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Perspectives |
AR Ward, PhD, is Associate Professor, Musculoskeletal Research Centre, Faculty of Health Sciences, La Trobe University, Victoria 3086, Australia. Address for correspondence: School of Human Biosciences, Faculty of Health Sciences, La Trobe University, Victoria 3086, Australia.
Address all correspondence to Dr Ward at: a.ward{at}latrobe.edu.au
Submitted February 24, 2008;
Accepted November 8, 2008
 | Abstract |
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| Introduction |
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 TopAbstract Introduction BMAC Stimulus ParametersBMAC Stimulation Types Used...Importance of ModulationThe "Conventional Wisdom"Discussion--The Known...ConclusionReferences |
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The response of nerve and muscle to PC electrical stimulation has been studied by physiologists since the late 19th century.4,5 Consequently, our present understanding of the effects of PC is relatively good. The physiological response to BMAC stimulation is less-well understood.
This article reviews the known physiology and clinically relevant, laboratory-based studies of electrical stimulation, which offer some insight into the mechanism of action of BMAC and provide some answers to the questions "Does BMAC stimulation have an advantage over PC?" and "What are the optimum treatment parameters for BMAC stimulation?"
| BMAC Stimulus Parameters |
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TopAbstractIntroductionBMAC Stimulus ParametersBMAC Stimulation Types Used...Importance of ModulationThe "Conventional Wisdom"Discussion--The Known...ConclusionReferences |
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Pulsed current is easily described by specifying 3 things: (1) the waveform (eg, rectangular and monophasic, as in Fig. 1B), (2) the pulse duration (normally in the range of 50 microseconds to 1 millisecond), and (3) the pulse frequency (normally in the range of 1 Hz to about 100 Hz).
The description of AC is more complex. Alternating current, by definition, is biphasic, and the biphasic waveform can be sinusoidal or rectangular. The current also can be delivered continuously (Fig. 1A), in rectangular bursts (Fig. 1C), or in sinusoidally modulated bursts (Fig. 1D). Thus, when describing the stimulus, there is the potential for confusion because several parameters must be specified to completely describe the waveform. Figure 2 shows an example of BMAC, with particular parameters identified.
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The greater the number of parameters, the greater the number of possible permutations and combinations. This raises the question of whether AC stimulators used clinically have the best combination of parameters for achieving the desired clinical outcome.
| BMAC Stimulation Types Used Clinically |
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TopAbstractIntroductionBMAC Stimulus ParametersBMAC Stimulation Types Used...Importance of ModulationThe "Conventional Wisdom"Discussion--The Known...ConclusionReferences |
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Russian currents became popular despite an equivocal evidence base due to the limited number of studies and their different findings.3,9 The balance of evidence supports the notion that strengthening can be produced, but at one extreme there is the single-case study reported by Delitto et al,10 which demonstrated a substantial strength gain, whereas at the other extreme there is the study by St Pierre et al,11 which demonstrated no strength gain. Other than the original Russian study,8,9 only 2 subsequent studies have addressed whether 2.5 kHz is the best AC frequency for muscle torque production.12,13 These 2 studies used 50-Hz bursts of kilohertz-frequency AC, and both studies showed that maximum torque was elicited at a frequency of 1 kHz. It is noteworthy that Andrianova et al8 reported that 2.5 kHz is optimum if stimulation is applied directly (over the muscle) but that if stimulation is applied indirectly (over the nerve trunk), the optimum frequency is 1 kHz. Thus, it might be concluded that "optimal stimulus parameters" may well depend on electrode positioning and that the popular frequency (2.5 kHz) could be suboptimal for commonly used electrode placements.
Interferential Currents
Interferential currents are reported to be the most popular form of electrical stimulation used in clinical practice in the United Kingdom and other European countries and in Australia.1 Interferential stimulators produce 2 independent kilohertz-frequency AC currents of constant intensity (Fig. 1A) applied by 2 separate pairs of electrodes, which are positioned diagonally opposed to produce an "interference" effect (Fig. 1D) in the central region of intersection of the currents (Fig. 3).1,2,7
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Premodulated Interferential Current
Most interferential stimulators also offer premodulated interferential current. The term "premodulated interferential" is something of a misnomer because it refers to current that is fully modulated (as in Fig. 1D) and applied between one pair of electrodes. Thus, by definition, this current is no longer interferential (ie, no longer produced by the interference of 2 currents). The current is simply kilohertz-frequency AC, modulated at a low frequency, typically in the range of 1 to 120 Hz.1,2,7 Unlike "true" interferential current, the amount of modulation of the stimulation waveform does not depend on the nerve fiber orientation relative to the electrodes. The stimulus waveform is simply that provided by the stimulator and, therefore, is predictable.
If the "premodulated" current is sinusoidally modulated (as produced by traditional interferential stimulators and shown in Fig. 1D), some parts of the burst will be below threshold while other parts of the burst will be above threshold. Thus, the effective burst duration for any given nerve fiber is uncertain and will vary with stimulation intensity, which varies with proximity to the electrodes. Nerve fibers close to the electrodes will be stimulated suprathreshold for a larger part of each burst than those further away; thus, the effective burst duration will vary. Some modern interferential stimulators use rectangular burst modulation (Fig. 1C), so there is no uncertainty as to the effective duration: the burst is either fully "on" or "off."
| Importance of Modulation |
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TopAbstractIntroductionBMAC Stimulus ParametersBMAC Stimulation Types Used...Importance of ModulationThe "Conventional Wisdom"Discussion--The Known...ConclusionReferences |
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The first published report of the effect of modulation appears to be the report by Soloviev published in 1963.16 Soloviev used AC stimulation over the frequency range of 2 to 8 kHz and found that there was little difference in motor threshold regardless of whether the current applied was continuous or burst modulated at 50 Hz with a 50% duty cycle. A 2001 study of motor thresholds by Ward and Robertson17 again showed little difference, this time over the frequency range of 1 to 25 kHz. It should be noted, however, that only continuous AC and 50% duty cycle, 50-Hz bursts were compared, so the comparison was between 10-millisecond bursts and continuous AC.
In 2007, Ward and Lucas-Toumbourou18 reported a study of sensory, motor, and pain thresholds using AC frequencies of 1 kHz and 4 kHz applied as 50-Hz bursts. They used burst durations in the range of 0.25 to 20 milliseconds and found that thresholds decreased to a plateau with increasing burst duration. An interesting finding of this study was that the plateau in threshold with burst duration depended on the response evoked (ie, sensory, motor, or pain threshold, with values of 5 to 7, 10, and >20 milliseconds, respectively). Thus, motor thresholds decrease with increasing burst duration, but at burst durations above about 10 milliseconds, there is no further decrease. These findings explain the lack of differences found in the earlier studies, when only 10-millisecond bursts and continuous AC were compared. The most intriguing finding of the study, however, was that the burst duration plateaus were different for sensory, motor, and pain thresholds. This means that there will be optimal burst durations where the pain/sensory threshold and pain/motor threshold ratios are maximum. Ward and Lucas-Toumbourou estimated an optimal burst duration for both sensory and motor stimulation as 2 to 3 milliseconds. This is appreciably shorter than the burst durations commonly used clinically (typically 10 milliseconds for Russian current and greater or similar for interferential currents).
Effect of Burst Duration on MEIT and Discomfort
Andrianova et al8 used different AC frequencies in the range of 100 Hz to 5 kHz and compared not thresholds but maximum torque production using continuous (unmodulated) AC and AC bursts (modulated at 50 Hz with a 50% duty cycle [ie, 10-millisecond burst duration]). They concluded that there was little difference in MEIT with burst-mode or continuous stimulation, but they did not make any statistical comparisons. Their published data (reproduced in Tab. 3 of the perspective article by Ward and Shkuratova9) however, show that across the frequency range, the torques produced by burst-modulated currents were, on average, 14% higher (SD=12%). Ward and Shkuratova9 conducted a paired t-test comparison across frequencies, using Andrianova and colleagues' published data,8 and found that this difference is significant (P=.03) (ie, torques are significantly higher when a rectangular burst-modulated stimulus of 10 milliseconds' duration is used rather than a continuous AC stimulus).
Bankov,19 in 1980, compared 5-kHz AC, modulated at 60 Hz, using stimulation intensities that produced just enough contraction of the biceps brachii muscle to maintain the elbow at 90 degrees of flexion with the upper arm vertical (an antigravity flexion level of muscle activity). He compared rectangular bursts of 1, 2, and 5 milliseconds' duration and reported that the 1-millisecond burst was the most comfortable. Another study reported by Bankov in the same year20 compared 60-Hz sinusoidally modulated bursts of AC, which varied in their modulation depth from 0% (steady, continuous AC; Fig. 1A) to 100% (fully modulated; Fig. 1D), and hypermodulated bursts of AC (gaps between bursts). He reported that force increased with the degree of modulation but that the associated discomfort showed little variation. A conclusion is that shorter burst durations produce more force at the same level of discomfort. In 1981, Bankov and Daskalov21 compared 5-kHz AC applied in 2-millisecond bursts with PC of varying pulse widths. Each was applied 3 seconds on and 3 seconds off at an intensity that produced antigravity flexion of the biceps muscle. The 5-kHz stimulus was found to be more comfortable. These early studies, thus, had 2 major findings: (1) that for a given level of force production, burst-modulated AC is preferable to continuous AC or PC, and (2) a short AC burst duration (1 or 2 milliseconds) is optimal for least discomfort.
A recent study13 measured MEIT and relative discomfort using 50-Hz bursts of AC in the frequency range of 0.5 to 20 kHz. Burst durations ranging from the shortest possible (1 cycle) to the longest (continuous AC) were used. Maximum torque was produced at a frequency of 1 kHz and a burst duration of 2 milliseconds (10% duty cycle). Minimum discomfort occurred at a frequency of 4 kHz and a burst duration of 4 milliseconds (20% duty cycle). Continuous AC produced the least torque and the greatest discomfort at all frequencies. Single cycles (biphasic PC) produced significantly less torque than 2-millisecond bursts and were more uncomfortable. A later study22 compared Russian current (2.5-kHz AC applied in 10-millisecond bursts) and "Aussie current" (1-kHz AC applied in 4-millisecond bursts) with PC of the same phase duration (200 and 500 microseconds, respectively) in terms of discomfort and torque production. The AC bursts (Fig. 1C) were more comfortable than their PC counterparts. Both Aussie current and the 2 forms of PC produced similarly high torques, but, perhaps surprisingly, Russian current evoked less.
Thus, it seems reasonable to conclude that a stimulus waveform that consists of kilohertz-frequency AC in short-duration bursts (2–4 milliseconds) is more comfortable and elicits greater MEIT than PC, continuous AC, Russian current, or interferential current stimulation.
| The "Conventional Wisdom" |
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TopAbstractIntroductionBMAC Stimulus ParametersBMAC Stimulation Types Used...Importance of ModulationThe "Conventional Wisdom"Discussion--The Known...ConclusionReferences |
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Nemec23–26 gave 3 arguments for the use of interferential current rather than PC:
The first point is incorrect for 2 reasons. First, the skin impedance to PC depends on the phase duration, not the pulse frequency.1,2,5,27–29 The skin impedance to low-frequency AC is much higher than to kilohertz-frequency AC because the phase duration is much longer. If the PC has the same phase duration as the kilohertz-frequency AC, the skin impedance is the same even if the pulse frequency is low.1,2,5,27–29 Conventional PC typically has a phase duration similar to that of interferential current. Thus, the argument that interferential current would meet with a lower impedance is without any basis. Second, a lower skin impedance does not mean less stimulation of sensory and pain fibers in the skin and, therefore, less discomfort. The high skin impedance with long phase durations (eg, with low-frequency AC) is due to the skin capacitance, which is due almost entirely to the stratum corneum: the dead, scaly, relatively dry, outermost layer.1,2,5,27,28 The stratum corneum has no sensory, pain, or other kind of nerve fibers.30,31 These fibers are located beneath, in the dermis, which is well hydrated and of similar conductivity to the deeper tissues.5,30,31
The second and third points are oversimplifications. There are 3 important things to consider with interferential stimulation:
Thus, the depth efficiency claims for interferential current are not substantiated. This, together with the uncertain degree of modulation of the stimulus, calls into question whether the "interference" effect of interferential current is of any value. Ozcan et al32 addressed this question when they assessed the relative discomfort of true and premodulated interferential currents (delivered in 50-Hz bursts, 10 milliseconds on and 10 milliseconds off). Premodulated interferential current was found to be significantly more comfortable than true interferential current and more effective for muscle contraction.
Historical Claims Concerning Russian Current
A talk given by Kots,33 of the Central Institute of Physical Culture, Moscow, at a conference hosted by Concordia University, Montreal, in 1977 laid the foundation for what became known in the Western world as Russian current electrical stimulation.9 Kots reported strength gains of up to 40% in elite Russian athletes stimulated with 2.5-kHz AC applied in 10-millisecond rectangular bursts at a frequency of 50 Hz. His protocol used currents with a 10-second on period followed by a 50-second rest period, applied 10 times in each stimulation session (ie, 10-minute treatment sessions). Treatment was applied daily over a period of weeks.
As noted previously, Russian currents became popular despite an equivocal evidence base due to the limited number of studies and their different findings.3,9 The choice, by Kots' group, of 10-millisecond bursts (50% duty cycle) was because of their observation that it evoked just as much muscle torque as continuous AC but, because of the burst modulation, the average current applied to tissue was halved. The effect of different burst durations was not explored. Bankov19,20 and Bankov and Daskalov,21 in the 1980s, examined the effect of burst duration and found that, for the same level of force production, short-duration bursts are more comfortable. An inference is that greater levels of force would be produced at the same level of discomfort if short-duration bursts were used. This is supported by the recent work of Ward et al,13 who measured torque at the pain tolerance limit and found that the greatest MEIT is produced using 2-millisecond bursts of AC with a frequency of 1 kHz.
Thus, the rationale for the clinical use of Russian current is called into question. The evidence is that stimulation with short-duration bursts of kilohertz-frequency AC would be preferable and that a burst duration of 2 milliseconds appears to be optimal for torque production.
| Discussion—The Known Electrophysiology |
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TopAbstractIntroductionBMAC Stimulus ParametersBMAC Stimulation Types Used...Importance of ModulationThe "Conventional Wisdom"Discussion--The Known...ConclusionReferences |
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Summation
With kilohertz-frequency AC stimulation, there is the possibility of summation, a phenomenon first described by Gildemeister.34,35 Gildemeister reported that when bursts of kilohertz-frequency AC are applied transcutaneously, the threshold voltage for sensory nerve excitation decreases as the burst duration is increased. This phenomenon, later called the "Gildemeister Effect," occurs because, with each successive pulse in the AC wave-train, the nerve fiber membrane is pushed closer to threshold. Membrane threshold is reached when successive pulses result in sufficient depolarization to produce an action potential. Gildemeister observed a limit to the summation effect. As the number of cycles per burst was increased, the threshold decreased, but only up to a point. Beyond a certain burst duration, no further decrease in threshold was observed. He called this maximum burst duration (ie, time over which pulses could summate) the "Nutzzeit" or "utilization time."
As noted previously, a recent study by Ward and Lucas-Toumbourou18 showed that the apparent utilization time was different for sensory, motor, and pain thresholds and, consequently, that relative thresholds (pain/motor and pain/sensory) vary with burst duration. These authors found that optimum discrimination (biggest separation between thresholds [ie, maximum relative thresholds]) occurred at burst durations of 2 to 4 milliseconds.
High-Frequency Fatigue
When electrical stimulation is applied to elicit a motor response using PC frequencies higher than physiological or at the high end of the physiological range (ie, greater than about 50 Hz), it is possible to produce a blockage of muscle activity due to propagation failure or neurotransmitter depletion.36–38 This is responsible for the phenomenon of "high-frequency fatigue,"38,39 which is characterized by its associated rapid recovery. If a stimulus frequency of 80 Hz, for instance, is used to elicit muscle contraction, the resulting muscle force declines rapidly, but if a brief rest period (a few seconds) is allowed, marked recovery occurs.38,39 This is quite different from "low-frequency fatigue," which is much more akin to normal physiological fatigue, where the force decline is much slower and the recovery time is much longer.
One form of high-frequency fatigue, propagation failure, can occur when action potentials are induced in motoneurons at sufficiently high frequency. This can result in action potential failure at branch points where a motor nerve divides to innervate individual muscle fibers. Failure also can occur at the neuromuscular junction because neurotransmitter depletion is possible at relatively high stimulation frequencies.40 Beyond the neuromuscular junction, transmission failure can occur at the level of the t-tubule system. Normally, the wave of depolarization of a muscle fiber action potential is transmitted over the muscle fiber membrane and throughout the t-tubule system, activating the contractile elements. When sufficiently high frequency action potentials are induced, the t-tubule membranes do not have time to recover between action potentials and muscle fiber contraction ceases.39,40 Whichever the mechanism, whether propagation failure or neurotransmitter depletion, a blockage of muscle contraction at stimulation frequencies around and above about 50 Hz is the result, and the effect is described as high-frequency fatigue.
Summation and Multiple Firing
When the stimulus is PC applied at low frequency (less than 100 Hz), it can be confidently concluded that, provided that the pulse intensity is sufficiently above threshold, the nerve fiber firing frequency will equal the pulse frequency. The firing frequency could be less if successive pulses occur within the relative refractory period and the stimulus intensity is not sufficiently high, but the firing frequency could never be higher than the PC frequency. With bursts of AC, however, there is the possibility that a single burst will result in multiple action potentials as a result of summation41–44; therefore, the firing frequency could be some multiple of the burst frequency. If the first few pulses in a burst summate, the nerve fiber could fire, go through a brief period of refractoriness, and then fire again. If this process happens rapidly and, therefore, is repeated during the burst, the nerve fiber firing frequency will be a multiple of the burst frequency. There is sound experimental evidence for this effect.41–45
A problem with multiple firing is that it could detract from the desired outcome. For example, a motoneuron firing frequency of 50 Hz might elicit an optimally forceful muscle contraction, so 50-Hz PC would be a good option. If long-duration 50-Hz bursts are used, however, the induced firing frequency could be a multiple of 50 Hz. This would initially result in a slightly greater muscle force, but the rate of fatigue would be higher. There also would be a greater amount of high-frequency fatigue. A recent study by Laufer and Elboim44 compared fatigue rates using 50-Hz bursts of 2.5-kHz AC with a burst duration of 10 milliseconds (Russian current), 50-Hz biphasic PC with the same phase duration (200 microseconds), 50-Hz bursts with a burst duration of 4 milliseconds, and 20-Hz bursts with a burst duration of 10 milliseconds. They reported that Russian current was the most rapidly fatiguing, PC was the least rapidly fatiguing, and the 2 currents of shorter burst duration were intermediate and equally fatiguing. A conclusion is that for motor stimulation using kilohertz-frequency AC bursts, if the duration is greater than 2 milliseconds, multiple firing is likely to occur and the fatigue rate will be compromised.
Neural Block
With kilohertz-frequency AC stimulation, another effect can be produced: direct conduction block of the nerve fiber. A direct observation of neural block was reported by Tanner,46 who measured compound action potentials produced in exposed sciatic nerve in response to direct, repetitive stimuli from a low-frequency pulse generator and found that neural activity could be blocked using a 20-kHz AC stimulus applied to the nerve trunk between the pulse generator and the recording electrodes. As the AC stimulus intensity was progressively increased, first the fast (large-diameter) fiber responses disappeared, followed by the slower (intermediate-diameter) fiber responses and then the slowest (small-diameter) fiber responses.
Bowman and McNeal45 examined the 
-motoneuron response to blocking signals in the frequency range of 100 Hz to 10 kHz. With high-intensity 2-kHz AC stimulation, they observed that following a brief period of firing at a very high rate (about 1 kHz), there was a progressive decrease in firing frequency, which occurred over a time frame of tens of seconds, after which activity ceased and complete conduction block occurred. At higher AC frequencies (4 kHz or more), the rate of decrease in activity was higher, with the firing frequency dropping to zero in less than a second and with stimulus intensities of 5 times the threshold. Bowman and McNeal concluded that neural block occurs more readily at multiples of threshold stimulation intensities and that the effects occur more rapidly at higher kilohertz frequencies.
Direct studies of neural block with AC stimulation, to date, have all used continuous AC. There do not appear to be any reported studies of the blocking effectiveness of burst-modulated AC, so it is not known to what extent neural block contributes to the effects observed. Indirect evidence for neural block was found by Ward and Robertson,17 who measured motor thresholds using continuous kilohertz-frequency AC, 50-Hz bursts, and single sine waves in the range of 1 to 25 kHz. Irregularities in the graphs of force versus stimulus intensity were consistent with multiple firing followed by nerve block. The effects were more pronounced at higher kHz frequencies and were greater with continuous stimulation than with 50-Hz bursts.
Whether neural block is of practical significance with electrical stimulation as used clinically thus remains uncertain, but it would affect MEIT, as -motoneurons are more susceptible to neural block than pain (A-
and C) fibers because of their larger diameter. This means that muscle force could be diminished without any diminution of pain sensation.
| Conclusion |
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TopAbstractIntroductionBMAC Stimulus ParametersBMAC Stimulation Types Used...Importance of ModulationThe "Conventional Wisdom"Discussion--The Known...ConclusionReferences |
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With kilohertz-frequency AC stimulation, summation and multiple firing, high-frequency fatigue, and neural block can potentially affect the neurophysiological response. The effects will vary, depending on the AC frequency and burst duration.
Both the historical evidence and more recent findings indicate that the stimulation parameters commonly used clinically (Russian and interferential currents) are suboptimal for achieving their stated goals and that greater benefit would be obtained using short-duration (2- to 4-millisecond) bursts of kilohertz-frequency AC, with a frequency chosen to maximize the desired outcome. For maximum muscle torque production, a frequency of 1 to 2.5 kHz is indicated, with a burst duration of 2 milliseconds or so. For minimal discomfort (but less muscle torque), a frequency of 4 kHz is indicated, with a burst duration of 4 milliseconds.
| References |
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TopAbstractIntroductionBMAC Stimulus ParametersBMAC Stimulation Types Used...Importance of ModulationThe "Conventional Wisdom"Discussion--The Known...ConclusionReferences |
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