromuscular control
and/or
increasing strength of a muscle
• Relaxation of muscle spasm or muscle
guarding
• Pain reduction
•