Low Frequency Currents Definition .
Low frequency currents are therapeutically used currents whose frequency is in the range of 0 to 100 cycles per second. The primary use of low frequency current is the stimulation of nerves and muscles. Electrical current impulses having pulse frequency between 1 and 250 Hz, used for stimulation of nerve or muscles, are known as low frequency currents .
Various currents in this category which are used for the physiotherapeutic treatments include, direct current, interrupted direct current, sinusoidal current, diadynamic current, high voltage pulsed galvanic current, microamperage electrical nerve stimulation, transcutaneous electrical nerve stimulation etc.
Classification of Low frequency current.
1. Interrupted galvanic current.
Interrupted galvanic current is a monophasic direct or galvanic current, broken at preset intervals and allowed to flow for a preset pulse duration with a frequency between 1 and 6 Hz, and a pulse duration between 1 and 300 msec. These are also known as long duration currents, specifically used for stimulation of denervated muscles or motor point detection in innervated muscles. It is called as long duration current having duration of more than 1 ms upto 300 or 600 ms.
Interruption is the most usual modification of direct current, the flow of current commencing and ceasing at regular intervals. The rise and fall of intensity may be sudden and may be of rectangular, saw-tooth, triangular and trapezoidal type . The impulse in which the current rises gradually are often termed “selective” because a contraction of denervated muscle can often be produced with an intensity of current that is insufficient to stimulate the motor nerve. This occurs due to accommodation. Read more :-Physical Principles of Light
It is often found that the more long-standing the denervation, the slower the rise in intensity of current that is required. An impulse of 100 ms duration is often used which requires frequency of 30 Hz. But as you increase the duration, frequency must be reduced. To eliminate the danger of chemical burn reverse wave of current, i.e. depolarized impulses should be used, which also reduces the skin irritation. Production of interrupted DC is usually accomplished in modern apparatus by circuits, which employ transistors and timing devices. Current is always applied to the patient via potentiometer as this allows the intensity of current to be turned up from zero.
2. Faradic type of current.
Faradic type of current is very short duration monophasic interrupted galvanic current, with frequency between 50 and 100 Hz and Pulse duration between 0.01 and 1 msec. These are also known as short duration current, specifically used as surged faradic current for stimulation of innervated muscles.
Faradic type current is short duration interrupted direct current with pulse duration of 0.1–1 ms and frequencies between 50–100 Hz, used for the stimulation of innervated muscles. The term faradism was previously used to signify the type of current produced by the first faradic coil and was unevenly alternating current with each cycle consisting of two unequal phases .
1. Low intensity long duration current.
2. High intensity short duration current.
Faradic coils have now been replaced by electronic stimulators which almost have the same physiological effect but differs in the waveform. The features essential for the production of these physiological effects are the impulses with duration of 0.1–1 ms with a frequency of 50–100 Hz .
3. Pure faradic current.
Pure faradic current is a biphasic current with a sharp negative spike of 1 msec, followed by a gentle positive trough of 4 msec; with a frequency of 50 Hz. Traditionally it was produced by the Smart-Bristow coil, though not used any more.
4. Modified Faradic Current.
For better results in the treatment, faradic current is always surged to produce a near-normal tetanic-like contraction and relaxation of the muscle. The apparatus should have sufficient control to surge the current so that the intensity of successive impulses increases gradually with surges varying in waveform to provide satisfactory muscle contraction and relaxation.
In the original faradic coils, the current was surged by hand but in modern stimulators an electronic device is used. The circuit can be modified to give surges of various durations, frequencies and waveforms. Various forms of surge are available, such as trapezoidal, triangular and saw-tooth impulses, and that most suitable for each patient must be selected.
Classification of Alternating Current.
1. Sinusoidal currents.
Sinusoidal currents are evenly alternating sine wave currents of 50 Hz. This gives 100 pulses or phases in each second of 10 ms each, 50 in one direction and 50 in another . It is produced from the mains by reducing the voltage to 60–80 V with a step-down transformer. It is usually surged to cause rhythmical muscle contractions. It relieves pain and reduces edema. Because of marked sensory stimulation this current is often applied to large areas and rarely used for local muscle stimulation. Physical Principles of Light
2. Diadynamic currents.
Diadynamic current is also called Bernard current. It is unidirectional sinusoidal current with a frequency of 50 to 100 Hz and pulse duration of 10 ms. Diadynamic current is used for pain relief, minimizes inflammation, swelling, facilitation of healing, increased circulation and motor re-education. It is commonly used in the treatment of painful and inflammatory disorders of muscles, ligaments, joints and peripheral nerves. It is also used in the treatment of contusion, hematoma, myalgia, muscle atrophy due to immobilization, inactivity and epicondylitis.
Diadynamic current is applied for ten minutes, once or twice daily with perceptible intensity. Various modulations of diadynamic currents are fix monophase, fix diaphase, short periods and long periods, syncopated rhythm and modulated monophase. Introduced by Pierre Bernard nearly 70 years ago, they are sinusoidal, direct currents being rectified mains type currents with frequency of 50-100 Hz. There are six (6) different types of currents, which are each used for different purposes.
a. The MF (monophase) Current.
The MF (monophase) is a half-sinusoidal alternating current, which is created by a one-way DC converter of 50 Hz, with an impulse length and interruption of 10 ms each. The primary effect of this type of current is muscle stimulation .
b. The DF (diphase) Current.
The DF (diphase) type of current is created by an alternating current of 50 Hz by means of a two-way DC converter, so that a current of 100 Hz is achieved. The patient feels a stabbing sensation in the treated area. The stimulus is less than that of the MF and primarily affects the autonomic nervous system in the sense of lowering the increased sympathetic tone .
c. The short-period Current (SP).
The short-period current (SP) involves a sudden alternation of MF and DF currents. The patient senses the abrupt change between the tensing MF current and relaxing DF current .
d. The long-period (LP) current.
The long-period (LP) current, the MF current is mixed with a second modulated MF. The gradual raising and lowering of the amplitude is experienced by the patient as a more pleasant sensation than that produced by SP .
e. The syncopated Rhythm (RS) Current.
The syncopated rhythm (RS) is the current is interrupted by a pause of 0.9 second after a current flow of 1.1 second. This type of current is used for the electrical stimulus of the muscles .
f. The modulated monophase (MM) current.
The modulated monophase (MM) current is a logical extension of Monophase currents and Syncopated Rhythm. In the MM the RS is gradually reduced in stepwise fashion. Like the RS, the MM is suited for the treatment of muscular atrophies, but the faradic excitability of the particular muscles must be maintained .
Therapeutic effects of the diadynamic currents .
- Pain relief.
- Decrease inflammation and swelling.
- Muscle reeducation.
- Increase local circulation.
- Facilitation of tissue healing.
Nerve Transmission .
In normal nerve, there is difference of concentration of ions inside and outside the nerve. Due to this there is difference of potential called as potential difference between inside and outside of the nerve.
Nerve remains in two states ; these are given below.
1. Resting state.
2. Stimulated state.
1. Resting state of Nerve.
In resting nerve, the nerve is positive outside and negative inside. At this time, the nerve is not permeable to Na+ ions, so it is called as polarized state of nerve. When a nerve is stimulated, it causes fall in potential difference (PD). When the fall reaches to a certain level, it provides the permeability of sodium ions. This permeability causes the difference in concentration of ions inside and outside the nerve and thus further fall of PD until reversal of polarity occurs.
Now the membrane is positive inside and negative outside . Immediately after this activity the Na+ ions are pumped again and the stimulated part again comes to resting state. Now the difference between the active and resting part of the nerve causes the local electron flow between the active and resting part of the nerve.
The direction of electron flow through the membrane is opposite to the PD across the fiber. The fiber acts as a resistance to current so that current flow lowers the PD, this again make the membrane permeable to Na+ ions and cause the reversal of PD as before. These changes of PD are then propagated along the length of nerve fiber. This change of polarized stage causes the travel of impulse.
2. Electrical Stimulation of Nerves .
To initiate the nerve impulse, varying current of adequate intensity must be applied. Potential difference (PD) is being formed when current flows in plasma membrane of nerve fibers and resistance lies in series with other tissues. The membrane nearer to cathode will be negative and is denoted by ’n’, whereas, surface nearer to anode will be positive and is denoted as ‘p’.
Increase in PD occurs on the nerves nearer to anode, whereas the PD decreases in the membrane nearer to the cathode because of opposite polarity. When the membrane becomes permeable to Na+ ions by fall in PD to a certain level, then the ions enter the axon and initiate the nerve impulse. When cathode is applied to superficial nerve then nearest side will get activated but the anode can only initiate the nerve impulse.
Therefore, further aspect of anode is activated. Due to this, the density of current is less in further aspect of nerve fiber than near one. So for initiating impulse cathode is more effective than anode. In some apparatus, polarity of terminals is marked which is beneficial for high peak of current for effective stimulus. To get contraction of innervated muscle in less current, cathode should be connected to active electrode.
Accommodation .
When a constant current flows, the nerve adapts itself. This phenomenon is known as accommodation.
Effect of Frequency of Stimulation.
The muscle responds with a large contraction and then rotates to its resting state. It is called twitch contraction. When single stimulus is applied per second then there is contraction followed by immediate relaxation. Increase in the frequency of stimuli up to 20 Hz shortens
the period of relaxation.
If stimuli are given more than 20 Hz then there is no time for complete relaxation between the contraction and another impulse. At more than 60 Hz, there is no relaxation at all and current flows smoothly leading to tetanic contraction.
Strength of Contraction .
Strength of Contraction depends on:
1. Quantity of motor nerve activated.
2. Rate of change of current.
If intensity of current rises suddenly, less intensity is required for muscle contraction as there is no time for accommodation but if current rises slowly greater intensity is required as in trapezoidal, triangular current, etc.
Current Modulation Definition .
The current modulation is an important phenomenon because the physiologic response to the various waveforms depends largely on current modulation. Modulation refers to any alteration in the magnitude or any variation in duration of these pulses. Modulation may be continuous, interrupted, burst, or ramped. According to various treatment goals the parameters of current modulation must be established.
Continuous Modulation.
Continuous modulation means that the amplitude of current flow remains the same for several seconds or minutes. Continuous modulation is usually associated with long-pulse duration direct current . With direct current, flow is always in a uniform direction. The positive and negative accumulation of charged ions over a period of time creates either an acidic or alkaline environment that may be of therapeutic value. This therapeutic technique has been referred to as medical galvanism.
The technique of iontophoresis also uses continuous direct current to drive ions into the tissues. If the amplitude is great enough to produce a muscle contraction, the contraction will occur only when the current flow is turned on or off. Thus with direct current continuous modulation, there will be a muscle contraction both when the current is turned on and when it is turned off. Continuous modulation is also used with alternating current primarily to elicit muscle contractions.
Interrupted Modulation .
In interrupted modulation, current flows for some period of time called the on-time, and is then periodically turned off during the off-time. On-time and off-time can be prefixed in some devices or can be altered by the operator. Interrupted modulation is used with monophasic as well as for biphasic currents. Currents with sine, rectangular, or triangular-shaped waveforms may be interrupted. Interrupted modulation is used clinically for muscle reeducation and strengthening and for improving range of motion .
Burst Modulation .
Burst modulation occurs when pulsed current flows for a short duration and then is turned off for a short duration and in a repetitive cycle. With polyphasic current, sets of pulses are combined. These combined pulses are most commonly referred to as bursts. These are also called pulse packets, envelopes, pulse trains, or beats .
The interruptions between individual bursts are called interburst intervals. The interburst interval may be too short to have any effect on a muscle contraction. Thus, the physiologic effects of a burst of pulses will be the same as with a single pulse. Bursts may be used with monophasic and biphasic currents as well.
Ramping Modulation .
In ramping modulation which is also called sometimes as surging modulation, current amplitude increases gradually or decreases gradually in its intensity. It is also called ramping-up or ramping-down of current modulation . Ramp-up time is usually preset at about one-third of the on-time.
The ramp-down option is not available on all machines. This type of modulation gives the patient a very comfortable feeling because of the very gradual rise of intensity of the current. Ramping modulation is used clinically to elicit muscle contraction and is generally considered to be a very comfortable type of current.
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