MUSCLE STIMULATOR

Electronic muscle stimulation (EMS). SPORTS

EMS has been used in former Communist Block countries for sport training since the early 1950s, but Western countries only became aware of its use in 1973, when Dr. Y. Kots of the Central Institute of Physical Culture in the former USSR outlined the tremendous potential for strength enhancement beyond that which was possible by traditional (voluntary) training methods.

His claims raised many eyebrows and considerable effort was expended in an attempt to validate Kots' claims. Crude studies that pitted EMS-fired muscles against voluntarily contracted muscles (using Cybex machines for measurement) appeared to show that EMS wasn't as effective as Kots had claimed. However, the design of these studies was flawed due to a poor understanding of how EMS works on the neuromuscular system.

An EMS stimulus fires all the motor neurons in the treated area simultaneously, creating an uncoordinated contraction, which is primarily isometric in nature. Voluntary muscular contractions, on the other hand, roll through the muscle in a wave to generate a coordinated, directed force.

Kots was able to show, using a tensiometric device, that the muscle tension produced in a maximal EMS contraction can be up to 30% higher than a maximal voluntary contraction. This finding was corroborated by independent studies and makes intuitive sense, given the nature of the body's energy conservation system.

Since individual muscle fibers can be completely exhausted in just a few seconds, the body has adopted several strategies to prolong endurance. Slow twitch (red) fiber is used first in voluntary contractions, as it is energy efficient, though not very powerful. Then only enough strong, but voracious, fast twitch (white) fiber is added to handle the load.

In addition, muscles work their individual fibers in relays, always holding some back from even the most demanding load to maintain a reserve. Therefore, it's impossible to voluntarily contract all fibers simultaneously. The order of recruitment makes it likely that most of the fibers held in reserve will be white.

EMS works directly on the muscles, bypassing the body's energy conservation system, thus there's no limit to the percentage of fiber that can be activated. The EMS stimulus "spills over" from fully contracted fiber to activate remaining fiber (given sufficient current) allowing the athlete to experience a training stimulus that's unattainable by any other means.

The supra-maximal nature of this exercise enhances the strength to weight ratio by favoring enhanced recruitment over cross-sectional growth and also optimizes fiber splitting and the conversion of intermediate fiber to white fiber, the "Holy Grail" of power training.

Recruitment Velocity

Recruitment velocity is the rate at which a muscle fiber can achieve maximum tension, varying from 20 milliseconds for white fiber to 65 milliseconds for red fiber. Recruitment rates vary since red fiber gets a "head start" in voluntary contractions as white fiber is only added in as needed once the load has been determined.

EMS reverses the natural recruitment order, as its nonspecific current flows more easily through the bigger neuron of the white fiber (less resistance) forcing red and intermediate fibers to shorten their recruitment rates in response to white fiber recruitment, which now precedes rather than follows in the contraction.

The reversed recruitment order combined with the positive effects of high intensity make EMS ideal for improving recruitment velocity across all fiber types, a key factor in explosive events.

From Theoretical to Practical

The benefits of EMS have been discussed extensively in theory but the real challenge is the successful incorporation of EMS into a training program. There are four main uses for EMS in sport training. First, for the enhancement of maximum strength; second, as a means of recovery; third, as a rehabilitation tool; and fourth, as a motor learning and muscle recruitment tool.

Maximal Strength Enhancement

EMS is the single most intense strength building method and has the briefest improvement period of all training modalities. Kots' literature describes a maximum strength gain plateau after twenty-five treatments (which could be administered over four to seven weeks); however, in my experience, most of the benefits available were achieved within ten treatments and strength gains beyond fifteen treatments were negligible. And since ten to fifteen treatments maximize recruitment velocity, it seems logical to work between these numbers.

Long-Term Planning

Strength is the foundation for sport-specific tasks, therefore it must be established early, in both general and specific terms. Generally, strength improvement needs are very high in the early stages of a career and diminish through the years until the athlete fulfills his strength requirements and merely must maintain them (keep in mind that this point applies to non-strength training athletes).

Strength gains on the order of 25% per year, or even higher, may be required in the first few years, though the requirement drops rapidly until top international athletes factor in improvements of 6% per year or less. This leads to the question: why don't athletes continue to push their strength work to the limit throughout their careers?

High intensity training elements must compete for central nervous system energy. A novice sprinter can't tax the CNS significantly no matter how hard he tries, but as he improves, the CNS demand rises exponentially, even if the volume of sprinting remains constant. Therefore, the degree of intensification of other factors must be reduced over time if speed is to improve further.

As a result, EMS should be used for strength development as soon as fitness fundamentals are in place, with a diminishing role in routine strength enhancement as the career advances. A quadrennial plan for a top sprinter might include EMS strength building twice per year during years one and two, reducing to once during year three and only if needed in year four.

Special strength requirements, such as secondary hip extension by the hamstring, must be in place early to facilitate the correct technique needed for the development of top speed. These special strengths can be developed even before the athlete is fast enough or skilled enough to develop them through voluntary means. EMS also facilitates the optimal fiber-type ratio, which should be in place early to aid in performance over time.

Incorporation into the Training Plan

EMS strength training should coincide with maximal strength weight lifting. The two modalities are synergistic, though the introduction of EMS must be phased in to allow a smooth progression of the workload. Modern sprint training uses a triple-periodized annual plan, with three maximum strength phases, though only the first two include EMS. The third maximum strength phase is shorter, with a more moderate strength improvement goal.

In our case, the first two maximum strength weightlifting phases lasted seven weeks with a "313" loading system, that is, three weeks of high intensity lifting, followed by one week of medium intensity, followed by another three weeks of high intensity lifting to maximize adaptation.

Apply EMS work during the second and third weeks of each three week high intensity block. As our speed work, followed by lifting, occurred on Monday, Wednesday, and Friday, with speed endurance work on Saturday (Tuesday and Thursday were reserved for low intensity work, with Sunday off), we used EMS on Monday, Wednesday, and Friday, which gave us a total of twelve EMS sessions during the whole max strength phase.

This sequence allowed for the optimal number of EMS sessions in the phase with optimal recovery. (EMS doesn't require 48 hours for recovery, as it bypasses the central nervous system; however, this schedule optimized the recovery for the other training elements). The volume of explosive power and sprint work must increase seven to ten days after completion of the max strength/EMS phase for the optimal incorporation of the new abilities and to compensate for the drop in CNS stress.

When adding EMS to a program, expect your peak performance up to two weeks later than before, as you're now tapering from a much higher workload.

Selection of Muscle Groups

Maximum strength EMS is applied to the quads, hamstrings, glutes, and the erector spinae. These muscle groups play the main role in power development around the hip joint, where, at maximum speed, the power output is seven times higher than around any other joint. The abs play a major role also, and they can be treated as well, but their rotational movement and primary support, rather than power role, favor traditional high rep training. More on this later. The soles of the feet can even be treated in cases of insufficient foot strength.

Individual Sessions – Preparation

EMS works best as the last training element of the day, separated from other work by at least two hours. This is usually done at night before bed, as it can be done at home and the supra-maximal stimulus it provides is excellent for promoting the release of growth hormone during sleep.

Use a hot shower as warm up preparation, being careful to remove any oils or creams from the areas to be treated to ensure proper conductivity (oils left on the skin can cause the current to jump around the skin surface causing considerable discomfort). The increased blood flow in the muscles after the shower heats the muscle motor neurons, lowers electrical resistance, and makes them more receptive to EMS.

Start the EMS session with a gentle pulsing mode for three to five minutes to complete the warm-up before starting the maximal contractions. Warm down using the same pulsing method.

Contractions – Timing

Each muscle group is stimulated maximally for ten reps of ten seconds duration with a fifty second rest period between contractions. It's critical to maintain the rest periods as prescribed as this is the absolute minimum recovery time needed to maintain a maximal contraction on the next rep. A shortened rest period may, in fact, change the nature of the exercise so that it enhances the wrong fiber type.

Sprinters use the full ten second contraction time, though shot putters and linemen find that six seconds is about the longest they can maintain a maximal contraction. In either case, the same fifty second rest period must be maintained.

Procedure

Though modern stim equipment allows for a number of muscle groups to be stimulated simultaneously, never work more than two muscle groups at a time. The athlete must be able to determine where the stimulus is coming from. Keep the limbs straight and unsecured. Never stimulate antagonists at the same time for safety reasons. This also allows the athlete to concentrate on the contraction in isolation for learning reasons.

When stimulating the soles of the feet, have the athlete stand on the pads to prevent cramping. The athlete must always control the intensity of the contraction as the amount of current necessary for a maximal contraction varies widely between individuals depending on fiber type, fat distribution (fat is an insulator), muscle size, and injury history.

As a rule, sprinters require much less current to achieve a maximal contraction because their higher percentage of white fiber provides less resistance. The better the sprinter, the more this is so. EMS units have a "rise-time" feature (the time it takes to ramp up the contraction from zero to max) that is either preset or adjustable. Where it's adjustable, choose the shortest time the athlete can tolerate, usually half to three-fourths of a second.

Crank It Up

Most users never come near the level of contraction they need for best results, especially in clinical settings. To understand the intensity the athlete needs to experience, have him contract the quads as hard as he possibly can voluntarily, and then have him imagine a goal 30% higher than that! The contraction is massive, and it feels that way! Don't worry about "burning" the muscle though, as it takes only five-millionths of an amp to maximally contract the quad.

To give you an example of what I mean by cranking it up, my athletes would often have to bite down on a piece of leather or a stick while being "stimmed." Is it really that painful? Well, it should feel like riding up a very steep and long hill on a bike. That's the type of "burn" you should feel.

Pad Placement

Most EMS machines come with a series of electrode pads secured by Velcro straps. This is a very cumbersome and time-consuming arrangement that can be greatly improved with the purchase of after-market adhesive pads. 3M makes good ones. Choose pads that are four inches square and be sure to replace them when they lose their stickiness. (Really hairy guys may need to use contact gel with a traditional pad.)

When choosing pad placements, a lot of experimentation will be needed to find the most comfortable and effective setup, though four pads per large muscle group usually helps. Since the EMS contraction is always strongest around the negative pole, you should place the negative pad over the largest bulk of the muscle to keep the contraction even throughout the muscle.

When treating the quad muscles, keep the pads towards the outside part of the upper quads to keep the current from jumping over into the groin area unexpectedly. In a four pad setup, crossing the pairs of leads in an "X" pattern may help ensure a tolerable, but complete contraction.

Recovery with EMS: The Fallacy

The search for ever greater specificity, compounded by a basic misunderstanding of exercise itself, has driven athletes away from low intensity work, much to their detriment. For decades, exercise programs have been based on the false premise that exercise doesn't count unless it's carried out at 70% of maximum voluntary contractile force or lasts at least twenty minutes, the threshold for protealysis (the breakdown of proteins in the muscle).

Once considered to be the precursor of muscular development, it's now known to be a side effect to be avoided if possible. In fact, steroids eliminate proteolysis and no one would suggest that steroids limit muscular growth!

Twofold Effect

Low intensity exercise has a positive effect not only on recovery from high intensity work but on the high intensity work itself. While high intensity exercise is anti- circulatory as it pumps up the muscles (restricting blood flow), low intensity exercise promotes circulation, which aids in nutrient transfer and hastens recovery.

Exercise of a low enough intensity will not lead to detrimental fiber type changes! In fact, the enhanced capillary density it creates leads to precisely the opposite effect! The enhanced capillary density raises the temperature around the motor neurons, lowering electrical resistance, allowing more fiber to take on the characteristics of fast-twitch fiber in response to high intensity work.

The Treatment

EMS used in a pulsing mode for ten to twenty minutes at very low intensity assists with recovery by stimulating circulation and the exercise it provides promotes capillary density. The effects can be enhanced if the legs are slightly elevated during treatment. These sessions can be carried out at the end of the day, before bed, at least two hours after your last workout.

Rehabilitation

EMS can play a role in the rehab of a variety of injuries and is used extensively in clinics to treat the VMO with knee cases. But its value in the rehab of hamstring injuries is poorly understood and under appreciated.

The selection of isokenetic machines over EMS contributes to a lengthening of injury downtime as the fluid resistance on which these machines rely hits the muscle all at once, causing muscle shortening and irritation. Often, effective treatment including EMS can have the athlete back in action within ten days.

Injury Assessment

Immediately after the injury, with the leg in its normal straight position, run your hand along the hamstring to feel for a depression in the muscle to determine if there's been actual fiber separation (a third degree tear). In all but severe cases this won't have happened, meaning it's a first or second degree strain where a quick recovery can be expected.

This must be checked before swelling sets in and fills up any depression and afterwards the muscle should be wrapped, iced, and elevated in the usual fashion. Do not test or stretch the muscle, as further damage could occur and, regardless of the findings, the initial treatment remains the same. Surprisingly, it usually takes only 72 hours for the injury to heal, but extension injuries can occur above and below the original site and adhesions can form if the tissue isn't mobilized sufficiently.

During the initial 72 hours, the athlete should stay off his feet as much as possible and an EMS pulsing mode can be applied above (not on) the injury site three to four times per day to reduce swelling and promote the transfer of nutrients to the site. After 72 hours, very gentle EMS pulsing can be applied to the injury site once per day while retaining the pulsing routine four times per day above the site.

From the third day on, high intensity EMS can be applied to all other muscles to maintain fitness during the recovery period. Additional therapy should include Active Release Technique (ART) if possible, to further reduce the prospect of adhesions.

Bodybuilding Applications

Yes, EMS does have its cosmetic uses. Much like high intensity weight training, EMS increases muscular density or "hardness". Think of Ben Johnson. If you slapped him on the back you'd think you were hitting a brick wall. Although he had a great physique, he wasn't "puffy" like Arnold; he was as hard as a rock. So the thing to think about with EMS is density, not size. Think of it as maximal strength training and not hypertrophy training.

Also keep in mind that EMS is for large muscle groups only. Although an expert might be able to pull it off, the average user will not be able to use it on small muscle groups like the biceps, triceps and calves. These muscles will "roll up" on you, plus even if you could do it (like by placing your foot in a ski boot for calves) it would be excruciating.

Bodybuilders could also use EMS to help them break through a barrier. For example, if a guy's upper body is weak as compared to his legs, he could use EMS to maintain his legs for a few weeks while focusing on upper body training. Basically, he'd be allowing all his body's recovery mechanisms and central nervous system to focus on his upper body. He wouldn't lose any size in his legs and may even see some improvement in density during this time of upper body specialization.

"Burning Off" Fat

Here's another trick that may help competitive bodybuilders. EMS can be used to temporarily "burn off" a layer of fat in small areas. What happens is that about two millimeters of subcutaneous fat is mobilized in the area directly under the pads. You can compare this to the effect seen when shooting growth hormone, i.e., there's local mobilization of the fat at the point of injection.

Now, since that layer of fat is a protective mechanism, this isn't permanent. In fact, the effect doesn't last long at all. Once the area under the pad starts to cool, the fat starts storing again. So if you're already very lean and are competing in a bodybuilding show, you'd have to use the machine (possibly even backstage) and then cover up and keep warm until you hit the stage.

Electronic muscle stimulation (EMS). Stroke Or Spinal Cord Injury

Introduction

Damage to the human nervous system during an event such as stroke or spinal cord injury (SCI) produces a rapid denervation of muscle resulting in weakness or paralysis. This lack of neural innervation renders muscle unable to produce the voluntary forces needed to create joint movement that will allow functional performance of daily tasks. Numerous scientific investigations have focused on devices, strategies, and regimens that may potentially restore body movement critically needed for daily function and quality of life.

Using electrical stimulation to produce human movement is not a novel procedure. In 1790, Luigi Galvani first observed motion after applying electrical wires to leg muscles severed from the body of frogs, and in 1831, Michael Faraday showed that electrical currents could stimulate nerves to create active movement. One of the earliest clinical experiments that used electrical stimulation for muscle function stimulated the peroneal nerve in the leg in an effort to correct foot drop in persons with stroke-related hemiplegia during ambulation.

Whether used alone to improve motor impairment or embedded within complex systems to create functional multi-joint movement, the potential that electrical stimulation holds for rehabilitation recovery is immeasurable. Electrical stimulation is currently used in many forms to facilitate changes in muscle action and performance. In clinical settings, electrical stimulation can be used for improving muscle strength, increasing range of motion, reducing edema, decreasing atrophy, healing tissue, and decreasing pain. Neuromuscular electrical stimulation (NMES), used interchangeably with electrical stimulation (ES), is typically provided at higher frequencies (20-50 Hz) expressly to produce muscle tetany and contraction that can be used for “functional” purposes and can be found in literature as early as 1964. TENS is an alternate form of electrical stimulation that historically used high frequencies for pain relief but is now also administered at very low frequencies (sensory level TENS, 2-10 Hz). TENS propagates along smaller afferent sensory fibers specifically to override pain impulses. When very low frequencies are used, TENS specifically targets sensory nerve fibers and does not activate motor fibers; therefore, no discernible muscle contraction is produced.

The acronym FES (functional electrical stimulation) is probably the most commonly used in the literature; however, a distinction should be made that this method of electrical stimulation usually refers to the process of pairing the stimulation simultaneously or intermittently with a functional task as initially described by Moe and Post.

The delivery of electrical stimulation can be customized to reduce fatigue and optimize force output by adjusting the associated stimulation parameters. A full understanding of the settings that govern the stimulation is vital for the safety of the patient and the success of the intervention. Consideration should be given to the frequency, pulse width/duration, duty cycle, intensity/amplitude, ramp time, pulse pattern, program duration, program frequency, and muscle group activated.

Parameters of Electrical Stimulation

Frequency

Frequency refers to the pulses produced per second during stimulation and is stated in units of Hertz (Hz, e.g., 40 Hz = 40 pulses per second). The frequencies of electrical stimulation used can vary widely depending on the goals of the task or intervention, but most clinical regimens use 20-50Hz patterns for optimal results. In order to avoid fatigue or discomfort, constant low frequency stimulation is typically used, which produces a smooth contraction at low force levels. In a study comparing several different frequencies and stimulation patterns, frequencies under 16Hz were not sufficient to elicit a strong enough contraction to allow the quadriceps to extend to a target of 40º. Interestingly, lower frequencies of stimulation have been shown to impart a long-lasting depression of force output known as “low-frequency fatigue,” first described by Edwards, Hill, Jones, and Merton (1977). These researchers observed that fatigued muscle stimulated with lower frequencies (10-30Hz) had the potential to produce lower forces, a condition that lasted for 24 hours or longer; the same effect was not seen when the muscle was stimulated with higher frequencies. Later work by Bigland-Ritchie, Jones, and Woods (1979) showed that higher frequencies of stimulation (50 Hz and 80 Hz) administered to hand muscles resulted in a rapid decline in force after approximately 20s. More recently, stimulation frequency rates closely aligned with physiological rates of motor unit discharge were studied in the hand that showed a consistent frequency of 30 Hz preserved force better than a decreasing frequency pattern (30 Hz decreasing to 15 Hz) . High frequencies of peripheral stimulation can have central contributions as well; activation of motor neurons in the spinal pool was highest when the tibialis anterior muscle was stimulated with 100Hz as compared to stimulation at 10 and 50 Hz. Higher frequencies are generally reported to be more comfortable because the force response is smoothed and has a tingling effect, whereas lower frequencies elicit a tapping effect where individual pulses can be distinguished.

Ramping of Stimulation Frequency

Frequently, a gradation of stimulation up to the desired frequency and intensity is used for patient comfort. Ramp time refers to the period of time from when the stimulation is turned on until the actual onset of the desired frequency. Ramp time is used in clinical applications when a patient may have increased tone that creates resistance against the stimulated movement. For instance, a person with flexor hypertonicity at the elbow would benefit from a gradual ramping up of stimulation frequency to allow more time to activate elbow extensors moving in opposition to tightened flexors to successfully complete the movement. Ramp times of 1 to 3 seconds are common in rehabilitation regimens with longer ramp times sometimes used for hypertonic or spastic musculature or for the patient with an increased sensitivity to stimulation. Ramp times also can be modulated in multiple-muscle applications such as standing and walking to produce smooth gradations of tetany between individual muscles and more closely replicate natural movement.

Pulse Width/Duration

Electrical stimulation devices deliver pulses in waveform patterns that are often represented by geometric shapes such as square, peaked, or sine wave. These shapes characterize electrical current that rises above a zero baseline for the extent of the stimulation paradigm (uniphasic; e.g., direct current) or current that alternates above and below the baseline (biphasic or alternating current). Biphasic and uniphasic waveforms were noted to produce greater torque than polyphasic waveforms when administered to the quadriceps muscles of young healthy individuals.

The time span of a single pulse is known as the pulse width or pulse duration. In biphasic (a positive phase combined with a negative) pulses, the pulse duration considers both phases. Typically, dynamic quadriceps extensions similar to those used in FES cycling tests exhibit pulse widths between 300µs-600µs . Some investigators have suggested that low frequency stimulation with short pulse durations (500µs-1000µs) will exhibit a lower fatigue index. However, even shorter pulse widths (10µs-50µs) have been shown to affect the recruitment of muscle fibers and can generate a larger maximum torque in a smaller number of fibers before causing a contraction in another muscle fascicle [36]. This is important as a greater recruitment ratio within muscle fascicles can possibly increase performance time; therefore, pulse width can be increased to potentially recruit more fibers in the surrounding area as fatigue ensues. Recent work comparing 50, 200, 500, and 1000µs pulse widths when 20 Hz stimulation was delivered to the soleus muscle found that the wider pulse widths produced stronger contractions of plantarflexion and additionally augmented overall contractile properties. In addition, longer pulse durations will typically penetrate more deeply into subcutaneous tissues, so these widths should be used when trying to impact secondary tissue layers.

Duty Cycle

Early work in persons with SCI demonstrated that when periods of force development were interrupted with silent periods, muscle tissue was able to recover more quickly and produce greater torque as compared to when constant stimulation patterns were used. Cycling pulses on and off (intermittent stimulation) is a common practice to preserve force development and simultaneously increase comfort for the patient. Duty cycle describes the actual on and off time of an NMES program and is usually stated in ratio form, such as 1:2 (10 seconds on, 20 seconds off) or percentages such as 70 percent, indicating time on percentage when compared to total on and off time combined. Common clinical applications use a 1:3 duty cycle as standard, but this ratio can be modified to accommodate the needs of the patient as well as the goals of the treatment.

Amplitude/Intensity

Another parameter that will contribute to fatigue is the strength of the current being administered or the intensity/amplitude (usually reported in milliamperes, mA) with which the stimulation is delivered. The higher the intensity, the stronger the depolarizing effect in the structures underlying the electrodes . Higher intensities can foster increases in strength; strength gains are consistently found following training with electrical stimulation programs Recent work examining the optimal parameters for stimulation has suggested that lower intensities can induce more central nervous system input than higher intensities. Higher amplitudes of NMES activate a large number of muscle fibers that create forceful peripheral-mediated contractions, but antidromic transmission can occur (neural transmission toward the cell body rather than normal orthodromic transmission away from the cell body). Antidromic transmission blocks both motor and sensory impulses emanating from the spinal motor pool, resulting in less overall CNS activation. The impact of stimulation amplitude on fatigue remains unclear. Downey et al. found that when both frequency and amplitude were varied during a stimulation regimen of knee extension in healthy adults, more contractions were performed as compared to when a constant frequency and amplitude program was used. In contrast, when NMES was delivered to the knee extensors of seven healthy participants and the influence of frequency, pulse width, and amplitude on fatigue was studied, investigators found that fatigue decreased only when frequency was decreased; lowering the other parameters had no appreciable effect on reducing fatigue. Stimulation frequency rates closely aligned with physiological rates of motor unit discharge were studied in the hand that showed a consistent frequency of 30 Hz preserved force better than a decreasing frequency pattern (30 Hz decreasing to 15 Hz). Intensity will also factor into patient comfort with higher intensities being typically less tolerated; however, frequency and intensity inevitably will determine the quality of muscle contraction produced.

Stimulation Pulse Patterns

Several investigations have examined the effects of various stimulation patterns on force output and neuromuscular fatigue. Common stimulation patterns studied are constant frequency trains (CFTs), variable frequency trains (VFTs), and doublet frequency trains (DFTs). CFTs are stimulation trains in which the frequency remains constant throughout the entire train. In contrast, VFTs are usually trains that begin with an initial doublet, (two closely spaced pulses, typically 5-10 µs apart) followed by pulses at a chosen frequency. The idea of VFT comes from studies where it was found that muscles have a “catchlike property,” a unique mechanical response to stimulation that allows muscle to hold a higher force level than normal (van Lunteren, JAP 2000). This response enhances muscle tension prior to contraction when a brief, high frequency burst is followed by a train of subtetanic pulses. The phenomenon does notappear to be a result of greater muscle fiber recruitment but an inherent property of the individual muscle cells.

In an isometric contraction of the thenar muscles of the hand, Bigland-Ritchie and colleagues showed that pulse trains that began with a doublet resulted in slower rates of force attenuation, suggesting a slower time to fatigue . A similar study of isometric contraction of the thenar muscles of the hand examined variable patterns where a 20Hz CFT fatigue task was compared to two other fatigue tasks; a 20Hz CFT was administered for the first half of the fatigue task and then the frequency was increased gradually to 40Hz frequency or a 20Hz doublet train was added. The findings of this study concluded that during submaximal stimulation, the doublet train was most effective in producing higher average forces and force-time integrals. These studies propose that using VFTs may be more beneficial in reducing fatigue in intrinsic hand muscles than CFTs alone.

Other studies have observed the lower limb comparing CFTs, DFTs, and VFTs. In particular, one study fatigued the quadriceps muscle using CFTs and VFTs with varying interpulse intervals. The fatigued muscle was then stimulated with either a CFT of 14 or 18 Hz or a VFT (consisting of a train that used an initial doublet followed by a CFT). The results showed that VFT trains are more effective in producing higher peak forces, maintaining force output, and eliciting a more rapid rate of rise after being fatigued with a CFT as compared to using a VFT. Another investigation studied the effect of using CFTs, VFTs, and DFTs with the same interpulse interval (50 ms, 20 Hz frequency) to elicit dynamic leg extension. DFTs had the best overall performance in time to reach target. These findings suggest that there may be several optimal stimulation patterns, but these will be dependent on the task, population studied, and the muscle group being investigated.

Electrode Placement

The success of the device to reach underlying tissue is highly related to electrode size and placement, as well as the conductivity of the skin-electrode interface. In the past, a conductive gel was applied to the surface of electrodes to improve transmission of the current; typical stimulating electrodes used now are pre-gelled for convenience. Larger surface electrodes will activate more muscle tissue but will disperse the current over a wider surface area, decreasing current density. Smaller electrodes will concentrate current densities, allowing for focal concentration of current with less chance of stimulation crossover into nearby muscles, but dense current increases the chance for discomfort or pain. Placement of electrodes will also markedly influence the muscle response and should be carefully considered. Contention regarding optimal placement of electrodes is prevalent throughout the literature, with much of the debate centering on whether the muscle belly or the motor point is the preferential location.

Stimulation Intensity

Stimulation can be delivered by means of constant voltage or constant current. The small portable units used in clinics and given to patients for home use are normally battery-operated and have modifiable current settings usually delivered through a constant voltage system of approximately 150V. These units use transcutaneous surface electrodes that adhere to the skin and can be easily removed. The contact area of the electrode is usually lined with the conductive gel described earlier that facilitates movement of the current from the electrode into the skin. Because the units use alternating current (AC) with a high degree of adjustability, muscle activation through these devices can be sometimes be variable and inconsistent; outcomes will depend on the quality of the skin-electrode interface and consistent placement of electrodes for repeatability.

Dosing of Stimulation

Dosing of programs can vary greatly and will ultimately depend on the muscle being stimulated, parameters used, and overall goal of the intervention. A review of the use of FES for motor recovery of the upper extremity in stroke examined several investigations and found an array of dosing protocols used. Program duration ranged from 30 minutes one time per day to an hour at each session for three times per day. Overall period of treatment varied from 2 weeks to 3 months