Medium frequency currents Definition .
Medium frequency currents are therapeutically used currents whose frequency is in the range of 1,000 to 10,000 cycles per second. They are basically used to stimulate deeply-situated muscles and nerves, since it is difficult to stimulate these muscles and nerves by low frequency currents without the spread of current.
Clinically, they are used for muscle re-education, to retard the rate of muscle atrophy and for pain relief. Various currents in this category, which are used for physiotherapeutic purpose are Russian current, Interferential current, etc. Sine wave current, in the frequency range of 2000 to 5000 Hz, modulated to produce physiological response in nerves are called the medium frequency currents.
The basic advantage of medium frequency stimulation over conventional surged faradic current, is its ability to produce the strong physiological effects of low frequency electrical stimulation, in much deeper muscle and nerve tissues, without the associated painful and unpleasant sensation of low frequency stimulation. To produce low frequency effects at sufficient intensity at depth, most patients experience considerable discomfort in the superficial tissues (i.e. the skin). This is due to the resistance (impedance) of the skin being inversely proportional to the frequency of the stimulation.
In other words, the lower the stimulation frequency, the greater the resistance to the passage of the current and so, more discomfort is experienced. The skin impedance at 50 Hz is approximately 3200 ohms whilst at 4000 Hz it is reduced to approximately 40 ohms. The result of applying such medium frequency current is that it will pass more easily through the skin, requiring less electrical energy to reach the deeper tissues, therefore producing less discomfort.
Types of Medium Frequency Current .
Medium frequency current can be broadly categorized in to two forms:
1. Medium frequency surge current.
2. Interferential current.
Russian Current .
It is a polyphasic sine wave continuous current having a basic or carrier frequency of 2500 Hz. The current is frequency modulated to produce a train of pulses with a pulse duration of 10 ms and a pulse interval of 10 to 50 ms. Such frequency modulation produces 50 to 10 pulses in one second, each pulse lasting for 10 ms. It is applied in bi-polar mode, usually with carbon rubber or vacuum electrodes. It is effective in muscle strengthening and for relief of muscle spasm.
Russian current is a medium frequency alternating current with a frequency of 2.5 KHz; burst modulation of 50 Hz with a duty cycle fifty percent. Russian forms of electrical stimulation became popular to a large extent as a result of the activities of Dr Yakov Kots, who claimed force gains of up to 40 percent in elite athletes as a result of what was then a new form of stimulation. The 10/50/10 stimulation protocol suggested is optimal. Whereas, the 10/50/10 regimen is in the form of stimulus. 10 sec/on period, 50 sec/off period follows stimulus.
Treatment time is 10 minutes and when applied once daily over a period of weeks surely increases strength. Dr Yakov Kots argued that increasing a muscle force generating capability can be achieved by two means: learning and central nervous system (CNS) adaptation and increase in physical bulk of the muscle. Greater maximal volitional contraction is produced by CNS learning and adaptation of the pattern of excitation.
In this case, force gains are achieved by greater and more effective recruitment of muscle fibers. Increase in physical bulk of the muscle produces a greater force output for the same neural input. In this case, the muscle fiber grows in size and muscle volume increases. The increase in limb circumference and thus by inference, muscle bulk parallels the increase in muscle force, so the Russian author concluded that the force gains were predominantly peripheral in origin.
Method of Application .
For muscles strengthening, the intensity is adjusted to produce strong titanic muscle contraction, using a pulse rate of 50 to 70 pulses per second with pulse duration of 150 to 200 microseconds. The current is applied during volitional activities like isometric contractions in different ROM, slow speed isokinetic and short arc isotonic movements. Primary effects are to build up muscle power delivery in different range of motion or mobilize stiff joints.
The stimulation is applied to produce contraction for 15 seconds and relaxation for 50 seconds. For relief of muscle spasm, titanic contraction is produced to the limit of tolerance a pulse rate of 50 to 70 pulses per second, with pulse duration of 50 to 170 microseconds. The stimulation is applied to provide brief isometric contractions for 5 to 12 seconds and 8 to 15 seconds of relaxation. Physical Principles of Light
Medium Frequency Surge Current .
It is a polyphasic sine wave continuous current having a basic or carrier frequency of 4000 Hz. It is more useful for relief of pain and improving circulation in muscles. Though it causes significant muscle contraction, it is less powerful than produced with Russian current. Medium frequency surge current is a two pole interference current, which increases and decreases in intensity over a set duration and interval, like the surged faradic current. It is far well-tolerated by the patient than faradic stimulation and the etching effect of IG stimulation are not seen.
It is available in two formats;
1. Russian current with carrier frequency of 2500 Hz.
2. MF surge current with carrier frequency of 4000 Hz.
Interferential Current .
Interferential current utilizes two amplitude modulated medium frequency sinusoidal currents; in the frequency range of 4000 to 5000 Hz. These are called the carrier currents. The carrier current in both channels has the same amplitude but the frequencies of are kept slightly out of synchronization. The carrier currents are passed through the tissues simultaneously, so that their paths cross and interfere with each other’s field deep within the tissues.
This interference gives rise to amplitude modulated frequency, like the strings of a sitar, to produce a pulsing effect or beat; wherever they cross each other. When the two carrier waves are in phase, constructive interference takes place. The resultant beat frequency has an amplitude (intensity) that is the sum total of the amplitudes of the carrier currents when the carrier waves are 180 degree out of phase, destructive interference cancels out the carrier amplitudes. The resultant beat frequency has amplitude (intensity) of zero.
Beat Frequency of Interferential Current .
The beat frequency current has the characteristics of low frequency stimulation. The exact frequency of the beat frequency can be controlled by the input frequencies. If one carrier current is at 4000 Hz and its companion current at 3900 Hz, the resultant beat frequency would be the difference of the two carrier frequencies i.e. 100Hz, spreading in a typical clover leaf pattern . The amplitude modulated beat frequency may be constant or variable. Constant beat current is obtained when both the carrier frequencies remain fixed.
Variable beat current is obtained when one carrier remains fixed and the other keeps on changing in frequency at regular intervals from a lower to a higher level and back, known as sweep. This produces a spectrum of frequencies in the beat current at regular sweep. Such sweep prevents accommodation of nerves. By careful manipulation of the input currents it is possible to achieve any beat frequency to use clinically. Classical interferential (static) field is generated when the beat current remains constant. Vector current (dynamic) field is produced when the interferential field rotates by 45 in clockwise or anticlockwise direction within the tissue, constantly changing the stimulation zone.
Modern machines usually offer frequencies of 1 to 150 Hz, though some offer a choice of up to 250 Hz or more. To a greater extent, the therapist does not have to concern themselves with the input frequencies, but simply with the appropriate beat frequency, which is selected directly from the machine. In 2 pole interferential stimulation, where there is clearly no interference within the body, is made possible by electronic manipulation of the currents, i.e. the interference occurs within the machine. This is suitable for small areas like sinus, temporo-mandibular joints etc.
Physiological Effects of Interferential Current .
Interferential Current as a therapy has been used for the Past decades ; the Physiological effect of Interferential current are not established enough to explain the analgesic effect . Goats in 1990 reported the interferential current as a therapy decrease the stimulation of cutaneous sensory nerves close to the electrodes while increasing the effect on deep tissues.
[1]. Gate Control Theory.
It has been suggested that an IFC frequency of 100Hz can activate the large diameter . low threshold nerve fibers and that It uses the Pain gating system to achieve analgesia.
[2]. Descending pain suppression pathway.
It involves endogenous opioid (endorphins, dynorphins and enkephalins) release from periaqueductal grey matter (PAG) and the rostral ventral medulla (RVM) (nucleus raphe magnus (NRM), reticular nuclei and the spinal dorsal horn). These endogenous opioids play an important role in the control of nociceptive messages from primary afferent nerves.
A pulse duration range of 100–200 µs may activate large-diameter fibers, once their threshold is lower than that of the small-diameter A-delta and C fibers. Most IFC devices have a fixed pulse duration of 125 µs. However, it is not clear how IFC therapy can selectively activate the different nerve fiber types.
[3]. Physiological blockage (Wedensky inhibition).
C and A δ fibers may not conduct nociceptive impulses after frequency stimuli greater than approximately 15 Hz and 40 Hz, respectively.
[4]. Placebo effect.
Refers to the patient–therapist relationship and types of modalities used during treatment. IFC devices are technically advanced and visually impressive, which may help convince patients that they are receiving an effective treatment.
- Sympathetic nerve: 1 to 5 Hz.
- Parasympathetic nerve: 10 to 150 Hz.
- Motor nerve: 10 to 50 Hz.
- Sensory nerve: 90 to 100 Hz.
- Nociceptive fibres: 90 to 150 Hz.
- Smooth muscle: 0 to 10 Hz.
Therapeutic Effects of Interferential Current .
The clinical application of IFT therapy is based on response threshold and the physiological behaviour of stimulated tissues. Selection of a wide treatment band can be considered less efficient than a smaller selective band because by treating with a frequency range of say 1 to 100 Hz, the appropriate treatment frequencies can be covered, but only for a relatively small percentage of the total treatment time. Additionally, some parts of the range might be counterproductive for the primary aims of the treatment.
Main clinical applications of IFT are
1. Pain relief.
2. Muscle stimulation.
3. Increased blood flow .
4. Reduction of oedema.
5. Tissue healing and repair.
Since IFT acts primarily on the excitable tissues like nerves and muscles, the strongest effects are likely to be those produced by such stimulation, i.e. pain relief and muscle contraction. The other effects like drainage of fluid and reduction in muscle spasm are secondary consequences of the primary effects.
Instrumentation of IFT .
Most modern IFT units allow the therapist to get tailor made current, suitable to treat a specific disorder, which may be built in to the memory of the software based equipments or by adjustment of following machine parameters:
1. Amplitude modulation parameter (AMF).
Amplitude modulation parameter (AMF), to choose the basic value of the low frequency modulation that is desired.
2. Spectrum parameter.
Spectrum parameter, to set the range of variation in the AMF value that is desired; setting the AMF at 100 Hz and spectrum at 50 Hz will give an AMF variation from 100 Hz upto150 Hz and back to 100 Hz. The spectrum is useful in preventing accommodation in nerves .
3. Sweep time parameter.
Sweep time parameter sets the time period for the AMF to change from base to peak frequency. Faster the sweep less painful is the stimulation. However, if strong muscle contraction or sensory input is desired, then the sweep must be slow, to ensure aggressive stimulation.
4. Contour parameter .
Contour parameter sets the rate of change of the AMF from base to peak frequency. This is expressed in percentage of time taken to reach from base to peak of AMF. Greater the percentage, the gentler is the stimulation.
5. Rotation parameter .
Rotation parameter is applicable in case of vector currents only and sets the rate of rotation and the direction of change of the AMF field within the tissues.
Treatment Parameters to Achieve Pain Relief with IFT .
Electrical stimulation for pain relief has widespread clinical use. Direct research evidence for the use of IFT in pain relief is limited. One could use the higher frequencies (90-150 Hz) to stimulate the pain gate mechanisms and thereby mask the pain symptoms for the duration of application. Alternatively, stimulation with lower frequencies (1-5 Hz) can be used to activate the release of indigenous opiates, providing long-term relief of pain.
These two different modes of action can be explained physiologically. Each has different latent periods and varying duration of effect. Relief of pain may be achieved by stimulation of the reticular formation at frequencies of 10 to 25 Hz or by blocking C fibre transmission at frequencies greater than 50 Hz.
Treatment Parameters to Achieve Muscle Stimulation with IFT .
Stimulation of the motor nerves can be achieved with a wide range of frequencies. Stimulation at low frequency (e.g. 1 Hz) will result in a series of twitches. Stimulation at 50 Hz will result in a titanic contraction. The choice of treatment parameters will depend on the desired effect. To combine muscle stimulation with an increase in blood flow and a possible reduction in oedema, selecting a frequency range which does not produce strong sustained titanic muscle contraction.
In such cases, a sweep of 10 to 25 Hz is often used, to produce pumping effect on the target muscles, which will help in drainage of fluid from the interstitial space. There is no primary nervous control of oedema re-absorption and the direct electrical stimulation of blood flow is limited in its effectiveness. It is suggested therefore, that in order to achieve these effects, suitable combinations of muscle stimulation should be made.
Treatment Techniques for IFT .
Preparation of the patient and the machine is done, as before any low frequency electrical stimulation The same local precautions, general contraindications, environment and the safety considerations apply for IFT, as in case of low frequency stimulation. The IFT is usually applied through four carbon rubber electrode plates between 5 and 15 sq cm in size.
These are applied on the prepared skin, with a coating of conductive electrode gel and fixed with elastic strap. Some IFT machines have built-in or optional vacuum suction pumps, for application of vacuum cup electrodes, with moist sponge fillers to maintain electrical contact with the skin. Fixing such electrodes is easy, particularly over odd-shaped areas like the shoulder joint. Electrode positioning should ensure adequate coverage of the area for stimulation .
In some circumstances, a bipolar method is preferable if a longitudinal zone requires stimulation rather than an isolated tissue area. Placement of the electrodes should be such that a crossover effect is achieved in the desired area. If the electrodes are not placed so that a crossover is achieved, the physiological effects of I/F cannot be achieved. Nerves will accommodate to a constant signal and, A sweep (or gradually changing frequency) is often used to overcome this problem (as well as generating a range of effects).
The sweep (range) should be appropriate to the desired physiological effects, though again it is suggested that an excessive range may minimize the clinical effect. The mode of delivery of the selected sweep varies with machines. The most common application is the 6 second rise and fall between the pre-set frequencies. For example, if a 10 to 25 Hz range has been selected, the machine will deliver a changing frequency, starting at 10 Hz, rising to 25 Hz over a 6 second period. Once this upper limit has been achieved, the frequency will once again fall, over a 6 second period to its starting point at 10 Hz. This pattern is repeated throughout the treatment session . Physical Principles of Light
Treatment times vary widely according to the usual clinical parameters of acute/chronic conditions and the type of physiological effect desired. In acute conditions, shorter treatment times of 5 to 10 minutes may be sufficient to achieve the effect. In other circumstances, it may be necessary to stimulate the tissues for 20 to 30 minutes. It is suggested that short treatment times are initially adopted especially with the acute case in case of symptom exacerbation. These can be progressed if the aim has not been achieved and no untoward side effects have been produced. There is no research evidence to support the continuous progression of a treatment dose in order to increase or maintain its effect.
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