September 2, 2008
ELECTRICAL STIMULATION©
Lyn Paul Taylor, A.A., B.A., M.A., R.P.T.
(Editing Assistant and Computer Consultant: Joanna Soon, B.S.)
Electrical
stimulation of human tissues is an old procedure dating from the attempts of
the early Greeks to use electric eels for therapeutic purposes. The modern rediscovery of electricity and its uses in physical medicine dates
from the early eighteenth century, when, again, electrical
stimulation generated by electric eels was applied therapeutically to relieve
headaches and to affect neuromuscular paralysis.
Therapeutically applied
electrical stimulation has had a checkered history, enjoying long periods of
popularity and respectable use interspersed with periods of widespread misuse
by medical charlatans who treated everything from psychiatric
conditions to cancerous tumors. One must suppose that the simplicity of electricity producing equipment (especially static or
direct current) coupled with the seemingly magical effects that
electrical currents have on human tissues, unavoidably led to its exploitation
by unscrupulous "practitioners" at the expense of the
unsophisticated and gullible.
Often billed as a near panacea
for the cure of most human physical ills, over time the therapeutic use of
electrical stimulation, in the minds of many, gradually became associated with quackery,
and therefore considered beneath the use of scrupulous and sophisticated
practitioners. This state of affairs was, and is, unfortunate, since there are
many physical ills that humans are afflicted with that may be improved
or corrected by appropriately applied electrical stimulation.
Electrical
Stimulation Related Terms
Modern
instrumentation used to apply electricity to the body is designed for users who
are without detailed knowledge of the instrument's internal circuitry or the
physics responsible for the production of electricity. However, some knowledge
of the basic principles that govern electrical stimulation is useful for an
understanding of the variable results that come from the necessary “trial
and error” that is a regular feature of its therapeutic use. A few
terms are defined below to help those of us who have little or no education in
this area.
Ampere:
the unit of flowing charge (current). Most therapeutic electrical stimulators
have a low average current of less than 1.5 milliamperes (mA) and relatively high peak currents of between 60 and 100 mA. (Amps =
coulomb/second).
Bipolar
Electrode Placement: both cathode (negative) and anode (positive) electrodes are placed on the treatment area in relative proximity to
each other. This arrangement provides for rather specific stimulation of
structures with few variations in responses.
Burst frequency:
the number of trains of impulses produced per second; it
is dependent on the “stimulation on and stimulation off” duty cycle
selected.
Coulomb:
a basic unit of charge theoretically produced by 6.28 x 1018 electrons.
Most therapeutic electrical stimulators have a low pulse charge, expressed in
micro-coulombs (10-6 coulombs).
Current
density: the amount of current per unit area; i.e., the
smaller the electrode the greater the current density, making the
stimulus perceptually stronger to the recipient.
Joule:
a basic expression for work performed by electricity; i.e., work is the
force required to move a charge. (Joule = coulomb * voltage).
Monopolar
electrode placement: one electrode is placed on the treatment area
and the other is placed on a remote location on the body. This arrangement
provides a rather general stimulation pattern because of the multiple parallel
pathways the current that may be take from one electrode to the other. One may
also expect variations in the responses produced by the electrical stimulation
applied this way because of the number of nerves and other structures the
current may pass through.
Ohm:
a unit expressing the amount of resistance offered by a
current conductor (the recipient’s soft tissues).
Ohm's Law:
“The current (amps) is directly proportional to potential (volts) and
inversely proportional to resistance (ohms). Current = potential/resistance.”
Amps (amperes) = volts/ohms.
Pulse duration:
the amount of time the current flows in one direction.
Pulse duration is measured when the current level is at 50% of its peak,
usually expressed in microseconds.
Pulse frequency:
the number of pulses produced per second, hertz
(Hz) or cycles per second (c/s).
Resistance
to current:
The body is made up of tissues and fluids that vary in
their electrical conductivity and, conversely, their resistance to the passage
of electricity. Tissue conductivity is proportionally related to
the tissue's water content; the higher the water content the greater the
conductivity and the lower the tissue's resistance. The water content of
muscle is 72 to 75%, the brain is 68%, fat is 14 to 15%, and of the peripheral
nerve, skin, and bone is five to 16%. Resistance varies in direct proportion
to the distance between electrodes. The resistance
increases, as the distance the electrical stimulus must travel
increases.
Volt:
A volt is a unit of measure that indicates the amount of potential energy (Joule)
each unit of charge (coulomb) contains (Voltage = Joule/coulomb).
Electrotherapeutic
currents are generally derived from the commercial lighting circuit (alternating
current in the United States or direct current in some
other parts of the world) or from the direct current (d/c) provided by
batteries. Transformers, electromagnetic or thermionic devices, or complex
circuitry (beyond our scope here) modify these basic currents to produce
various therapeutic current forms. The therapeutic current forms include
galvanic (square wave), interrupted galvanic, surged
interrupted galvanic, sinusoidal, alternating, surged
alternating, faradic, surged faradic and
other hybrid waveforms (generally produced by combining two or
more waveforms). The variables manipulated to produce the various waveforms
include: voltage, amperage, mode flow (direction),
pulse frequency, and pulse duration (pulse width).
Electrotherapeutic
Effects on Soft Tissues
Electrical
stimulation is applied through a pair of electrodes placed on the body. The
electricity is passed from the cathode (negative) pole electrode through the tissues to the anode (positive) pole electrode (sometimes called the dispersive), thus completing an electrical
circuit.
Electrical
currents passed through muscle or nervous tissue from an external source (electrical
stimulator) will be partially depolarized in the region
of the negative and hyperpolarized in the region of
the positive. If the current is sufficiently strong, the degree
of depolarization will reach or exceed the critical level necessary to produce
a muscle contraction or firing of the nerve. At
the positive, as the circuit is completed, the body
overcompensates for electrical changes induced by the current, so that some degree
of irritability is present. If sufficiently great, the
irritability will also cause a muscle contraction or nerve firing under the
anode. The current level required to produce a single neuron impulse
or single muscle fiber contraction is called a minimal stimulus. If a stronger stimulus is required to excite all of a group of nerve fibers or
denervated muscle fibers, it is called a maximal stimulus. Any stimulus greater than the maximal stimulus is
called a supramaximal stimulus.
The factors
that determine the adequacy of a stimulus to either elicit a muscle contraction
or provoke firing of nervous tissue are pulse frequency, pulse
duration, and the amplitude of the current. The minimal
duration of an effective electrical stimulus, sufficient to provoke a
muscle contraction or nerve firing, is 1.0 microsecond for a normal innervated
muscle fiber and 0.03 microseconds for a normal nerve fiber. The strength
of an electrically induced muscle contraction is related to the intensity and
pulse duration of the stimulus: the greater the intensity and
pulse duration, the greater the strength of contraction.
Equipment
Utilized in Electrical Stimulation
The electrical units currently used
for the stimulation of muscle or deep tissues can generally be classified
into five categories:
High Frequency
(Medium Frequency) Stimulators:
These units by definition generate more then 1000 c/s with popular models producing 2500 c/s. The 2500 c/s
units employ a usual duty cycle of 10 milliseconds (msec) on and
10 msec off. In this case, the 2500 c/s is interrupted at 1/100
of a second on and 1/100 of a second off with a 50%
duty cycle producing 50 bursts per second with 25 cycles per burst. The 2500
c/s unit generally has a peak current of 130 mA with an average current level
of from 80 to 100 mA root mean square (RMS). These units provide a variety of duty
cycles, ramps, and peak currents from which
to choose. They can create a muscle contraction that is 60% (or greater) of
that produced by a maximal isometric contraction.
An
electrical stimulator with variable current forms
High Voltage
Stimulators:
These units have a high peak current of 500 mA or greater with a low
average current of less than one mA. They are constant voltage
generators with a pulse charge of approximately four micro coulombs,
and their pulse durations usually range from five to eight microseconds. They
generally provide a variety of duty cycles and pulse-frequencies from which to
choose.
Interferential
Stimulators:
These units are constant current generators
that create a pulse frequency of 4000 to 5000 c/s. The
interferential units generally employ two electrical sine wave circuits,
one of which has a fixed frequency while the other varies its frequency; when
the two waveforms intersect, an interferential frequency is said
to result. The interferential unit usually has a peak current of 60 mA.
Low Voltage
Electrical Stimulators:
These units have low peak currents, low voltage driving forces that can be alternate or direct currents, and their pulse
durations are usually large, measured in msec or seconds. If using a direct
current, they can produce thermal and chemical
effects and can be used to produce iontophoresis.
Portable
Neuromuscular Stimulators:
These units employ a constant current, which generally has a peak of 100 mA with a driving force of from 50 to 100 volts. They
generally provide a choice of duty cycles, pulse frequencies, peak currents,
and ramps (the time it takes for the current level to rise from zero
to its peak).
The electrical energy from
electrical stimulators is conveyed to the subject by conducting cables.
The cables are plastic or rubber insulated flexible copper or silver wires.
The thickness of the cable depends on the amount of current to be carried by
the conductor: the greater the current, the thicker the cable
needs to be. These cables may be a uniform color or color-coded according to
function. If color-coded, the wire to the negative electrode is
conventionally black, and that to the positive
electrode is red.
An electrode
is a medium that intervenes between the cable from the electrical
stimulator and the subject's body (only surface electrodes will be discussed).
It generally consists of a good conducting material whose shape and form can be
adapted to conform to contours of the body. Electrode mediums include water,
metal foil (usually made from an alloy of lead, tin, and zinc), moist-pads,
or flexible carbon or silicone pads.
Flexible
electrode pads
Electrode pads are usually employed in pairs, often of equal size. Between two electrode pads
of equal size, the current density beneath each of them is equal. If one is twice as large as the other is, the current density under the
smaller one will be twice as great as that under the larger. As
the current spreads between two electrodes pads, across the body, its density
must gradually decrease so that midway between them the density is the least.
The closer the electrodes are to one another, the greater
the density of the current that passes between them. The higher the
current density, the greater the effect on the tissues stimulated.
The electric current
carried along the cable length eventually leads to some crystallization of
the conducting wire and to breaks in the wires at the sites where
the most bending or movement of the cable occurs, usually close to the
electrode connections at both ends.
Application:
- If low frequency sponge pad electrodes are being used, they
should be well moistened with a saline solution (or water) and placed over the
chosen treatment sites. If carbon or silicone pads are used, care should be
taken that the skin between the electrodes remains dry to avoid an electrode
bridge that would decrease or preclude effective electrical stimulation
(the electricity would pass through the water to complete the circuit, having
no effect on the body).
- Generally, the negative electrode should be placed
on the muscle's motor point (where the motor nerve is most
superficial as it innervates the muscle) so that when stimulated the greatest
muscle contraction is provoked. Once the best sites for electrode placement
have been determined, elastic strapping, weighting, or taping should be applied to ensure good electrode contact.
- A watch or timer should be set for the length of treatment. The
electrical stimulator should be turned on, and the amplitude (intensity)
increased until a visible contraction takes place, always staying
within the subject’s range of tolerance.
- The subject should be allowed to become comfortable with the
current before additional intensity increases are slowly made. This process
should be continued until the desired degree of contraction is
reached. The subject should be closely monitored for excessive muscle spasm,
cramping, joint compression, or pain. The subject should never be left out of
sight or hearing range once the treatment has been started.
- At the end of the session (if not automatically shut off) the
intensity of the stimulator should be gradually decreased until it is switched
off. Return all controls to zero. Remove all electrodes from
the subject. The subject should rest for several minutes before being allowed
to exercise.
Precautions:
Electrical burns may occur if continuous
uninterrupted galvanic current is used and an excess of current density is applied to the skin or mucous membrane. If an electrical
burn results, the tissue damage produced occurs in a roughly conical area,
extending from the apex of the cone on the skin's surface (where the original
electrical contact occurred), and fanning out into the deeper layers. Just
following the injury, the burn site appears rather small and inconsequential
but becomes more alarming as the damaged tissues are subsequently sloughed off
and ulceration occurs. Electrical burns are slow to heal, prone
to infection, and (if sufficiently deep) may be followed by extensive unsightly
scarring.
Electric shock may be caused if the subject touches a grounded object (a water pipe,
radiator, or electric circuit) while being stimulated. This is especially
serious if a large area is subjected to the shock. Electric shock
may also occur if the electrical stimulator suffers transformer breakdown
(which is unlikely with modern units). Should this hap-pen, the high-tension,
low frequency current may jump to the subject and produce an electrical
burn as well as shock.
Care should be taken to avoid over-fatigue
of the muscles stimulated. Stimulation should stop when the muscle begins to
respond with less vigor.
Generally, electrodes
should not be placed over scar tissue, skin
irritations or open skin lesions (unless used to help
fight infection or foster healing). If increased sweating, salivation
or signs of nausea occur stimulation should be discontinued.
Electrical current should
not be allowed to flow across a pregnant uterus or a cardiac pacemaker.
When applying electrodes,
care should be taken to avoid overlapping negative and positive
electrodes and conductive materials (electrode cream, water or gel) should not
be allowed to form an electrode (conductive) bridge
between the two. Either situation will cause a completion of the circuit
without involving or affecting the subject's tissues.
When applying electrical
stimulation, a gradual increase of intensity is preferred because
of the tendency of natural skin resistance to suddenly break
down after being exposed to an electric current for several minutes.
If the apparent lack of tissue response persuades the practitioner to increase
the intensity to a relatively high level before skin resistance breaks down,
the subject may pay for the practitioner's lack of patience by
experiencing additional pain or discomfort. Future treatment may be put in
jeopardy because of the subject’s acquired fear.
References:
G. Alon, "High
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Pp. 890-895
L.L. Baker,
K. Parker and D. Sanderson, "Neuromuscular Electrical Stimulation
for the Head-Injured Patient," Physical Therapy, 63:12,
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S.A. Binder-Macleod,
L.R. McDermond, "Changes in the Force-Frequency Relationship of
the Human Quadriceps Femoris Muscle Following Electrically and Voluntarily
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D.P. Currier
and R. Mann, "Muscular Strength Development by Electrical Stimulation
in Healthy Individuals," Physical Therapy, 63:6, June 1983.
Pp. 915-921
A. Delitto,
J.M. McKowen, J.A. McCarthy, R.A. Shively and S.J. Rose, "Electrically
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and S.J. Rose, "Comparative Comfort of Three Waveforms Used in
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L.F. Eckerson
and J. Axelgaard, "Lateral Electrical Surface Stimulation as an
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C.B. Killian,
"Electrical Stimulation Overview Introduction to High Frequency
Stimulation," Presented at a Combined Section Meeting in Orlando,
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Department of Physical Therapy, 1400 East Hanna Avenue, Indianapolis,
In, 46227.]
T. Mohr, B.
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R.A. Newton
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T. Malone, "Treatment Parameters of High Frequency Electrical Stimulation
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"Relationship Between Functional Electrical Stimulation Duty Cycle
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D.M. Selkowitz,
"Improvement in Isometric Strength of the Quadriceps Femoris Muscle
After Training with Electrical Stimulation," Physical Therapy,
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W.J. Shriber,
A Manual of Electrotherapy, Lea & Febiger, Philadelphia, Pa.,
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D.R. Sinacore,
A. Delitto, D.S. King, S.J. Rose, "Type II Fiber Activation with
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G.K. Stillwell,
Therapeutic Electricity and Ultraviolet Radiation, The Williams
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L.P. Taylor, T. Hui, The Taylor Technique of Soft
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R.A. Wong,
"High Voltage Versus Low Voltage Electrical Stimulation,"
Physical Therapy, 66:8, August 1986. Pp. 1209-1214
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