Neuromuscular electrical stimulation (NMES) is a versatile therapeutic modality used for a variety of purposes. These purposes include aid of regaining muscle strength,1–9 enhancing recovery of motor control,6,8 and retarding muscle atrophy.5,10–14 Stimulation electrodes play the important role of interfacing tissue with the stimulation unit.15 Currently, the most commonly used electrode in physical medicine and rehabilitation is a reusable, adhesive-backed, silicon-impregnated hydrogel electrode.16 Theoretically, the multiple-use electrode will degrade over time with frequency of use leading to a decrease in the effectiveness of NMES.
An inexpensive, single-use electrode has been produced to eliminate the possibility of cross-contamination and degradation. With a single-use electrode, the patient receives a new electrode for each treatment. It is important to understand how single-use electrodes compare with multiple-use electrodes, both initially and after degradation of the multiple-use electrodes.
The purpose of this study was to compare single-use and multiple-use electrodes in 3 areas: amplitudes at which the patient first perceives sensation, muscle twitch response, and the force produced at a specific intensity. Single-use electrodes were tested once and multiple-use electrodes were tested both at the 1st and 10th use (to simulate degradation).
The experiment used a 2 × 2 factorial design with repeated measures. The independent variables were types of electrodes (multiple-use, self-adhesive electrodes and single-use, self-adhesive electrodes) and number of trials (initial use and 10th use). Dependent variables were amplitudes of perceived sensation, muscle twitch, and the force produced at a specific intensity.
Description of Participants
Study participants were 7 men (mean age, 24.7 ± 2.3 years; mean height, 72.1 ± 2.4 inches; mean weight, 192.9 ± 39.8 lbs; and mean skinfold thickness, 5.9 ± 2.4 mm) and 13 women (mean age, 21.5 ± 2.3 years; mean height 67.4 ± 4.0 inches; mean weight, 160.8 ± 25.1 lbs; mean skinfold thickness, 10.7 ± 4.1 mm) who were healthy and active. Exclusion factors were compromised circulation of the upper extremity, serious injury, surgery, impairment of the upper extremity within the past 6 months, skin disease or lesion on the upper extremity, infection of the upper extremity, or internal or external fixation devices of the upper extremity.
The university’s institutional review board for human subject research approved the study, and participants gave informed consent.
Description of Equipment
The multiple-use electrodes were 2 × 2-inch squares (Dynatronics Corp, Salt Lake City, Utah). The single-use electrodes were 2 × 2-inch squares (Accelerated Care Plus, Reno, Nevada) (Figure 1). The NMES device used was the Omnistim FX2 Pro Sport (Accelerated Care Plus). The strain gauge used was the LC-100 (Omega Engineering Inc, Stamford, Connecticut).
Figure 1. Single-use electrode (left) and multiple-use electrode (right).
All testing was performed in the Therapeutic Modalities Research Laboratory, and the same clinician (L.M.) performed all tests to maintain uniformity of the testing procedures.
As the participants registered for the study, they were randomly assigned by the clinician in a random draw to either the single-use or multiple-use electrode group. The participants remained in their assigned group throughout the study. Participants were instructed to select testing days 1 week apart for the initial and final trials. To normalize the trials, they were performed at the same time each day for each participant. Participants were instructed to attempt to maintain uniformity of their schedules on those days.
Before the initial trial, skinfold measurements were obtained over the wrist extensors at the muscle belly of the extensor carpi radialis brevis using baseline skinfold calipers (Fabrication Enterprises Inc, Irvington, New York). Prior to applying the electrodes to the participant’s skin, his or her arm was shaved and cleaned with alcohol. The skin was allowed to air dry. Participants had 1 electrode placed on their wrist extensor muscle belly just distal to the lateral epicondyle on the posterior forearm and another was placed distally over the wrist extensor tendons. Outlines of the electrode were traced with a permanent marker on the participants’ skin to act as a template for the next session’s trials. Using a curtain to obstruct the participants’ view of the electrodes, the participants were masked to the type of electrode being tested.
Participants were seated in a customized chair. The upper arm and forearm were stabilized with straps to eliminate involvement of the shoulder or elbow. The wrist was supported in a neutral, relaxed position. A strain gauge was attached at the participants’ dominant hand. A strap, placed around the participants’ hand, was connected to a turn block that attached to the strain gauge. This allowed the clinician to modify the length to accommodate for the various sizes of the participants. The strain gauge reported muscle force production to a customized computer program.
Participants were told to first expect a mild tingling sensation followed by an involuntary twitch in the wrist extensor muscles as the electrical intensity was increased. Participants were instructed by the clinician to verbally announce when they first perceive the electrical stimulation and when they first feel the muscle twitch due to electrical stimulation. Participants verbally announced the first perceived sensation of muscle twitch, but the first observed twitch of the wrist extensor muscles between the electrode pair was recorded by the clinician. This intensity in milliamper-age (mA) was recorded by the clinician for perceived sensation and muscle twitch.
Participants were then informed that the clinician would increase the intensity to 40 mA (this intensity was derived from an average intensity during a pilot study conducted previously), and the force produced on the strain gauge held for 10 seconds was recorded by a custom computer program (Visual Studio 2008; Microsoft Corporation Inc, Redmond, Washington). After the trial began, there were no visual or verbal cues given. Due to a clicking sound emitted by the machine when the buttons were pushed, each participant wore headphones to stop any sounds that may have indicated the intensity was being increased.
The intensity was then turned off, and the procedure was repeated twice more with a 2-minute rest between procedures, for a total of 3 procedures per electrode pair. The electrodes were not removed during the 3 procedures. The average of the 3 procedures was used for statistical analysis.
Measurements were taken, and the intensity (in mA) at which participants perceived sensation, muscle twitch, and muscle force production at a specified intensity with both single-use and multiple-use electrodes was recorded. Multiple-use electrodes were tested on initial use and on the 10th use to determine whether there was decay in the integrity of the electrodes. Each pair of electrodes was used for the same participant for the initial trial and the 10th-use final trial. The following treatment parameters were used: pulse width, 150 μs16; medium frequency; carrier frequency of 5000 Hz; and beat frequency of 75 Hz.
The same procedures were followed at both the initial trial and the final trial. After the multiple-use electrodes were tested initially, they were drained of current and used in the university’s athletic training clinic for eight 15-minute treatments (2-hour total) of nonparticipants. When the electrodes were drained out, it occurred at the same level of typical treatments. The electrodes were assigned an identification number to ensure that the same electrodes were used on the same study participant during testing. After the electrodes were returned by the athletic training clinic to the study, the pretest procedures were repeated (10th-use electrode) as the participants’ final trial.
This was a repeated measures study that used mixed models analysis of variance with blocked randomization of participants. The blocking of participants allowed us to better control for any possible between-participant difference and improve the precision for treatment comparisons.17 Tukey-Kramer post hoc tests were used to determine differences between groups. Alpha was set at P < .05. Data were analyzed using SAS 9.1 software (SAS Institute Inc, Cary, North Carolina).
Regressions analysis was performed to determine variance due to group or demographic factors (gender, age, height, weight, and skinfold measurements). Any variance that resulted from group (Fgroup = .01, P = .9103) or demographic factors (Fgender = .39, P = .5377; Fage = .76, P = .3895; Fheight = .05, P = .8293; Fweight = .30, P = .5873; and Fskinfold = 1.70, P = .2002) was not found to be significant.
On average, the values for the single-use electrode were 9.73 mA for perceived sensory, 15.87 mA for observed muscle twitch, and 2.33 kg for force produced at a specific intensity. The average values for the multiple-use electrode initial trial were 16.70 mA for perceived sensory, 29.16 mA for observed muscle twitch, and 1.03 kg for force produced at a specific intensity of 40 mA. The average values for the multiple-use electrode final trial were 21.03 mA for perceived sensory, 31.78 mA for observed muscle twitch, and 0.51 kg for force produced at a specific intensity.
A statistical difference was found between groups for perceived sensation (F2,38 = 55.57, P < .0001) (Figure 2). The only sign of decay exhibited by the multiple-use electrodes when testing perceived sensation was when the initial multiple-use electrode condition had a higher perceived sensation than on the 10th use in the final multiple-use electrode condition (P = .0007). Also, there was a significant difference between the single-use and the multiple-use electrodes for both the initial trial (P < .0001) and the final trial (P < .0001).
Figure 2. Mean values of perceived sensation with standard error bars (1 standard deviation). * Significant difference between single-use electrodes and both trials (initial and final) of multiple-use electrodes. † Significant difference between the initial and final trial of multiple-use electrodes.
A statistical difference was found between groups for muscle twitch produced (F2,35 = 108.26, P < .0001) (Figure 3). No statistically significant difference was noted between the measures taken during the initial trial and final trial of the multiple-use electrodes for muscle twitch (P = .0634, Power = 0.42). A statistical difference was noted between the multiple-use electrodes and the single-use electrodes (P < .0001).
Figure 3. Mean values of muscle twitch with standard error bars (1 standard deviation). * Significant difference between single-use electrodes and both trials (initial and final) of multiple-use electrodes.
A statistical difference was found between groups for muscle force production (F2,38 = 24.22, P < .0001) (Figure 4). No difference was noted between the initial and final trials of the multiple-use electrodes for muscle force production (P = .1494, Power = 0.99). A statistical difference was noted between the multiple-use electrodes and the single-use electrodes for muscle force production (P < .001).
Figure 4. Mean values of force production with standard error bars (1 standard deviation). * Significant difference between single-use electrodes and both trials (initial and final) of multiple-use electrodes.
This study confirmed the viability of single-use electrodes compared with the common multiple-use electrodes. The single-use electrodes not only favorably compared with the multiple-use electrodes but they significantly outperformed the multiple-use electrodes on every test. To our knowledge, no other study has established the efficacy of single-use electrodes. Also, we could find no study where researchers specifically investigated the effects of multiple treatments on electrode degradation at 40-mA intensity. Many studies have been conducted on of the effects of electrode placement,1,4,18 position,1,4 size,18 shape,18 and type,15 but no study has evaluated the effects of degradation. Statistically, we found no electrode degradation for muscle twitch and muscle force production after 2 hours of use. Even with the decrease in adhesive quality with regular removal and reapplication of electrode pairs during the 2 hours, there was no significant degradation for muscle twitch and muscle force production in the multiple-use electrodes.
The only test that showed a significant difference between the initial trial and the final trial for the multiple-use electrodes was the perception of when the sensation of the stimulation began. Perception of the sensation caused by electrical stimulation was significantly reduced in the final trial compared with the first trial, with many participants stating they felt little electrical sensation during the final trial. Why perception of sensation should be significantly reduced is not immediately apparent. Because participants were not exposed to more than a total of 4 minutes of electrical stimulation during either the initial or final trial, an increase in tolerance is not a viable possibility due to the lack of exposure time.2 Outlines of the electrode pairs were traced on the participants’ arms so there would be uniformity of positioning and motor unit stimulation. The waveform and carrier frequency did not change between the trials, so perception should have remained uniform.2–4,15,19,20 However, in the general population, the strength–duration time constant is longer for cutaneous afferents than for motor axons, probably because the cutaneous afferents express a greater non-inactivating sodium ion conductance that is active at threshold.21 Therefore, it may be possible that sensation would be decreased, whereas the amount of electricity conducted through the skin would remain relatively constant.
Muscle twitch and force production did remain statistically uniform, with only the perception of sensation decreasing. It is important to note that the sample size may have been a factor associated with no statistical difference of muscle twitch intensity between the initial and final trials of multiple-use electrodes. The low power result for this analysis suggests that it is unclear whether multiple-use degradation occurs with respect to muscle twitch.
However, it is also our belief that 2 hours is not enough time to truly simulate clinical use of multiple-use electrodes. The decrease in sensation may have been the first sign of a trend of degradation. Many times in the clinical setting, electrical stimulation treatments are performed for longer than 15 minutes (the amount of time of each simulated treatment in our study) in 1 treatment period. Often, electrical stimulation treatments are combined with heat or ice, either of which could affect the degradation of the electrode. In our study, the participants’ and nonparticipants’ skin was shaved and cleaned with alcohol prior to any application of the electrodes. This may have also affected the integrity of the electrode. Perceived sensation decreased despite these precautions, but its effect on muscle twitch or force production is undetermined. Body oils, hair, lotions, or dead skin may play a role in the degradation of electrodes in a clinical setting. Our study showed that there is little degradation to a multiple-use electrode pair with only 2 hours of use in a laboratory setting.
A key aim of our study was to form a baseline for the efficacy of single-use electrodes compared with the multiple-use electrodes commonly used in clinical settings. Originally, the study was conducted to determine whether single-use electrodes satisfactorily compared to multiple-use electrodes. However, after completing the data collection, it became obvious that the single-use electrodes were capable of surpassing the multiple-use electrodes for perceived sensation and motor thresholds and in muscle force production. The single-use electrodes produced a lower threshold of sensation and muscle twitch and more force for every participant tested. A significant difference was noted between the results of the single-use electrodes and the multiple-use electrodes for both the initial and final trials. This study showed that the single-use electrodes are comparable to a commonly used brand of multiple-use electrodes.
However, the single-use and the multiple-use electrodes were produced by different manufacturers. Therefore, the difference in manufacturing and materials used in the electrodes may have played a role in the results. Further study is necessary to determine whether single-use electrodes can favorably compare with multiple-use electrodes produced by the same manufacturer.
Our study investigated single-use electrodes for sensation, muscle twitch, and muscle force production and found them to be as effective as multiple-use electrodes. Our study revealed the efficacy of single-use electrodes.
Implications for Clinical Practice
Single-use electrodes are a viable option for clinicians. In this study, single-use electrodes performed as well as or better than the multiple-use electrodes. Single-use electrodes produced sensation and visible muscle twitch at a lower threshold, and they produced more muscle force production. It is our opinion that single-use electrodes are an effective alternative to the commonly used multiple-use electrodes.
- Alon G, McCombe SA, Koutsantonis S, et al. Comparison of the effects of electrical stimulation and exercise on abdominal musculature. J Orthop Sports Phys Ther. 1987;8(12):567–573.
- Alon G, Smith GV. Tolerance and conditioning to neuro-muscular electrical stimulation within and between sessions and gender. J Sports Sci Med. 2005;4(4):395–405.
- Eriksson E, Häggmark T, Kiessling KH, Karlsson J. Effect of electrical stimulation on human skeletal muscle. Int J Sports Med. 1981;2(1):18–22 doi:10.1055/s-2008-1034578 [CrossRef] .
- Fitzgerald GK, Piva SR, Irrgang JJ. A modified neuromuscular electrical stimulation protocol for quadriceps strength training following anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther. 2003;33(9):492–501.
- Gould N, Donnermeyer D, Gammon GG, Pope M, Ashikaga T. Transcutaneous muscle stimulation to retard disuse atrophy after open meniscectomy. Clin Orthop Relat Res. 1983;(178):190–197.
- Holcomb WR. A practical guide to electrical therapy. J Sport Rehabil. 1997;6(3):272–282.
- Lieber RL, Silva PD, Daniel DM. Equal effectiveness of electrical and volitional strength training for quadriceps femoris muscles after anterior cruciate ligament surgery. J Orthop Res. 1996;14(1):131–138 doi:10.1002/jor.1100140121 [CrossRef] .
- Morrissey MC, Brewster CE, Shields CL Jr, Brown M. The effects of electric stimulation on the quadriceps during postoperative knee immobilization. Am J Sports Med. 1985;13(1):40–45 doi:10.1177/036354658501300107 [CrossRef] .
- Parker MG, Bennett MJ, Hieb MA, Hollar AC, Roe AA. Strength response in human femoris muscle during 2 neuromuscular electrical stimulation programs. J Orthop Sports Phys Ther. 2003;33(12):719–726.
- Arvidsson I, Arvidsson H, Eriksson E, Jansson E. Prevention of quadriceps wasting after immobilization: an evaluation of the effect of electrical stimulation. Orthopedics. 1986;9(11):1519–1528.
- Delitto A, Rose SJ, McKowen JM, Lehman RC, Thomas JA, Shively RA. Electrical stimulation versus voluntary exercise in strengthening thigh musculature after anterior cruciate ligament surgery. Phys Ther. 1988;68(5):660–663. Erratum in Phys Ther. 1988;68(7):1145.
- Neder JA, Sword D, Ward SA, Mackay E, Cochrane LM, Clark CJ. Home based neuromuscular electrical stimulation as a new rehabilitative strategy for severely disabled patients with chronic obstructive pulmonary disease (COPD). Thorax. 2002;57(4):333–337 doi:10.1136/thorax.57.4.333 [CrossRef] .
- Oldham JA, Stanley JK. Rehabilitation of atrophied muscle in the rheumatoid arthritic hand: a comparison of two methods of electrical stimulation. J Hand Surg Br. 1989;14(3):294–297 doi:10.1016/0266-7681(89)90085-5 [CrossRef] .
- Wigerstad-Lossing I, Grimby G, Jonsson T, Morelli B, Peterson L, Renström P. Effects of electrical muscle stimulation combined with voluntary contractions after knee ligament surgery. Med Sci Sports Exerc. 1988;20(1):93–98 doi:10.1249/00005768-198802000-00014 [CrossRef] .
- Keller T, Kuhn A. Electrodes for transcutaneous (surface) electrical stimulation. Journal of Automatic Control. 2008;18(2):35–45 doi:10.2298/JAC0802035K [CrossRef] .
- Knight KL, Draper DO. Therapeutic Modalities: The Art and Science. Philadelphia, PA: Lippincott Williams & Wilkins; 2008.
- Ramsey FL, Schafer DW. The Statistical Sleuth: A Course in Methods of Data Analysis. 2nd ed. Pacific Grove, CA: Duxbury; 2002.
- Forrester BJ, Petrofsky JS. Effect of electrode size, shape, and placement during electrical stimulation. J Appl Res2004;4(2):346–354.
- Prodanov D, Marani E, Holsheimer J. Functional electrical stimulation for sensory and motor functions: progress and problems. Biomedical Reviews. 2003;14:23–50.
- Rooney JG, Currier DP, Nitz AJ. Effect of variation in the burst and carrier frequency modes of neuromuscular electrical stimulation on pain perception of healthy subjects. Phys Ther. 1992;72(11):800–806.
- Mogyoros I, Kiernan MC, Burke D, Bostock H. Strength-duration properties of sensory and motor axons in amyotrophic lateral sclerosis. Brain. 1998;121(Pt 5):851–859 doi:10.1093/brain/121.5.851 [CrossRef] .