1. Introduction
Neuromuscular electrical stimulation (NMES) of the quadriceps following knee surgery is often poorly tolerated by many patients, particularly in the early post-operative phases of recovery when such stimulation may be most beneficial. These patients report an intolerable level of discomfort from NMES before visible muscle contractions can be achieved. Intolerance for NMES could potentially delay these patients’ recovery. Dry needling is routinely used with electrical stimulation for pain relief, set at parameters designed to achieve twitch contractions, usually in the range of 2 – 4 pulses per second (pps). Since dry needling with electrical stimulation is useful for pain relief, the purpose of this study was to investigate the clinical question, can dry needling be used with NMES, using the same equipment and parameters as conventional NMES, to achieve quadriceps contractions at lower stimulator amplitude levels and with less discomfort than conventional NMES? If so, then patients who do not tolerate conventional NMES may experience the same benefits as those who tolerate that intervention.
Background.
The sequelae of trauma and/or surgical interventions to the knee often includes a significant decrease in voluntary quadriceps activation,1 a consequence called arthrogenic muscle inhibition (AMI),2 reflex inhibition,3 or reflex muscle inhibition.4 Neural inhibition is suspected as the primary cause of AMI and is supported by its observance in both involved and uninvolved limbs.5 It is present in spite of maximal voluntary effort to contract the quadriceps.2,6 AMI may be a primitive defense mechanism to protect the injured joint from further activity,7 triggered by pain and edema.4,8 AMI is associated with a variety of knee pathologies, including total knee arthroplasty, anterior cruciate ligament injury or reconstruction, osteoarthritis, and anterior knee pain.1–4,6,8 AMI accounts for long-term deficits in strength and function.2,3,9 It is, therefore, a primary focus of concern in rehabilitating these patients.2,8–10
Neuromuscular electrical stimulation (NMES) is a common intervention used to address impairments of muscle strength and to promote functional recovery after injury or surgery, especially those involving AMI of the quadriceps.3,11,12 NMES, as it is conventionally applied in rehabilitation settings, uses surface electrodes applied as closely as possible to motor units of the targeted muscle with the goal of achieving a muscle contraction.11 Thus, if NMES is able to recruit inhibited motor units, it may help mitigate the effects of AMI.8 Although some studies question the effectiveness of NMES,12 the preponderance of evidence supports its use.2,13 Despite evidence favoring success of NMES, there are clinical challenges to achieving strong muscle contractions with NMES. Perhaps the most commonly reported challenge is tolerance to the discomfort of NMES.11 For some patients, the discomfort is intolerable, rendering NMES impractical.14 Discomfort seems to be correlated to the amount of subcutaneous adipose tissue, which acts as an electrical insulator between the surface electrodes and the underlying muscle.14,15 Individuals with less adipose tissue in their thighs, therefore, generally have better tolerance for NMES.3 Obese women tend to have the lowest tolerance of all.11 The correlation between adipose tissue and pain tolerance does not, of course, rule out the possibility of other factors affecting pain tolerance.
Another clinical challenge with the use of NMES is muscle fatigue.11 NMES recruits motor units in a pattern contrary to that of voluntary contraction. Voluntary contraction recruits small diameter fibers first, saving large diameter fibers for more forceful contractions. Beckel, et al., describe motor unit recruitment with NMES as occurring in a nonselective, spatially fixed, and synchronous pattern.16 Additionally, NMES activates only the motor units that are found between the electrodes, potentially leaving portions of the muscle dormant during NMES-induced contractions.11 For these reasons, the muscle is likely to fatigue faster with NMES than with voluntary, active exercise.
Some possible adverse effects of NMES include histological changes to the muscle fibers, increased creatine kinase levels, and delayed onset muscle soreness.11 These effects may be attributed to high mechanical stresses on the muscle fibers related to the specificity of motor unit recruitment of NMES as previously discussed.17 It should be noted that these changes are not limited to NMES, as similar changes occur during maximal voluntary eccentric contractions.17
Intramuscular electrical stimulation (IMES) is an alternative to conventional NMES proposed to improve tolerance. IMES uses an insulated wire(s) threaded into the targeted motor unit and held in place by a small bend in the wire(s). IMES is reported to be better tolerated than NMES because it bypasses intervening tissues and delivers the current directly to the motor unit.18–20 Consequently, some studies have shown that it takes only one-tenth the voltage to produce contractions with IMES versus NMES.7 Since insulated, percutaneous electrodes bypass the impedance of intervening tissues and cutaneous nociceptors while using substantially lower current to produce results, it is clear that IMES has the potential to be better tolerated than surface NMES. However, it is not as clear that IMES has advantages in regard to muscle fatigue or histological changes.21 It should be noted that available studies of IMES involve motor recovery after hemiparesis rather than muscular retraining of AMI or strength gains following a musculoskeletal injury
Before discussing the possibility of using dry needling with NMES (DN-NMES), the differences in electrodes between IMES and DN-NMES should be noted. IMES uses either a single or, more commonly, a bundle of thin wires threaded through an insulating sheath into the target area of the muscle. Dry needling, in contrast, uses a single, solid, uninsulated monofilament needle passed through the skin into the target tissue. Dry needling can be performed with electrical stimulation via alligator clips attached to the needles. When used for trigger point pain relief, dry needling can deliver electrical stimulation directly to the targeted trigger point, bypassing the impedance of superficial tissue.22
Although dry needling is widely used with electrical stimulation to promote a variety of positive effects including pain relief and muscle relaxation, there is a dearth of research investigating its use for NMES. A case report by Hollis and McClure discusses the use of DN-NMES to promote voluntary activation of the tibialis anterior following surgical repair.7 Increased strength, increased range of motion, and improved gait were noted after only five sessions of DN-NMES. Beyond this encouraging case report, little is known about the potential for DN-NMES.
Purpose / hypothesis. The purpose of this study was to determine the feasibility of using dry needling with NMES versus conventional NMES (C-NMES) to achieve a specific target level muscle contraction of the quadriceps with less discomfort. We hypothesized that we could achieve a muscle contraction with DN-NMES at a percentage of maximum voluntary isometric contraction (MVIC) typically achieved with C-NMES. We further hypothesized that this contraction could be achieved with stimulator amplitudes sufficiently low enough to be better tolerated than C-NMES. These findings may suggest DN-NMES as an alternative to C-NMES for addressing muscle strength or performance impairments
2. Materials and Methods
Experimental design. A quasi-experimental one group design with one-way repeated measures of each factor was used. The experimental procedures were approved by a university Institutional Review Board prior to initiation of the study.
Investigators. The study was conducted by four doctor of physical therapy students along with a faculty member who has over 26 years of clinical experience as a physical therapist and is certified in dry needling (Cert DN).
Participants. Sixty-five healthy participants completed the study. The participants’ ages ranged from 18 to 65 years old (average age 26 years, standard deviation 8.5 years). There were 23 male and 42 female participants. Participants were recruited as a sample of convenience through advertising and word of mouth on a university campus as well as local fitness centers. The participants were screened according to health history and inclusion/exclusion criteria. Inclusion criteria included the age range of 18 to 65 years old as well as the ability to understand and consent to participation. Exclusion criteria included any outstanding health issues such as a history of fainting, cardiac or vascular disease, electrical implants, impaired wound healing, local or systemic infection, anticoagulant therapy, pregnancy, metal allergies, needle phobia/aversion, or previous experience with dry needling or NMES. Sample size was estimated for a paired t-test across two conditions within one group. Allowing for a power of 0.8 and a moderate effect size, we determined that 64 participants were needed. Sixty-six participants began the study with one dropping out before completion of data collection.
Maximum Voluntary Isometric Contraction.
A handheld dynamometer has been proven as a reliable device for measuring MVIC.23,24 The microFET2, in particular, has been shown to have excellent inter- and intra-rater reliability (ICC 0.95 – 0.99) for the measurement of quadriceps MVIC.23 To further ensure intra-rater reliability during this study, a single investigator measured MVIC for all participants. Strength was measured in kilograms. For the measurement of quadriceps strength, participants were seated on a mat table with their hips and knees flexed to 90 degrees and their feet hanging free. These joint angles are consistent with, or similar to, other studies which measured quadriceps strength and its relationship with electrical stimulation.3,6,15 The handheld dynamometer was positioned on the anterior tibia, approximately 13 cm distal to the tibial tuberosity. The participants were instructed to cross their arms over their chests and to maximally contract the quadriceps by extending the knee against the handheld dynamometer. This measurement was taken three times and the average was recorded as the MVIC
The MVIC was needed in order to calculate a target-level contraction for electrical stimulation. Although the percentage range varies by source, most researchers report that electrical contractions of 40 – 60% MVIC should be achieved in order to stimulate muscle hypertrophy.3,10,25 The therapeutic window may, however, extend as low as 25% MVIC.2,26,27 While we hoped to achieve sufficient muscle contractions with both C-NMES and DN-NMES to promote hypertrophy, it should be noted that reversal of AMI by achieving any contraction is a worthy clinical goal itself. Achieving that goal may improve patients’ ability to perform voluntary exercise for strengthening and hypertrophy. A study of critically ill cardiothoracic patients demonstrated that, while NMES had no effect on muscle layer thickness, NMES was associated with a higher rate of regaining muscle strength in the ICU.28
Neuromuscular Electrical Stimulation. The Chattanooga Vectra Genisys was used to deliver neuromuscular electrical stimulation for both the superficial and dry needling trials, with the operator’s manuals referenced for proper use of the equipment.29,30 Symmetrical, biphasic pulsed current with 480 microseconds pulse duration, 35 pps frequency, no modulation, and a continuous cycle was utilized for this study. Various parameters are acceptable for NMES, including pulse durations of 200-600 microseconds and frequencies of 20-100 pps.31 A pulse duration of 480 microseconds was chosen to match the pulse duration used with dry needling to produce twitch contractions for electroanalgesia. Given that frequencies of 50-75 pps are typically used for NMES, a frequency of 35 pps was chosen for two reasons: it matches the stimulator’s preprogrammed setting for muscle re-education of the quadriceps, and it more closely approximates the frequency used with dry needling to produce twitch contractions for electroanalgesia. These parameters were used for both phases of the study so that current amplitude and delivery method (superficial electrodes vs dry needles) were the only variables.
Electrodes. Standard 2” x 4” self-adhesive electrodes were used for the delivery of CNMES (see Figure 1). Given that current density was a consideration in this study, the 2” x 4” electrodes were chosen because they would mitigate the disparity with dry needles more so than larger electrodes. Seirin J-Type acupuncture needles with a 0.30 mm diameter and either 50 or 60 mm length were used for the delivery of DN-NMES (see Figure 2).
Figure 1. Conventional NMES
Figure 2. NMES with dry needling NMES
Self-Reported Discomfort. Discomfort was measured with the Visual Analog Scale (VAS). The VAS consists of a horizontal 10 cm line with marks at both ends to define the limits of the pain experience where the zero point is labeled “No pain” and the end point is labeled “Worst imaginable pain.” Patients are asked to place a vertical mark on the line which corresponds to their pain experience. The VAS has been determined to be both valid and reliable.32
Procedures. Participants were screened for inclusion / exclusion criteria, received an explanation of procedures, and signed an informed consent agreement to participate in the study. They were assigned an identification number to protect their confidentiality in the data collected. To obtain MVIC data, participants were seated unsupported on a mat table with their hips and knees flexed to 90 degrees, their arms crossed over their chests, and their feet hanging free. They were then asked to extend a knee as forcefully as possible against the microFET2. The average of three trials was recorded as the MVIC. They received a short rest break to allow the quadriceps to recover before beginning the next phase of the study. To obtain C-NMES data, conventional surface electrodes were placed over the vastus medialis oblique and the belly of the rectus femoris, with one electrode at each location. Participants were asked to not actively contract their quadriceps at any time during the procedure. Electrical stimulation was applied at the above-listed parameters with amplitude increased until the participants reported that they had reached the threshold of maximal discomfort that they were willing to tolerate. At this point, the microFET2 was applied to measure the strength of the quadriceps contraction achieved through stimulation. The stimulation was discontinued, and the participants were immediately asked to rate their level of discomfort at the point of maximal stimulation using the VAS. They were then given a five-minute rest break to recover from the trial. The electrodes were outlined with a pen before removal to ensure needle placement at the exact locations during the next phase of the trial. The area inside the outlines was disinfected with cotton balls soaked in isopropyl alcohol. To obtain DN-NMES data, needles were placed within the vastus medialis oblique and rectus femoris, with one needle at each point. Needles were connected to the electrical stimulator via alligator clips, and the trial proceeded in the same manner as the C-NMES trial. After their testing was concluded, participants were asked to remain in close proximity for a few minutes to make sure that they did not experience any adverse events from the procedure.
Data analysis. Data recorded included participant’s age, gender, MVIC, stimulation amplitude, discomfort level, and strength of quadriceps contraction during stimulation. Statistical analyses were performed using SPSS (version 27; SPSS Inc., Chicago, IL). Paired t-tests were used to compare the means of discomfort (Table 1) and stimulation amplitude (Table 2). The strength of the quadriceps contraction achieved during CNMES and DN-NMES was calculated as a percentage of each participant’s MVIC. A one-way repeated measures ANOVA was used to compare those differences (Table 3). An alpha level of 0.05 was used to determine statistical significance.
3. Results
Participants reported significantly higher discomfort levels (Table 1) at lower stimulation amplitudes (Table 2) for DN-NMES versus C-NMES, tolerating only 28% of stimulus amplitude through dry needles vs. surface electrodes. The strength of quadriceps contraction achieved during MVIC testing was significantly higher than that achieved during C-NMES and DN-NMES, and the strength of contraction during C-NMES was significantly higher than during DN-NMES (Table 3).
4. Discussion
Some considerations may account for the findings of this study. Discomfort is wellestablished as a limiting factor in the delivery of NMES. It is for this reason that this study was conducted. Electrode size seems to have an effect on discomfort. Smaller electrodes have a higher current density such that the stimulator amplitude required to achieve a muscle contraction is intolerable for many patients.3 Larger electrodes, on the other hand, distribute current over a wider area, recruiting more motor units, and achieving a stronger contraction at lower stimulator amplitudes with correspondingly less discomfort.2 One study found that electrode size did not have an effect on discomfort, but the difference in size between the smallest and largest electrodes used in that study was only 1.42 cm2 .15 The researchers in that study did suggest that the small difference in electrode size may account for the statistical insignificance of the difference in discomfort. However, the difference in surface area between a dry needle and a standard self-adhesive electrode is substantial. Therefore, we may theorize that any advantage gained in bypassing tissue impedance via a dry needle is overwhelmed by the corresponding increase in current density.
The stimulator parameters used in this study may play a role in the finding that DNNMES was less tolerable than C-NMES. Since the purpose of this study was to investigate whether dry needles could take the place of surface electrodes in the delivery of NMES, we used a direct comparison between the two types of current delivery. The stimulator parameters for this study, therefore, were suitable for C-NMES with surface electrodes. As noted in the introduction, electrical stimulation through dry needling is typically delivered at a rate of 2 – 4 pps for electroanalgesia and produces twitch contractions rather than tetanic contractions. The rate used for this study was 35 pps. Adjustments in stimulator parameters are often used clinically to maximize the comfort and effectiveness of NMES for individual patients.11 Therefore, we may theorize that DN-NMES might be more tolerable and effective at parameters other than those used in this study.
Neither the C-NMES or DN-NMES trials achieved quadriceps contractions in the 25 –60% MVIC threshold proposed for quadriceps strengthening. However, it may not be reasonable to expect this achievement in only one session. Subjects of NMES should be conditioned over the course of multiple sessions to tolerate this level of stimulation.2,11,33 As mentioned previously, females tend to be less tolerant of NMES than males. One study found that after six sessions all males but only 64% of females were able to tolerate the 25% MVIC threshold.33 Motivation may also play a critical role in tolerance of NMES. Study participants who expect no therapeutic value from NMES may have a lower tolerance than patients for whom NMES is a means of recovery.33 The participants in our study not only perceived no personal therapeutic benefit from electrical stimulation but also had to mitigate any aversion to being needled.
5. Conclusion
The results of this study do not support our hypothesis that NMES through dry needling could achieve a target-level muscle contraction of the quadriceps at lower stimulation amplitude and with less discomfort than conventional NMES with surface electrodes. Therefore, NMES through dry needling is not a feasible alternative to conventional NMES, at least not at parameters suitable for conventional NMES. Future studies may suggest alternative parameters more tolerable and effective for NMES through dry needling
Acknowledgments
The authors would like to acknowledge Elizabeth Norris for her guidance in developing the study design. This study was supported by a Q-TAG (Quick Turn-Around Grant) from Western Kentucky University which afforded a $25 Amazon gift certificate to each participant in appreciation for their enrollment in the study. This study has been presented as a poster (on-line format) at the American Physical Therapy Association Combined Sections Meeting in February 2022. The authors have no commercial interests associated with this study
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